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Rule syntax

tip

Getting started with rule writing? Try the Semgrep Tutorial 🎓

This document describes the YAML rule syntax of Semgrep.

Schema​

Required​

All required fields must be present at the top-level of a rule, immediately under the rules key.

FieldTypeDescription
idstringUnique, descriptive identifier, for example: no-unused-variable
messagestringMessage that includes why Semgrep matched this pattern and how to remediate it. See also Rule messages.
severitystringOne of the following values: Low, Medium, High, Critical. The severity key specifies how critical are the issues that a rule potentially detects. Note: Semgrep Supply Chain differs, as its rules use CVE assignments for severity. For more information, see Filters section in Semgrep Supply Chain documentation.
languagesarraySee language extensions and tags
pattern*stringFind code matching this expression
patterns*arrayLogical AND of multiple patterns
pattern-either*arrayLogical OR of multiple patterns
pattern-regex*stringFind code matching this PCRE2-compatible pattern in multiline mode
info

Only one of the following is required: pattern, patterns, pattern-either, pattern-regex

Language extensions and languages key values​

The following table includes languages supported by Semgrep, accepted file extensions for test files that accompany rules, and valid values that Semgrep rules require in the languages key.

LanguageExtensionslanguages key values
Apex (only in Semgrep Pro Engine).clsapex
Bash.bash, .shbash, sh
C.cc
Cairo.cairocairo
Clojure.clj, .cljs, .cljc, .ednclojure
C++.cc, .cppcpp, c++
C#.cscsharp, c#
Dart.dartdart
Dockerfile.dockerfile, .Dockerfiledockerfile, docker
Elixir.ex, .exsex, elixir
Genericgeneric
Go.gogo, golang
HTML.htm, .htmlhtml
Java.javajava
JavaScript.js, .jsxjs, javascript
JSON.json, .ipynbjson
Jsonnet.jsonnet, .libsonnetjsonnet
JSX.js, .jsxjs, javascript
Julia.jljulia
Kotlin.kt, .kts, .ktmkt, kotlin
Lisp.lisp, .cl, .ellisp
Lua.lualua
OCaml.ml, .mliocaml
PHP.php, .tplphp
Python.py, .pyipython, python2, python3, py
R.r, .Rr
Ruby.rbruby
Rust.rsrust
Scala.scalascala
Scheme.scm, .ssscheme
Solidity.solsolidity, sol
Swift.swiftswift
Terraform.tf, .hcltf, hcl, terraform
TypeScript.ts, .tsxts, typescript
YAML.yml, .yamlyaml
XML.xmlxml
info

To see the maturity level of each supported language, see the following references:

Optional​

FieldTypeDescription
optionsobjectOptions object to enable/disable certain matching features
fixobjectSimple search-and-replace autofix functionality
metadataobjectArbitrary user-provided data; attach data to rules without affecting Semgrep behavior
min-versionstringMinimum Semgrep version compatible with this rule
max-versionstringMaximum Semgrep version compatible with this rule
pathsobjectPaths to include or exclude when running this rule

The below optional fields must reside underneath a patterns or pattern-either field.

FieldTypeDescription
pattern-insidestringKeep findings that lie inside this pattern

The below optional fields must reside underneath a patterns field.

FieldTypeDescription
metavariable-regexmapSearch metavariables for Python re compatible expressions; regex matching is left anchored
metavariable-patternmapMatches metavariables with a pattern formula
metavariable-comparisonmapCompare metavariables against basic Python expressions
metavariable-namemapMatches metavariables against constraints on what they name
pattern-notstringLogical NOT - remove findings matching this expression
pattern-not-insidestringKeep findings that do not lie inside this pattern
pattern-not-regexstringFilter results using a PCRE2-compatible pattern in multiline mode

Operators​

pattern​

The pattern operator looks for code matching its expression. This can be basic expressions like $X == $X or unwanted function calls like hashlib.md5(...).

rules:
  - id: md5-usage
    languages:
      - python
    message: Found md5 usage
    pattern: hashlib.md5(...)
    severity: ERROR

The pattern immediately above matches the following:

import hashlib
# ruleid: md5-usage
digest = hashlib.md5(b"test")
# ok: md5-usage
digest = hashlib.sha256(b"test")

patterns​

The patterns operator performs a logical AND operation on one or more child patterns. This is useful for chaining multiple patterns together that all must be true.

rules:
  - id: unverified-db-query
    patterns:
      - pattern: db_query(...)
      - pattern-not: db_query(..., verify=True, ...)
    message: Found unverified db query
    severity: ERROR
    languages:
      - python

