CVE-2015-8385 in PCRE
Summary
by MITRE
PCRE before 8.38 mishandles the /(?|(\k'Pm')|(?'Pm'))/ pattern and related patterns with certain forward references, 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-2015-8385 represents a critical buffer overflow flaw within the Perl Compatible Regular Expressions library version 8.37 and earlier. This issue specifically manifests when processing certain complex regex patterns involving conditional subpatterns and named references, creating a scenario where malicious input can trigger arbitrary code execution or system instability. The flaw occurs during the compilation phase of regular expressions, particularly when the pattern contains nested conditional constructs with forward references that create ambiguous parsing conditions within the PCRE engine's internal state management mechanisms. The vulnerability affects web browsers and applications that utilize PCRE for pattern matching, including the Konqueror browser which served as the initial demonstration platform for this flaw.
The technical root cause stems from improper handling of duplicate named subpattern references within conditional constructs, specifically when the pattern structure /(?|(\k'Pm')|(?'Pm'))/ creates a situation where the regex engine cannot properly resolve which subpattern should be referenced during backtracking operations. This condition results in the engine attempting to access memory locations beyond the allocated buffer boundaries, leading to memory corruption that can manifest as stack overflow, heap corruption, or other memory-related anomalies. The flaw is categorized under CWE-121 as a stack-based buffer overflow, though it can potentially evolve into more severe memory corruption patterns that may enable code execution. The vulnerability is particularly dangerous because it requires no special privileges to exploit, making it suitable for remote code execution attacks in web browser contexts where user input is processed through regex engines.
The operational impact of this vulnerability extends beyond simple denial of service scenarios to potentially enable arbitrary code execution in affected applications. When a malicious user crafts a regular expression containing the vulnerable pattern structure, the target application's PCRE library will attempt to process the input, leading to buffer overflows that can be leveraged by attackers to execute malicious code with the privileges of the affected process. Web browsers like Konqueror are particularly at risk since they process user-supplied JavaScript regex patterns, creating a direct attack surface for remote exploitation. The vulnerability demonstrates how complex regex processing logic can create security risks that are not immediately apparent during normal development and testing phases, as the problematic patterns are edge cases that may not be encountered during typical usage scenarios. This makes the vulnerability particularly insidious as it can remain undetected until exploited in real-world attacks.
Mitigation strategies for CVE-2015-8385 focus primarily on updating to PCRE version 8.38 or later, which includes specific fixes for the buffer overflow conditions in conditional pattern handling. Organizations should implement comprehensive patch management procedures to ensure all systems utilizing PCRE-based applications receive the necessary updates promptly. Additionally, input validation and sanitization measures should be strengthened to prevent malicious regex patterns from reaching the PCRE engine, including implementing regex length limits and restricting the use of complex conditional constructs in user-supplied input. The vulnerability also highlights the importance of runtime protection mechanisms such as address space layout randomization and stack canaries, which can help mitigate the impact of buffer overflow exploits even when the underlying vulnerability remains unpatched. Security monitoring should include detection of suspicious regex patterns that may trigger similar vulnerabilities in other regular expression engines, as similar issues may exist in other implementations following the same parsing logic patterns. This vulnerability serves as a reminder of the critical importance of thorough testing of edge cases in security-critical code components and the necessity of maintaining current security patches across all software dependencies.