Rule syntax

tip

Getting started with rule writing? Try the Semgrep Tutorial 🎓

This document describes Semgrep’s YAML rule syntax.

Schema

Required

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

Field	Type	Description
id	string	Unique, descriptive identifier, e.g., no-unused-variable
message	string	Message highlighting why this rule fired and how to remediate the issue
severity	string	One of: INFO, WARNING, or ERROR
languages	array	See supported languages
pattern*	string	Find code matching this expression
patterns*	array	Logical AND of multiple patterns
pattern-either*	array	Logical OR of multiple patterns
pattern-regex*	string	Search files for Python re compatible expressions

info

Only one of pattern, patterns, pattern-either, or pattern-regex is required.

Optional

Field	Type	Description
options	object	Options object to enable/disable certain matching features
fix	object	Simple search-and-replace autofix functionality
metadata	object	Arbitrary user-provided data; attach data to rules without affecting Semgrep’s behavior
paths	object	Paths to include or exclude when running this rule

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

Field	Type	Description
metavariable-regex	map	Search metavariables for Python re compatible expressions
metavariable-pattern	map	Matches metavariables with a pattern formula
metavariable-comparison	map	Compare metavariables against basic Python expressions
pattern-not	string	Logical NOT - remove findings matching this expression
pattern-inside	string	Keep findings that lie inside this pattern
pattern-not-inside	string	Keep findings that do not lie inside this pattern
pattern-not-regex	string	Filter results using Python re compatible expressions
pattern-where-python	string	Remove findings matching this Python expression

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(...).

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.

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.

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 a Python re compatible expression. This is useful for migrating existing regular expression code search functionality to Semgrep.

The pattern-regex operator can be combined with other pattern operators:

It can also be used as a standalone, top-level operator:

info

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

Note that if the regex uses groups, the metavariables $1, $2, etc. will be binded to the content of the captured group.

pattern-not-regex

The pattern-not-regex operator filters results using a Python re regular expression. 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 will work with regular pattern clauses, too.

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

This operator will filter 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.

metavariable-regex

The metavariable-regex operator searches metavariables for a Python re compatible 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.

info

Include quotes in your regular expression when using metavariable-regex to search string literals. See this snippet for more details. 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, it can be used to filter out matches that do not match certain criteria:

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.

It is also useful in combination with pattern-either:

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, will always evaluate to false.

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.

For example, we can match JavaScript code inside HTML:

We can also use this feature to filter regex matches:

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:

This will catch code like 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:

Building off of the previous example this will still catch code like set_port(80) but will no longer catch set_port(443) or set_port(8080).

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.

For example, base:

This will interpret metavariable values found in code as octal, so 0700 will be detected, but 0400 will not.

For example, strip:

This will remove quotes (', ", and `) from both ends of the metavariable content. So "2147483648" will be detected but "2147483646" will not. 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.

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.

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.

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.

pattern-where-python

The pattern-where-python is the most flexible operator. It allows for writing custom Python logic to filter findings. This is useful when none of the other operators provide the functionality needed to create a rule.

danger

Use caution with this operator. It allows for arbitrary Python code execution.

As a defensive measure, the --dangerously-allow-arbitrary-code-execution-from-rules flag must be passed to use rules containing pattern-where-python.

Example:

rules:  - id: use-decimalfield-for-money    patterns:      - pattern: $FIELD = django.db.models.FloatField(...)      - pattern-inside: |          class $CLASS(...):              ...      - pattern-where-python: "'price' in vars['$FIELD'] or 'salary' in vars['$FIELD']"    message: "use DecimalField for currency fields to avoid float-rounding errors"    languages: [python]    severity: ERROR

The above rule looks for use of Django’s FloatField model when storing currency information. FloatField can lead to rounding errors and should be avoided in favor of DecimalField when dealing with currency. Here the pattern-where-python operator allows us to utilize the Python in statement to filter findings that look like currency.

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 the following matching features:

Option	Default	Description
constant_propagation	true	Constant propagation, including intra-procedural flow-sensitive constant propagation.
ac_matching	true	Matching modulo associativity and commutativity, we treat Boolean AND/OR as associative, and bitwise AND/OR/XOR as both associative and commutative.
commutative_boolop	false	Treat Boolean AND/OR as commutative even if not semantically accurate.

The full list of available options can be consulted here. 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

metadata

To note extra information on a rule, such as a related CVE or the name of the security engineer who wrote the rule, use the metadata: key.

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 will also be shown in Semgrep’s output if you’re running it with --json.

paths

Excluding a rule in paths

To ignore a specific rule on specific files, set the paths: key with one or more filters.

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 will run on files inside project/, but no results will be 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 pathlib 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"

When invoked with semgrep -f rule.yaml project/, this rule will run on files inside project/, but results will be 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

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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