CVE-2018-20796 in C Library
Summary
by MITRE
In the GNU C Library (aka glibc or libc6) through 2.29, check_dst_limits_calc_pos_1 in posix/regexec.c has Uncontrolled Recursion, as demonstrated by '(\227|)(\\1\\1|t1|\\\2537)+' in grep.
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Analysis
by VulDB Data Team • 07/19/2023
The vulnerability CVE-2018-20796 represents a critical uncontrolled recursion flaw in the GNU C Library's regular expression engine, specifically within the posix/regexec.c file. This issue affects glibc versions through 2.29 and demonstrates a severe security weakness that can be exploited through carefully crafted regular expressions. The vulnerability manifests when processing certain regex patterns that create recursive call chains without proper termination conditions, leading to potential system instability and denial of service conditions.
The technical implementation of this flaw occurs in the check_dst_limits_calc_pos_1 function which handles regular expression matching operations. When the regex pattern '(\227|)(\1\1|t1|\\2537)+' is processed, it triggers an infinite recursive loop in the regular expression engine's state calculation mechanism. This pattern exploits the library's handling of backreferences and alternation constructs, creating a scenario where the engine repeatedly calls itself without reaching a base case termination condition. The flaw specifically targets the POSIX regular expression implementation within glibc, making it particularly dangerous for systems that rely heavily on standard library regex functions.
The operational impact of this vulnerability extends beyond simple denial of service scenarios to potentially enable more sophisticated attack vectors. Systems utilizing grep or other tools that depend on glibc's regex functionality become vulnerable to recursive exploitation, where malicious input can cause stack overflow conditions and system crashes. The vulnerability affects not just grep but any application that leverages the standard POSIX regex functions, including shell scripts, network security tools, and various system utilities. From an attacker's perspective, this represents a reliable method for causing system instability and can be used in conjunction with other techniques to build more complex attack chains. The issue has been classified under CWE-674, which specifically addresses Uncontrolled Recursion, and aligns with ATT&CK technique T1499.1 for Network Denial of Service attacks.
Mitigation strategies for this vulnerability require immediate patching of affected glibc versions to address the recursion control mechanisms. System administrators should prioritize updating their glibc installations to versions that contain fixes for this specific recursion handling issue. Additionally, input validation and sanitization should be implemented at application layers to prevent malicious regex patterns from reaching the vulnerable library functions. Organizations should also consider implementing rate limiting and resource monitoring to detect and prevent exploitation attempts. The fix typically involves adding proper recursion depth checking and termination conditions within the regex engine's state calculation functions, preventing the infinite loop scenario that leads to system instability. Security teams should monitor for exploitation attempts and implement network segmentation to limit the potential impact of successful attacks against vulnerable systems.