The pattern immediately above matches the following:

# ruleid: unverified-db-query
db_query("SELECT * FROM ...")
# ok: unverified-db-query
db_query("SELECT * FROM ...", verify=True, env="prod")

patterns operator evaluation strategy​

Note that the order in which the child patterns are declared in a patterns operator has no effect on the final result. A patterns operator is always evaluated in the same way:

  1. Semgrep evaluates all positive patterns, that is pattern-insides, patterns, pattern-regexes, and pattern-eithers. Each range matched by each one of these patterns is intersected with the ranges matched by the other operators. The result is a set of positive ranges. The positive ranges carry metavariable bindings. For example, in one range $X can be bound to the function call foo(), and in another range $X can be bound to the expression a + b.
  2. Semgrep evaluates all negative patterns, that is pattern-not-insides, pattern-nots, and pattern-not-regexes. This gives a set of negative ranges which are used to filter the positive ranges. This results in a strict subset of the positive ranges computed in the previous step.
  3. Semgrep evaluates all conditionals, that is metavariable-regexes, metavariable-patterns and metavariable-comparisons. These conditional operators can only examine the metavariables bound in the positive ranges in step 1, that passed through the filter of negative patterns in step 2. Note that metavariables bound by negative patterns are not available here.
  4. Semgrep applies all focus-metavariables, by computing the intersection of each positive range with the range of the metavariable on which we want to focus. Again, the only metavariables available to focus on are those bound by positive patterns.

pattern-either​

The pattern-either operator performs a logical OR operation on one or more child patterns. This is useful for chaining multiple patterns together where any may be true.

rules:
  - id: insecure-crypto-usage
    pattern-either:
      - pattern: hashlib.sha1(...)
      - pattern: hashlib.md5(...)
    message: Found insecure crypto usage
    languages:
      - python
    severity: ERROR

The pattern immediately above matches the following:

import hashlib
# ruleid: insecure-crypto-usage
digest = hashlib.md5(b"test")
# ruleid: insecure-crypto-usage
digest = hashlib.sha1(b"test")
# ok: insecure-crypto-usage
digest = hashlib.sha256(b"test")

This rule looks for usage of the Python standard library functions hashlib.md5 or hashlib.sha1. Depending on their usage, these hashing functions are considered insecure.

pattern-regex​

The pattern-regex operator searches files for substrings matching the given PCRE2 pattern. This is useful for migrating existing regular expression code search functionality to Semgrep. Perl-Compatible Regular Expressions (PCRE) is a full-featured regex library that is widely compatible with Perl, but also with the respective regex libraries of Python, JavaScript, Go, Ruby, and Java. Patterns are compiled in multiline mode, for example ^ and $ matches at the beginning and end of lines respectively in addition to the beginning and end of input.

caution

PCRE2 supports some Unicode character properties, but not some Perl properties. For example, \p{Egyptian_Hieroglyphs} is supported but \p{InMusicalSymbols} isn't.

Example: pattern-regex combined with other pattern operators​

rules:
  - id: boto-client-ip
    patterns:
      - pattern-inside: boto3.client(host="...")
      - pattern-regex: \d{1,3}\.\d{1,3}\.\d{1,3}\.\d{1,3}
    message: boto client using IP address
    languages:
      - python
    severity: ERROR

The pattern immediately above matches the following:

import boto3
# ruleid: boto-client-ip
client = boto3.client(host="192.168.1.200")
# ok: boto-client-ip
client = boto3.client(host="dev.internal.example.com")

Example: pattern-regex used as a standalone, top-level operator​

rules:
  - id: legacy-eval-search
    pattern-regex: eval\(
    message: Insecure code execution
    languages:
      - javascript
    severity: ERROR

The pattern immediately above matches the following:

# ruleid: legacy-eval-search
eval('var a = 5')
info

Single (') and double (") quotes behave differently in YAML syntax. Single quotes are typically preferred when using backslashes (\) with pattern-regex.

Note that you may bind a section of a regular expression to a metavariable, by using named capturing groups. In this case, the name of the capturing group must be a valid metavariable name.

rules:
  - id: my_pattern_id-copy
    patterns:
      - pattern-regex: a(?P<FIRST>.*)b(?P<SECOND>.*)
    message: Semgrep found a match, with $FIRST and $SECOND
    languages:
      - regex
    severity: WARNING

The pattern immediately above matches the following:

acbd

pattern-not-regex​

The pattern-not-regex operator filters results using a PCRE2 regular expression in multiline mode. This is most useful when combined with regular-expression only rules, providing an easy way to filter findings without having to use negative lookaheads. pattern-not-regex works with regular pattern clauses, too.

