CVE-2015-8386 in PCRE
Summary
by MITRE
PCRE before 8.38 mishandles the interaction of lookbehind assertions and mutually recursive subpatterns, which allows remote attackers to cause a denial of service (buffer overflow) 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-2015-8386 represents a critical buffer overflow condition within the Perl Compatible Regular Expressions library version 8.37 and earlier. This flaw specifically manifests when processing regular expressions containing lookbehind assertions in conjunction with mutually recursive subpatterns, creating a scenario where maliciously crafted patterns can trigger unintended memory corruption. The vulnerability was discovered through analysis of how PCRE handles complex pattern matching operations and demonstrates a fundamental issue in the library's handling of recursive pattern structures. Attackers can exploit this weakness by constructing carefully crafted regular expressions that, when processed by vulnerable PCRE implementations, lead to memory access violations. The vulnerability affects web browsers and applications that rely on PCRE for regular expression processing, particularly impacting Konqueror browser's JavaScript engine which processes RegExp objects using the affected library.
The technical implementation of this vulnerability stems from how PCRE manages the execution stack during pattern matching operations involving lookbehind assertions and recursive subpatterns. When a regular expression contains both lookbehind constructs and mutually recursive patterns, the library's internal state management becomes inconsistent, leading to improper memory allocation and access patterns. The lookbehind assertion requires the engine to examine text in reverse direction, while mutually recursive subpatterns create nested execution contexts that must be properly tracked. This combination causes the stack pointer to move beyond allocated memory boundaries, resulting in buffer overflow conditions. The flaw operates at the core level of PCRE's matching engine, specifically within the code that handles backtracking and state management during complex pattern evaluation. This vulnerability is classified under CWE-121 as a stack-based buffer overflow, representing a classic memory corruption issue where insufficient bounds checking allows data to be written beyond allocated buffer limits.
The operational impact of CVE-2015-8386 extends beyond simple denial of service conditions to potentially enable more sophisticated attacks depending on the target environment. Remote attackers can cause applications to crash through controlled buffer overflows, leading to persistent denial of service across web applications, email servers, and other systems relying on PCRE for text processing. In some scenarios, the vulnerability may allow for information disclosure or even code execution if the affected system's memory management allows exploitation of the buffer overflow conditions. The vulnerability affects web browsers such as Konqueror, but also impacts any application that utilizes PCRE's regular expression capabilities, including web servers, content management systems, and security tools. The specific nature of the flaw means that exploitation requires crafting of malicious regular expressions that leverage the interaction between lookbehind assertions and recursive patterns, making it somewhat more complex than typical buffer overflow vulnerabilities but still highly dangerous in practice.
Mitigation strategies for CVE-2015-8386 primarily focus on updating to PCRE version 8.38 or later, where the underlying buffer overflow conditions have been addressed through improved stack management and bounds checking. System administrators should prioritize patching all affected applications and services that utilize PCRE, particularly those handling untrusted input through regular expressions. Additionally, implementing input validation and sanitization measures can help reduce the attack surface by limiting the complexity and length of regular expressions processed by vulnerable systems. Organizations should also consider implementing regular expression timeouts or execution limits to prevent excessive processing time that could be exploited in conjunction with this vulnerability. The remediation process should include thorough testing of updated libraries to ensure compatibility with existing applications while maintaining the security improvements. Security monitoring should be enhanced to detect unusual patterns of regular expression processing that might indicate exploitation attempts, particularly in web applications that process user-supplied input through regex operations. This vulnerability demonstrates the importance of proper memory management in cryptographic and text processing libraries, emphasizing that even well-established software can contain critical flaws that require ongoing security maintenance and updates.