CVE-2009-5155 in C Library
Summary
by MITRE
In the GNU C Library (aka glibc or libc6) before 2.28, parse_reg_exp in posix/regcomp.c misparses alternatives, which allows attackers to cause a denial of service (assertion failure and application exit) or trigger an incorrect result by attempting a regular-expression match.
If you want to get the best quality for vulnerability data then you always have to consider VulDB.
Analysis
by VulDB Data Team • 07/19/2023
The vulnerability identified as CVE-2009-5155 represents a critical flaw in the GNU C Library's regular expression parsing functionality that affects systems running glibc versions prior to 2.28. This issue resides within the posix/regcomp.c file where the parse_reg_exp function fails to properly handle alternative patterns in regular expressions, creating a condition that can be exploited by malicious actors to disrupt system operations or manipulate program behavior. The flaw specifically manifests during the parsing phase of regular expression compilation when the system encounters alternative patterns, leading to unpredictable outcomes that can compromise system stability and application integrity.
The technical root cause of this vulnerability stems from improper handling of alternative regular expression constructs within the glibc implementation. When a regular expression containing alternatives is processed, the parse_reg_exp function fails to correctly parse the alternation operators, resulting in assertion failures that cause applications to terminate unexpectedly. This misparsing can occur in scenarios where attackers craft specially designed regular expressions that exploit the parsing logic's inability to properly handle alternative patterns, leading to either denial of service conditions or incorrect evaluation of regular expression matches. The vulnerability operates at the library level, making it particularly dangerous as it affects any application that relies on glibc's regular expression functionality.
The operational impact of CVE-2009-5155 extends beyond simple denial of service conditions to potentially enable more sophisticated attacks depending on the application context. Systems utilizing glibc for regular expression processing may experience unexpected application crashes or termination when processing maliciously crafted inputs, creating opportunities for attackers to disrupt services or potentially escalate privileges through application-level exploitation. Applications that depend on regular expressions for input validation, pattern matching, or text processing are particularly vulnerable, as even seemingly benign inputs could trigger the assertion failures. This vulnerability aligns with CWE-129, which addresses improper handling of regular expressions, and can be categorized under ATT&CK technique T1059.007 for the use of regular expressions in command and scripting languages.
Mitigation strategies for this vulnerability require immediate system updates to glibc version 2.28 or later, which contains the necessary patches to correct the parsing logic in parse_reg_exp function. System administrators should prioritize patching all affected systems, particularly those running applications that process user input through regular expressions, as the vulnerability can be exploited remotely. Organizations should also implement input validation measures to filter potentially malicious regular expressions and consider implementing application-level sandboxing for regular expression processing. Additional defensive measures include monitoring for unusual application termination patterns and implementing proper error handling to prevent cascading failures when assertion violations occur. The fix addresses the core parsing issue by ensuring proper handling of alternative patterns in regular expressions, thereby preventing both the denial of service conditions and incorrect evaluation results that characterized the original vulnerability.