The syntax for this operator is the same as pattern-regex.

This operator filters findings that have any overlap with the supplied regular expression. For example, if you use pattern-regex to detect Foo==1.1.1 and it also detects Foo-Bar==3.0.8 and Bar-Foo==3.0.8, you can use pattern-not-regex to filter the unwanted findings.

rules:
  - id: detect-only-foo-package
    languages:
      - regex
    message: Found foo package
    patterns:
      - pattern-regex: foo
      - pattern-not-regex: foo-
      - pattern-not-regex: -foo
    severity: ERROR

The pattern immediately above matches the following:

# ruleid: detect-only-foo-package
foo==1.1.1
# ok: detect-only-foo-package
foo-bar==3.0.8
# ok: detect-only-foo-package
bar-foo==3.0.8

focus-metavariable​

The focus-metavariable operator puts the focus, or zooms in, on the code region matched by a single metavariable or a list of metavariables. For example, to find all functions arguments annotated with the type bad you may write the following pattern:

pattern: |
  def $FUNC(..., $ARG : bad, ...):
    ...

This works but it matches the entire function definition. Sometimes, this is not desirable. If the definition spans hundreds of lines they are all matched. In particular, if you are using Semgrep AppSec Platform and you have triaged a finding generated by this pattern, the same finding shows up again as new if you make any change to the definition of the function!

To specify that you are only interested in the code matched by a particular metavariable, in our example $ARG, use focus-metavariable.

rules:
  - id: find-bad-args
    patterns:
      - pattern: |
          def $FUNC(..., $ARG : bad, ...):
            ...
      - focus-metavariable: $ARG
    message: |
      `$ARG' has a "bad" type!
    languages:
      - python
    severity: WARNING

The pattern immediately above matches the following:

def f(x : bad):
    return x

Note that focus-metavariable: $ARG is not the same as pattern: $ARG! Using pattern: $ARG finds all the uses of the parameter x which is not what we want! (Note that pattern: $ARG does not match the formal parameter declaration, because in this context $ARG only matches expressions.)

rules:
  - id: find-bad-args
    patterns:
      - pattern: |
          def $FUNC(..., $ARG : bad, ...):
            ...
      - pattern: $ARG
    message: |
      `$ARG' has a "bad" type!
    languages:
      - python
    severity: WARNING

The pattern immediately above matches the following:

def f(x : bad):
    return x

In short, focus-metavariable: $X is not a pattern in itself, it does not perform any matching, it only focuses the matching on the code already bound to $X by other patterns. Whereas pattern: $X matches $X against your code (and in this context, $X only matches expressions)!

Including multiple focus metavariables using set intersection semantics​

Include more focus-metavariable keys with different metavariables under the pattern to match results only for the overlapping region of all the focused code:

    patterns:
      - pattern: foo($X, ..., $Y)
      - focus-metavariable:
        - $X
        - $Y
rules:
  - id: intersect-focus-metavariable
    patterns:
      - pattern-inside: foo($X, ...)
      - focus-metavariable: $X
      - pattern: $Y + ...
      - focus-metavariable: $Y
      - pattern: "1"
    message: Like set intersection, only the overlapping region is highilighted
    languages:
      - python
    severity: ERROR

The pattern immediately above matches the following:

# ruleid: intersect-focus-metavariable
foo (
    1
    +
    2,
    1
)

# OK: test
foo (2+ 1, 1)
info

To make a list of multiple focus metavariables using set union semantics that matches the metavariables regardless of their position in code, see Including multiple focus metavariables using set union semantics documentation.

metavariable-regex​

The metavariable-regex operator searches metavariables for a PCRE2 regular expression. This is useful for filtering results based on a metavariable’s value. It requires the metavariable and regex keys and can be combined with other pattern operators.

rules:
  - id: insecure-methods
    patterns:
      - pattern: module.$METHOD(...)
      - metavariable-regex:
          metavariable: $METHOD
          regex: (insecure)
    message: module using insecure method call
    languages:
      - python
    severity: ERROR

The pattern immediately above matches the following:

# ruleid: insecure-methods
module.insecure1("test")
# ruleid: insecure-methods
module.insecure2("test")
# ruleid: insecure-methods
module.insecure3("test")
# ok: insecure-methods
module.secure("test")

