Regular Expression Denial of Service (ReDoS) vulnerabilities occur when poorly designed regular expressions can be exploited to cause excessive CPU consumption, potentially leading to application unavailability.
Regular Expression Denial of Service Overview
Regular Expression Denial of Service (ReDoS) is a vulnerability that occurs when a regular expression contains patterns that can cause exponential or catastrophic backtracking. When these regular expressions process maliciously crafted input, they can consume excessive CPU resources, potentially leading to denial of service.
The root cause of ReDoS is the backtracking behavior in regex engines. When a regex engine attempts to match a pattern but encounters a situation where multiple paths could be taken, it may need to try all possible paths before determining that a match is impossible. With carefully crafted input, this can lead to exponential time complexity.
Preventing ReDoS requires careful design of regular expressions, avoiding vulnerable patterns, and implementing proper timeouts and input validation.
Catastrophic Backtracking
Catastrophic backtracking occurs when a regular expression contains patterns that can match the same input in multiple ways, causing the regex engine to try all possible combinations.
Common patterns that can lead to catastrophic backtracking:
(a+)+
)To prevent catastrophic backtracking:
Vulnerable Regex Patterns
Certain regex patterns are particularly vulnerable to ReDoS attacks due to their backtracking behavior.
To identify and fix vulnerable patterns:
(a+)+
or (a*b*)*
(ab|abc)+
(?>...)
when available (in languages that support them)a++
when available (in languages that support them)[a]
instead of single characters with quantifiersTimeout Mechanisms
Implementing timeout mechanisms is crucial for mitigating the impact of ReDoS attacks, as they limit the resources that can be consumed by a single regex operation.
To implement effective timeout mechanisms:
Alternative Regex Engines
Alternative regex engines with guaranteed linear time complexity can eliminate the risk of ReDoS vulnerabilities.
Popular alternative regex engines:
Considerations when using alternative engines:
Input Validation Strategies
Proper input validation before applying regular expressions can significantly reduce the risk of ReDoS attacks.
Effective input validation strategies:
Testing for ReDoS Vulnerabilities
Testing for ReDoS vulnerabilities is essential to identify problematic regex patterns before they can be exploited.
Effective testing strategies:
Static Analysis for ReDoS
Static analysis tools can help identify potentially vulnerable regex patterns during development, before they can be exploited.
Popular static analysis tools for ReDoS:
Benefits of static analysis:
ReDoS Prevention Checklist
Preventing ReDoS vulnerabilities requires a comprehensive approach that addresses regex pattern design, input validation, execution controls, and monitoring.
Key prevention strategies: