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Semgrep developed metavariable analysis to support several metavariable inspection techniques that are difficult to express with existing rules, but have “simple” binary classifier behavior. Currently, this syntax supports two analyzers: redos and entropy.

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ReDoS

metavariable-analysis:
    analyzer: redos
    metavariable: $VARIABLE
Poorly constructed regular expressions that exhibit exponential runtime when fed specifically crafted inputs can cause RegEx denial of service. The redos analyzer uses known RegEx anti-patterns to determine if the target expression is potentially vulnerable to catastrophic backtracking.

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Entropy

metavariable-analysis:
    analyzer: entropy
    metavariable: $VARIABLE
Entropy is a common approach for detecting secret strings. Many existing tools utilize a combination of entropy calculations and regular expressions (RegEx) for secret detection. This analyzer returns true if a metavariable has high entropy, or randomness, relative to the English language.