Regex matching is left anchored. To allow prefixes, use .* at the beginning of the regex. To match the end of a string, use $. The next example, using the same expression as above but anchored on the right, finds no matches:

rules:
  - id: insecure-methods
    patterns:
      - pattern: module.$METHOD(...)
      - metavariable-regex:
          metavariable: $METHOD
          regex: (insecure$)
    message: module using insecure method call
    languages:
      - python
    severity: ERROR

The following example matches all of the function calls in the same code sample, returning a false positive on the module.secure call:

rules:
  - id: insecure-methods
    patterns:
      - pattern: module.$METHOD(...)
      - metavariable-regex:
          metavariable: $METHOD
          regex: (.*secure)
    message: module using insecure method call
    languages:
      - python
    severity: ERROR
info

Include quotes in your regular expression when using metavariable-regex to search string literals. For more details, see include-quotes code snippet.

metavariable-pattern​

The metavariable-pattern operator matches metavariables with a pattern formula. This is useful for filtering results based on a metavariable’s value. It requires the metavariable key, and exactly one key of pattern, patterns, pattern-either, or pattern-regex. This operator can be nested as well as combined with other operators.

For example, the metavariable-pattern can be used to filter out matches that do not match certain criteria:

rules:
  - id: disallow-old-tls-versions2
    languages:
      - javascript
    message: Match found
    patterns:
      - pattern: |
          $CONST = require('crypto');
          ...
          $OPTIONS = $OPTS;
          ...
          https.createServer($OPTIONS, ...);
      - metavariable-pattern:
          metavariable: $OPTS
          patterns:
            - pattern-not: >
                {secureOptions: $CONST.SSL_OP_NO_SSLv2 | $CONST.SSL_OP_NO_SSLv3
                | $CONST.SSL_OP_NO_TLSv1}
    severity: WARNING

The pattern immediately above matches the following:

function bad() {
    // ruleid:disallow-old-tls-versions2
    var constants = require('crypto');
    var sslOptions = {
    key: fs.readFileSync('/etc/ssl/private/private.key'),
    secureProtocol: 'SSLv23_server_method',
    secureOptions: constants.SSL_OP_NO_SSLv2 | constants.SSL_OP_NO_SSLv3
    };
    https.createServer(sslOptions);
}
info

In this case it is possible to start a patterns AND operation with a pattern-not, because there is an implicit pattern: ... that matches the content of the metavariable.

The metavariable-pattern is also useful in combination with pattern-either:

rules:
  - id: open-redirect
    languages:
      - python
    message: Match found
    patterns:
      - pattern-inside: |
          def $FUNC(...):
            ...
            return django.http.HttpResponseRedirect(..., $DATA, ...)
      - metavariable-pattern:
          metavariable: $DATA
          patterns:
            - pattern-either:
                - pattern: $REQUEST
                - pattern: $STR.format(..., $REQUEST, ...)
                - pattern: $STR % $REQUEST
                - pattern: $STR + $REQUEST
                - pattern: f"...{$REQUEST}..."
            - metavariable-pattern:
                metavariable: $REQUEST
                patterns:
                  - pattern-either:
                      - pattern: request.$W
                      - pattern: request.$W.get(...)
                      - pattern: request.$W(...)
                      - pattern: request.$W[...]
                  - metavariable-regex:
                      metavariable: $W
                      regex: (?!get_full_path)
    severity: WARNING

The pattern immediately above matches the following:

from django.http import HttpResponseRedirect
def unsafe(request):
    # ruleid:open-redirect
    return HttpResponseRedirect(request.POST.get("url"))
tip

It is possible to nest metavariable-pattern inside metavariable-pattern!

info

The metavariable should be bound to an expression, a statement, or a list of statements, for this test to be meaningful. A metavariable bound to a list of function arguments, a type, or a pattern, always evaluate to false.

metavariable-pattern with nested language​

If the metavariable's content is a string, then it is possible to use metavariable-pattern to match this string as code by specifying the target language via the language key. See the following examples of metavariable-pattern:

Examples of metavariable-pattern

Example: Match JavaScript code inside HTML​

rules:
  - id: test
    languages:
      - generic
    message: javascript inside html working!
    patterns:
      - pattern: |
          <script ...>$...JS</script>
      - metavariable-pattern:
          language: javascript
          metavariable: $...JS
          patterns:
            - pattern: |
                console.log(...)
    severity: WARNING

The pattern immediately above matches the following:

<!-- ruleid:test -->
<script>
console.log("hello")
</script>

Example: Filter regex matches​

rules:
  - id: test
    languages:
      - generic
    message: "Google dependency: $1 $2"
    patterns:
      - pattern-regex: gem "(.*)", "(.*)"
      - metavariable-pattern:
          metavariable: $1
          language: generic
          patterns:
            - pattern: google
    severity: INFO

The pattern immediately above matches the following:

source "https://rubygems.org"

#OK:test
gem "functions_framework", "~> 0.7"
#ruleid:test
gem "google-cloud-storage", "~> 1.29"

metavariable-comparison​

The metavariable-comparison operator compares metavariables against a basic Python comparison expression. This is useful for filtering results based on a metavariable's numeric value.

