Skip to main content

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.

FieldTypeDescription
idstringUnique, descriptive identifier, e.g., no-unused-variable
messagestringMessage highlighting why this rule fired and how to remediate the issue
severitystringOne of: INFO, WARNING, or ERROR
languagesarraySee supported languages
pattern*stringFind code matching this expression
patterns*arrayLogical AND of multiple patterns
pattern-either*arrayLogical OR of multiple patterns
pattern-regex*stringSearch files for Python re compatible expressions
info

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

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’s behavior
pathsobjectPaths to include or exclude when running this rule

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

FieldTypeDescription
metavariable-regexmapSearch metavariables for Python re compatible expressions
metavariable-patternmapMatches metavariables with a pattern formula
metavariable-comparisonmapCompare metavariables against basic Python expressions
pattern-notstringLogical NOT - remove findings matching this expression
pattern-insidestringKeep findings that lie inside this pattern
pattern-not-insidestringKeep findings that do not lie inside this pattern
pattern-not-regexstringFilter results using Python re compatible expressions
pattern-where-pythonstringRemove 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:

OptionDefaultDescription
constant_propagationtrueConstant propagation, including intra-procedural flow-sensitive constant propagation.
ac_matchingtrueMatching modulo associativity and commutativity, we treat Boolean AND/OR as associative, and bitwise AND/OR/XOR as both associative and commutative.
commutative_boolopfalseTreat 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.


Find what you needed in this doc? Join the Slack group to ask the maintainers and the community if you need help.