Redos

ReDoS Security Check (CWE-1333)

What this checks

Protects against Regular Expression Denial of Service where a crafted input causes catastrophic backtracking in regex engines. A single malicious string can pin a CPU core for minutes or hours, taking down the service.

Vulnerable patterns

• Nested quantifiers applied to user input — a quantified group whose body is itself quantified produces exponential state
• Overlapping alternation inside repetition — alternatives that can match the same prefix multiply backtracking work
• Common "email" or "URL" regexes built from quantified character classes wrapped in further repetition
• Regexes compiled from external configuration or user input where the pattern shape itself is attacker-controlled

Fix immediately

Flag the vulnerable regex, explain the risk, and suggest a fix establishing these properties. Translate the principle to the language and engine of the audited file — do not echo examples from another stack.

1. No nested quantifiers on user input. Collapse a quantified group whose body is also quantified into a single quantifier. Rewrite alternations so alternatives cannot match the same prefix.
2. Prefer a linear-time regex engine when one is available for the language under review. Engines that disallow backtracking guarantee linear matching regardless of pattern shape.
3. Bound regex execution. When the engine supports it, set a match timeout or run the match on a cancellable worker; when it does not, simplify the pattern until worst-case input completes in bounded time.
4. Treat patterns sourced from configuration or user input as untrusted. Either pre-compile a fixed allowlist of patterns, or run user-supplied patterns on a sandbox engine with a hard timeout.

Verification

• No regex applied to user input contains nested quantifiers or alternation inside repetition where alternatives share a prefix
• If the language offers a linear-time engine, the audited code uses it for user-input regexes; otherwise patterns are simplified or execution is time-bounded
• Regex patterns loaded from external configuration or user input run against an allowlist or under an enforced timeout

References

• CWE-1333 (Inefficient Regular Expression Complexity)
• CWE-400 (Uncontrolled Resource Consumption)