The metavariable-comparison operator is a mapping which requires the metavariable and comparison keys. It can be combined with other pattern operators in the following Semgrep Playground example.

This matches code such as set_port(80) or set_port(443), but not set_port(8080).

Comparison expressions support simple arithmetic as well as composition with Boolean operators to allow for more complex matching. This is particularly useful for checking that metavariables are divisible by particular values, such as enforcing that a particular value is even or odd.

rules:
  - id: superuser-port
    languages:
      - python
    message: module setting superuser port
    patterns:
      - pattern: set_port($ARG)
      - metavariable-comparison:
          comparison: $ARG < 1024 and $ARG % 2 == 0
          metavariable: $ARG
    severity: ERROR

The pattern immediately above matches the following:

# ok: superuser-port
set_port(443)
# ruleid: superuser-port
set_port(80)
# ok: superuser-port
set_port(8080)

Building on the previous example, this still matches code such as set_port(80) but it no longer matches set_port(443) or set_port(8080).

The comparison key accepts Python expression using:

  • Boolean, string, integer, and float literals.
  • Boolean operators not, or, and and.
  • Arithmetic operators +, -, *, /, and %.
  • Comparison operators ==, !=, <, <=, >, and >=.
  • Function int() to convert strings into integers.
  • Function str() to convert numbers into strings.
  • Function today() that gets today's date as a float representing epoch time.
  • Function strptime() that converts strings in the format "yyyy-mm-dd" to a float representing the date in epoch time.
  • Lists, together with the in, and not in infix operators.
  • Strings, together with the in and not in infix operators, for substring containment.
  • Function re.match() to match a regular expression (without the optional flags argument).

You can use Semgrep metavariables such as $MVAR, which Semgrep evaluates as follows:

  • If $MVAR binds to a literal, then that literal is the value assigned to $MVAR.
  • If $MVAR binds to a code variable that is a constant, and constant propagation is enabled (as it is by default), then that constant is the value assigned to $MVAR.
  • Otherwise the code bound to the $MVAR is kept unevaluated, and its string representation can be obtained using the str() function, as in str($MVAR). For example, if $MVAR binds to the code variable x, str($MVAR) evaluates to the string literal "x".

Legacy metavariable-comparison keys​

info

You can avoid the use of the legacy keys described below (base: int and strip: bool) by using the int() function, as in int($ARG) > 0o600 or int($ARG) > 2147483647.

The metavariable-comparison operator also takes optional base: int and strip: bool keys. These keys set the integer base the metavariable value should be interpreted as and remove quotes from the metavariable value, respectively.

rules:
  - id: excessive-permissions
    languages:
      - python
    message: module setting excessive permissions
    patterns:
      - pattern: set_permissions($ARG)
      - metavariable-comparison:
          comparison: $ARG > 0o600
          metavariable: $ARG
          base: 8
    severity: ERROR

The pattern immediately above matches the following:

# ruleid: excessive-permissions
set_permissions(0o700)
# ok: excessive-permissions
set_permissions(0o400)

This interprets metavariable values found in code as octal. As a result, Semgrep detects 0700, but it does not detect 0400.

rules:
  - id: int-overflow
    languages:
      - python
    message: Potential integer overflow
    patterns:
      - pattern: int($ARG)
      - metavariable-comparison:
          strip: true
          comparison: $ARG > 2147483647
          metavariable: $ARG
    severity: ERROR

The pattern immediately above matches the following:

# ruleid: int-overflow
int("2147483648")
# ok: int-overflow
int("2147483646")

This removes quotes (', ", and `) from both ends of the metavariable content. As a result, Semgrep detects "2147483648", but it does not detect "2147483646". This is useful when you expect strings to contain integer or float data.

metavariable-name​

tip
  • metavariable-name requires a Semgrep account and the use of Semgrep's proprietary engine since it requires name resolution information. This means that it does not work with the --oss-only flag.
  • While optional, you can improve the accuracy of metavariable-name by enabling cross-file analysis.

The metavariable-name operator adds a constraint to the types of identifiers a metavariable is able to match. Currently the only constraint supported is on the module or namespace an identifier originates from. This is useful for filtering results in languages which don't have a native syntax for fully qualified names, or languages where module names may contain characters which are not legal in identifiers, such as JavaScript or TypeScript.

rules:
  - id: insecure-method
    patterns:
      - pattern: $MODULE.insecure(...)
      - metavariable-name:
          metavariable: $MODULE
          module: "@foo-bar"
    message: Uses insecure method from @foo-bar.
    languages:
      - javascript
    severity: ERROR

The pattern immediately above matches the following:

// ECMAScript modules
import * as lib from '@foo-bar';
import * as lib2 from 'myotherlib';

// CommonJS modules
const { insecure } = require('@foo-bar');
const lib3 = require('myotherlib');

// ruleid: insecure-method
lib.insecure("test");
// ruleid: insecure-method
insecure("test");

// ok: insecure-method
lib.secure("test");
// ok: insecure-method
lib2.insecure("test");
// ok: insecure-method
lib3.insecure("test");

In the event that a match should occur if the metavariable matches one of a variety of matches, there is also a shorthand modules key, which takes a list of module names.

rules:
  - id: insecure-method
    patterns:
      - pattern: $MODULE.method(...)
      - metavariable-regex:
          metavariable: $MODULE
          modules:
           - foo
           - bar
    message: Uses insecure method from @foo-bar.
    languages:
      - javascript
    severity: ERROR

This can be useful in instances where there may be multiple API-compatible packages which share an issue.

pattern-not​

The pattern-not operator is the opposite of the pattern operator. It finds code that does not match its expression. This is useful for eliminating common false positives.

rules:
  - id: unverified-db-query
    patterns:
      - pattern: db_query(...)
      - pattern-not: db_query(..., verify=True, ...)
    message: Found unverified db query
    severity: ERROR
    languages:
      - python

The pattern immediately above matches the following:

# ruleid: unverified-db-query
db_query("SELECT * FROM ...")
# ok: unverified-db-query
db_query("SELECT * FROM ...", verify=True, env="prod")

pattern-inside​

The pattern-inside operator keeps matched findings that reside within its expression. This is useful for finding code inside other pieces of code like functions or if blocks.

rules:
  - id: return-in-init
    patterns:
      - pattern: return ...
      - pattern-inside: |
          class $CLASS:
            ...
      - pattern-inside: |
          def __init__(...):
              ...
    message: return should never appear inside a class __init__ function
    languages:
      - python
    severity: ERROR

The pattern immediately above matches the following:

class A:
    def __init__(self):
        # ruleid: return-in-init
        return None

class B:
    def __init__(self):
        # ok: return-in-init
        self.inited = True

def foo():
    # ok: return-in-init
    return 5

pattern-not-inside​

The pattern-not-inside operator keeps matched findings that do not reside within its expression. It is the opposite of pattern-inside. This is useful for finding code that’s missing a corresponding cleanup action like disconnect, close, or shutdown. It’s also useful for finding problematic code that isn't inside code that mitigates the issue.

rules:
  - id: open-never-closed
    patterns:
      - pattern: $F = open(...)
      - pattern-not-inside: |
          $F = open(...)
          ...
          $F.close()
    message: file object opened without corresponding close
    languages:
      - python
    severity: ERROR

The pattern immediately above matches the following:

def func1():
    # ruleid: open-never-closed
    fd = open('test.txt')
    results = fd.read()
    return results

def func2():
    # ok: open-never-closed
    fd = open('test.txt')
    results = fd.read()
    fd.close()
    return results

The above rule looks for files that are opened but never closed, possibly leading to resource exhaustion. It looks for the open(...) pattern and not a following close() pattern.

The $F metavariable ensures that the same variable name is used in the open and close calls. The ellipsis operator allows for any arguments to be passed to open and any sequence of code statements in-between the open and close calls. The rule ignores how open is called or what happens up to a close call—it only needs to make sure close is called.

Metavariable matching​

Metavariable matching operates differently for logical AND (patterns) and logical OR (pattern-either) parent operators. Behavior is consistent across all child operators: pattern, pattern-not, pattern-regex, pattern-inside, pattern-not-inside.

Metavariables in logical ANDs​

Metavariable values must be identical across sub-patterns when performing logical AND operations with the patterns operator.

Example:

rules:
  - id: function-args-to-open
    patterns:
      - pattern-inside: |
          def $F($X):
              ...
      - pattern: open($X)
    message: "Function argument passed to open() builtin"
    languages: [python]
    severity: ERROR

This rule matches the following code:

def foo(path):
    open(path)

The example rule doesn’t match this code:

def foo(path):
    open(something_else)

Metavariables in logical ORs​

Metavariable matching does not affect the matching of logical OR operations with the pattern-either operator.

