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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: INFO (Low severity), WARNING (Medium severity), or ERROR (High severity). 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 PCRE-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 sections in Supported languages document:

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 unanchored
metavariable-patternmapMatches metavariables with a pattern formula
metavariable-comparisonmapCompare metavariables against basic Python expressions
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 PCRE-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(...).

Example

Try this pattern in the Semgrep Playground.

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.

Example

Try this pattern in the Semgrep Playground.

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.

Example

Try this pattern in the Semgrep Playground.

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 PCRE 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

PCRE supports only a limited number of Unicode character properties. For example, \p{Egyptian_Hieroglyphs} is supported but \p{Bidi_Control} isn't.

Examples of the pattern-regex operator
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.

Example

Try this pattern in the Semgrep Playground.

pattern-not-regex​

The pattern-not-regex operator filters results using a PCRE 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.

Example

Try this pattern in the Semgrep Playground.

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 Cloud 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.

Example

Try this pattern in the Semgrep Playground.

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.)

Example

Try this pattern in the Semgrep Playground.

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
Example

Try this pattern in the Semgrep Playground.

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 PCRE 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.

Example

Try this pattern in the Semgrep Playground.

Regex matching is unanchored. For anchored matching, use \A for start-of-string anchoring and \Z for end-of-string anchoring. The next example, using the same expression as above but anchored, finds no matches:

Example

Try this pattern in the Semgrep Playground.

info

Include quotes in your regular expression when using metavariable-regex to search string literals. For more details, see include-quotes code snippet. String matching functionality can also be used to search string literals.

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:

Example

Try this pattern in the Semgrep Playground.

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:

Example

Try this pattern in the Semgrep Playground.

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

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.

Example

Try this pattern in the Semgrep Playground.

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.

Example of metavariable-comparison with base

Try this pattern in the Semgrep Playground.

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

Example of metavariable-comparison with strip

Try this pattern in the Semgrep Playground.

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.

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.

Example

Try this pattern in the Semgrep Playground.

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.

Example

Try this pattern in the Semgrep Playground.

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.

Example

Try this pattern in the Semgrep Playground.

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.
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.
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. Paths are relative to the root directory of the scanned project.

Example:

rules:
  - id: eqeq-is-bad
    pattern: $X == $X
    paths:
      exclude:
        - "*.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.

Find what you needed in this doc? Join the Semgrep Community Slack group to ask the maintainers and the community if you need help, or check out other ways to get help.