CVE-2015-8985 in C Library
Summary
by MITRE
The pop_fail_stack function in the GNU C Library (aka glibc or libc6) allows context-dependent attackers to cause a denial of service (assertion failure and application crash) via vectors related to extended regular expression processing.
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Analysis
by VulDB Data Team • 09/10/2020
The vulnerability identified as CVE-2015-8985 resides within the GNU C Library implementation of the pop_fail_stack function, which is part of the regular expression processing subsystem. This flaw manifests during the handling of extended regular expressions and represents a classic example of an assertion failure that can be exploited to trigger a denial of service condition. The vulnerability operates through context-dependent attack vectors that specifically target the regular expression engine's internal stack management mechanisms. The affected component is part of the widely deployed glibc library that serves as the foundation for numerous Unix-like operating systems and applications, making this a critical security concern for system stability.
The technical implementation of this vulnerability stems from improper handling of stack operations within the regular expression matching algorithm. When processing certain extended regular expressions, the pop_fail_stack function encounters a condition where an assertion fails, leading to an immediate application crash. This occurs because the function attempts to pop from an empty stack or accesses invalid stack memory locations during the backtracking process of regular expression matching. The flaw is categorized under CWE-682, which specifically addresses incorrect arithmetic operations and improper handling of stack operations. The assertion failure represents a fundamental breakdown in the library's error handling mechanisms, where the system fails to properly validate stack states before attempting operations.
The operational impact of CVE-2015-8985 extends far beyond simple denial of service, as it can affect any application that relies on glibc's regular expression processing capabilities. Systems running web servers, database applications, network daemons, and security tools that utilize regular expressions for input validation or pattern matching become vulnerable to this attack. The vulnerability can be exploited through various attack vectors including malformed input strings, crafted regular expressions in configuration files, or malicious user input that gets processed through regular expression engines. From an adversarial perspective, this vulnerability aligns with ATT&CK technique T1059.007, which involves the use of command and scripting interpreters, particularly when applications crash due to malformed inputs. The impact is particularly severe in server environments where denial of service can lead to complete system unavailability and potential cascading failures across dependent services.
Mitigation strategies for this vulnerability require immediate patching of affected glibc versions, as the flaw exists at the core library level where applications depend on its stability. System administrators should prioritize updating their glibc implementations to versions that contain the patched pop_fail_stack function and proper stack validation. Additionally, implementing input validation controls at application layers can provide defense-in-depth measures, though these are not complete solutions given the vulnerability's location in the underlying library. Network-based mitigations such as regular expression filtering, input sanitization, and rate limiting can help reduce the attack surface, but the fundamental fix requires updating the affected library components. Organizations should also consider monitoring for unusual application crashes or assertion failures that may indicate exploitation attempts, as these patterns often precede successful attacks. The vulnerability demonstrates the critical importance of maintaining up-to-date system libraries and highlights the potential for low-level library flaws to create widespread service disruption across entire application ecosystems.