Example:

rules:
  - id: insecure-function-call
    pattern-either:
      - pattern: insecure_func1($X)
      - pattern: insecure_func2($X)
    message: "Insecure function use"
    languages: [python]
    severity: ERROR

The above rule matches both examples below:

insecure_func1(something)
insecure_func2(something)
insecure_func1(something)
insecure_func2(something_else)

Metavariables in complex logic​

Metavariable matching still affects subsequent logical ORs if the parent is a logical AND.

Example:

patterns:
  - pattern-inside: |
      def $F($X):
        ...
  - pattern-either:
      - pattern: bar($X)
      - pattern: baz($X)

The above rule matches both examples below:

def foo(something):
    bar(something)
def foo(something):
    baz(something)

The example rule doesn’t match this code:

def foo(something):
    bar(something_else)

options​

Enable, disable, or modify the following matching features:

OptionDefaultDescription
ac_matchingtrueMatching modulo associativity and commutativity, treat Boolean AND/OR as associative, and bitwise AND/OR/XOR as both associative and commutative.
attr_exprtrueExpression patterns (for example: f($X)) matches attributes (for example: @f(a)).
commutative_boolopfalseTreat Boolean AND/OR as commutative even if not semantically accurate.
constant_propagationtrueConstant propagation, including intra-procedural flow-sensitive constant propagation.
decorators_order_mattersfalseMatch non-keyword attributes (for example: decorators in Python) in order, instead of the order-agnostic default. Keyword attributes (for example: static, inline, etc) are not affected.
generic_comment_stylenoneIn generic mode, assume that comments follow the specified syntax. They are then ignored for matching purposes. Allowed values for comment styles are:
  • c for traditional C-style comments (/* ... */).
  • cpp for modern C or C++ comments (// ... or /* ... */).
  • shell for shell-style comments (# ...).
By default, the generic mode does not recognize any comments. Available since Semgrep version 0.96. For more information about generic mode, see Generic pattern matching documentation.
generic_ellipsis_max_span10In generic mode, this is the maximum number of newlines that an ellipsis operator ... can match or equivalently, the maximum number of lines covered by the match minus one. The default value is 10 (newlines) for performance reasons. Increase it with caution. Note that the same effect as 20 can be achieved without changing this setting and by writing ... ... in the pattern instead of .... Setting it to 0 is useful with line-oriented languages (for example INI or key-value pairs in general) to force a match to not extend to the next line of code. Available since Semgrep 0.96. For more information about generic mode, see Generic pattern matching documentation.
implicit_returntrueReturn statement patterns (for example return $E) match expressions that may be evaluated last in a function as if there was a return keyword in front of those expressions. Only applies to certain expression-based languages, such as Ruby and Julia.
interfilefalseSet this value to true for Semgrep to run this rule with cross-function and cross-file analysis. It is required for rules that use cross-function, cross-file analysis.
symmetric_eqfalseTreat equal operations as symmetric (for example: a == b is equal to b == a).
taint_assume_safe_functionsfalseExperimental option which will be subject to future changes. Used in taint analysis. Assume that function calls do not propagate taint from their arguments to their output. Otherwise, Semgrep always assumes that functions may propagate taint. Can replace not-conflicting sanitizers added in v0.69.0 in the future.
taint_assume_safe_indexesfalseUsed in taint analysis. Assume that an array-access expression is safe even if the index expression is tainted. Otherwise Semgrep assumes that for example: a[i] is tainted if i is tainted, even if a is not. Enabling this option is recommended for high-signal rules, whereas disabling is preferred for audit rules. Currently, it is disabled by default to attain backwards compatibility, but this can change in the near future after some evaluation.
vardef_assigntrueAssignment patterns (for example $X = $E) match variable declarations (for example var x = 1;).
xml_attrs_implicit_ellipsistrueAny XML/JSX/HTML element patterns have implicit ellipsis for attributes (for example: <div /> matches <div foo="1">.

The full list of available options can be consulted in the Semgrep matching engine configuration module. Note that options not included in the table above are considered experimental, and they may change or be removed without notice.

fix​

The fix top-level key allows for simple autofixing of a pattern by suggesting an autofix for each match. Run semgrep with --autofix to apply the changes to the files.

