CVE-2015-8392 in PCRE
Summary
by MITRE
PCRE before 8.38 mishandles certain instances of the (?| substring, which allows remote attackers to cause a denial of service (unintended recursion and buffer overflow) or possibly have unspecified other impact via a crafted regular expression, as demonstrated by a JavaScript RegExp object encountered by Konqueror, a related issue to CVE-2015-8384 and CVE-2015-8395.
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Analysis
by VulDB Data Team • 06/28/2022
The vulnerability identified as CVE-2015-8392 affects the Perl Compatible Regular Expressions library version 8.37 and earlier, representing a critical flaw in regular expression processing that can be exploited remotely. This issue stems from improper handling of the (?| substring syntax within PCRE patterns, which creates unintended recursive behavior during pattern compilation and execution. The flaw specifically manifests when processing certain regular expressions that utilize the conditional pattern syntax, where the library fails to properly manage the state transitions between different branches of the pattern, leading to unpredictable execution paths that can result in either infinite recursion or buffer overflow conditions.
The technical implementation of this vulnerability exploits the way PCRE manages nested and conditional patterns within regular expressions, particularly when the (?| syntax is used to create conditional branches that share the same capture group numbers. When a maliciously crafted regular expression is processed, the library's internal state management becomes corrupted, causing the parser to enter recursive loops or allocate excessive memory resources. This behavior is particularly dangerous in web browsers and applications that process user-supplied regular expressions, such as the Konqueror web browser mentioned in the vulnerability description, where JavaScript RegExp objects can trigger the flaw during pattern compilation or execution phases.
The operational impact of CVE-2015-8392 extends beyond simple denial of service to potentially enable more sophisticated attacks depending on the execution environment. Remote attackers can leverage this vulnerability to consume excessive system resources through unintended recursion, leading to system instability or complete service unavailability. In some cases, the buffer overflow conditions may allow for memory corruption that could potentially be exploited for code execution, though this requires additional attack surface and is more difficult to achieve. The vulnerability affects not only web browsers but also any application that relies on PCRE for pattern matching, including mail servers, intrusion detection systems, and various network security tools that process regular expressions.
Mitigation strategies for CVE-2015-8392 primarily focus on immediate patching of affected PCRE installations to version 8.38 or later, which contains the necessary fixes for proper handling of the (?| substring syntax. Organizations should also implement input validation and sanitization measures to prevent untrusted regular expressions from being processed, particularly in web applications that accept user input for pattern matching operations. The vulnerability aligns with CWE-121, which addresses buffer overflow conditions, and CWE-674, which covers unintended recursion in software. From an ATT&CK perspective, this vulnerability maps to T1059.007 for script injection and T1499.004 for network denial of service, representing potential attack vectors that could be leveraged by adversaries to compromise system availability or execute malicious code through crafted regular expressions.
The broader implications of this vulnerability highlight the complexity of regular expression engines and the challenges in implementing secure pattern matching algorithms. The flaw demonstrates how seemingly innocuous syntax features can create security risks when not properly handled in the underlying implementation, particularly in libraries that are widely used across different platforms and applications. System administrators should monitor for any signs of resource exhaustion or unusual behavior in applications that process regular expressions, as these could indicate exploitation attempts. Additionally, organizations should conduct thorough security assessments of their applications to identify any potential dependencies on vulnerable PCRE versions and ensure comprehensive testing of regular expression processing functionality in security-sensitive contexts.