CVE-2015-8389 in PCRE
Summary
by MITRE
PCRE before 8.38 mishandles the /(?:|a|){100}x/ pattern and related patterns, which allows remote attackers to cause a denial of service (infinite recursion) or possibly have unspecified other impact via a crafted regular expression, as demonstrated by a JavaScript RegExp object encountered by Konqueror.
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Analysis
by VulDB Data Team • 06/28/2022
The vulnerability identified as CVE-2015-8389 represents a critical flaw in the Perl Compatible Regular Expressions library that affects versions prior to 8.38. This issue stems from the library's improper handling of specific regular expression patterns that contain nested quantifiers and alternation constructs. The problematic pattern structure /(?:|a|){100}x/ demonstrates how the regular expression engine can become trapped in infinite recursion when processing certain combinations of quantifiers and alternation operators. The vulnerability specifically affects the backtracking mechanism within PCRE's regex engine, where the parser fails to properly optimize or terminate recursive evaluation of complex pattern structures.
The technical implementation of this vulnerability exploits the fundamental design of how regular expression engines handle backtracking and quantifier evaluation. When processing the pattern /(?:|a|){100}x/, the engine attempts to evaluate all possible combinations of the alternation construct within the quantified group, leading to exponential growth in the number of potential match paths. This creates a scenario where the engine enters an infinite recursive loop, consuming system resources without termination. The issue is particularly dangerous because it can be triggered by user-supplied input through JavaScript RegExp objects, making it exploitable in web browser environments where regular expressions are commonly used for input validation and pattern matching.
The operational impact of CVE-2015-8389 extends beyond simple denial of service conditions to potentially enable more sophisticated attacks. While the primary effect manifests as infinite recursion leading to resource exhaustion and system instability, the vulnerability could theoretically be leveraged to create more complex attack vectors. The vulnerability affects web browsers and applications that rely on PCRE for regular expression processing, including Konqueror as demonstrated in the original exploit. This creates a significant risk for web applications that process untrusted user input through regular expression operations, as attackers could craft malicious patterns to consume system resources and potentially cause application crashes or system instability.
This vulnerability maps directly to CWE-674, which describes the "Uncontrolled Recursion" weakness in software systems, and aligns with ATT&CK technique T1059.007 for "Command and Scripting Interpreter: JavaScript.' The security implications extend to the broader category of denial of service attacks that target application logic rather than network infrastructure. Organizations using affected versions of PCRE should prioritize immediate patching to prevent exploitation, as the vulnerability can be triggered through common web application input validation mechanisms. Mitigation strategies include implementing input validation to limit regex complexity, using alternative regex engines with better recursion handling, and applying rate limiting to regex processing operations. The vulnerability highlights the importance of proper regex engine design and the need for comprehensive testing of regular expression patterns under various complexity conditions to prevent similar issues in the future.