Example:

rules:
  - id: use-dict-get
    patterns:
      - pattern: $DICT[$KEY]
    fix: $DICT.get($KEY)
    message: "Use `.get()` method to avoid a KeyNotFound error"
    languages: [python]
    severity: ERROR

For more information about fix and --autofix see Autofix documentation.

metadata​

Provide additional information for a rule with the metadata: key, such as a related CWE, likelihood, OWASP.

Example:

rules:
  - id: eqeq-is-bad
    patterns:
      - [...]
    message: "useless comparison operation `$X == $X` or `$X != $X`"
    metadata:
      cve: CVE-2077-1234
      discovered-by: Ikwa L'equale

The metadata are also displayed in the output of Semgrep if you’re running it with --json. Rules with category: security have additional metadata requirements. See Including fields required by security category for more information.

min-version and max-version​

Each rule supports optional fields min-version and max-version specifying minimum and maximum Semgrep versions. If the Semgrep version being used doesn't satisfy these constraints, the rule is skipped without causing a fatal error.

Example rule:

rules:
  - id: bad-goflags
    # earlier semgrep versions can't parse the pattern
    min-version: 1.31.0
    pattern: |
      ENV ... GOFLAGS='-tags=dynamic -buildvcs=false' ...
    languages: [dockerfile]
    message: "We should not use these flags"
    severity: WARNING

Another use case is when a newer version of a rule works better than before but relies on a new feature. In this case, we could use min-version and max-version to ensure that either the older or the newer rule is used but not both. The rules would look like this:

rules:
  - id: something-wrong-v1
    max-version: 1.72.999
    ...
  - id: something-wrong-v2
    min-version: 1.73.0
    # 10x faster than v1!
    ...

The min-version/max-version feature is available since Semgrep 1.38.0. It is intended primarily for publishing rules that rely on newly released features without causing errors in older Semgrep installations.

category​

Provide a category for users of the rule. For example: best-practice, correctness, maintainability. For more information, see Semgrep registry rule requirements.

paths​

Excluding a rule in paths​

To ignore a specific rule on specific files, set the paths: key with one or more filters. The patterns apply to the full file paths relative to the project root.

Example:

rules:
  - id: eqeq-is-bad
    pattern: $X == $X
    paths:
      exclude:
        - "src/**/*.jinja2"
        - "*_test.go"
        - "project/tests"
        - project/static/*.js

When invoked with semgrep -f rule.yaml project/, the above rule runs on files inside project/, but no results are returned for:

  • any file with a .jinja2 file extension
  • any file whose name ends in _test.go, such as project/backend/server_test.go
  • any file inside project/tests or its subdirectories
  • any file matching the project/static/*.js glob pattern
note

The glob syntax is from Python's wcmatch and is used to match against the given file and all its parent directories.

Limiting a rule to paths​

Conversely, to run a rule only on specific files, set a paths: key with one or more of these filters:

rules:
  - id: eqeq-is-bad
    pattern: $X == $X
    paths:
      include:
        - "*_test.go"
        - "project/server"
        - "project/schemata"
        - "project/static/*.js"
        - "tests/**/*.js"

When invoked with semgrep -f rule.yaml project/, this rule runs on files inside project/, but results are returned only for:

  • files whose name ends in _test.go, such as project/backend/server_test.go
  • files inside project/server, project/schemata, or their subdirectories
  • files matching the project/static/*.js glob pattern
  • all files with the .js extension, arbitrary depth inside the tests folder

If you are writing tests for your rules, add any test file or directory to the included paths as well.

note

When mixing inclusion and exclusion filters, the exclusion ones take precedence.

Example:

paths:
  include: "project/schemata"
  exclude: "*_internal.py"

The above rule returns results from project/schemata/scan.py but not from project/schemata/scan_internal.py.

Other examples​

This section contains more complex rules that perform advanced code searching.

Complete useless comparison​

rules:
  - id: eqeq-is-bad
    patterns:
      - pattern-not-inside: |
          def __eq__(...):
              ...
      - pattern-not-inside: assert(...)
      - pattern-not-inside: assertTrue(...)
      - pattern-not-inside: assertFalse(...)
      - pattern-either:
          - pattern: $X == $X
          - pattern: $X != $X
          - patterns:
              - pattern-inside: |
                  def __init__(...):
                       ...
              - pattern: self.$X == self.$X
      - pattern-not: 1 == 1
    message: "useless comparison operation `$X == $X` or `$X != $X`"

The above rule makes use of many operators. It uses pattern-either, patterns, pattern, and pattern-inside to carefully consider different cases, and uses pattern-not-inside and pattern-not to whitelist certain useless comparisons.

Full specification​

The full configuration-file format is defined as a jsonschema object.

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