CVE-2015-20109 in C Library
Summary
by MITRE • 06/25/2023
end_pattern (called from internal_fnmatch) in the GNU C Library (aka glibc or libc6) before 2.22 might allow context-dependent attackers to cause a denial of service (application crash), as demonstrated by use of the fnmatch library function with the **(!() pattern. NOTE: this is not the same as CVE-2015-8984; also, some Linux distributions have fixed CVE-2015-8984 but have not fixed this additional fnmatch issue.
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Analysis
by VulDB Data Team • 10/05/2023
The vulnerability identified as CVE-2015-20109 represents a critical denial of service flaw within the GNU C Library that affects systems running versions prior to 2.22. This issue specifically impacts the fnmatch library function which is commonly used for pattern matching operations in Unix-like operating systems. The vulnerability manifests when the end_pattern function processes certain complex pattern strings, particularly those containing the **(!()) construct, leading to application crashes and system instability. The flaw exists in the internal_fnmatch function which is part of glibc's pattern matching implementation, making it a fundamental component of the system's security infrastructure.
The technical root cause of this vulnerability lies in improper handling of nested pattern matching constructs within the fnmatch implementation. When the end_pattern function encounters the specific pattern **(!()), it enters an infinite recursion or stack overflow condition due to inadequate boundary checking and pattern parsing logic. This pattern structure creates a situation where the parser attempts to process nested negation and wildcard combinations in a manner that exhausts available system resources. The vulnerability is classified as context-dependent because it requires specific input patterns to trigger the exploitable condition, making it less likely to be exploited accidentally but still dangerous in environments where applications process untrusted pattern data. This flaw directly corresponds to CWE-674, which addresses "Uncontrolled Recursion" in software implementations, and aligns with ATT&CK technique T1499.004 for "Endpoint Denial of Service" through resource exhaustion attacks.
The operational impact of CVE-2015-20109 extends beyond simple application crashes to potentially affect entire system stability, particularly in environments where multiple applications rely on glibc's fnmatch functionality. Systems running affected versions of glibc may experience service disruption when processing maliciously crafted pattern strings, leading to cascading failures in applications that depend on pattern matching operations. The vulnerability is especially concerning in server environments where applications may receive pattern inputs from external sources, including web applications, file management systems, and configuration management tools. Additionally, this flaw demonstrates the importance of proper input validation and resource management in system libraries, as it shows how a single flawed function can compromise entire application ecosystems. Organizations using affected systems face significant risk of denial of service attacks that could be exploited by malicious actors to disrupt legitimate operations.
Mitigation strategies for CVE-2015-20109 primarily focus on immediate system updates and patches to glibc versions 2.22 or later where the vulnerability has been resolved. System administrators should prioritize patching all affected systems, particularly those handling untrusted pattern data inputs, and conduct thorough testing to ensure compatibility with updated library versions. Additional protective measures include implementing input validation controls at application layers to sanitize pattern strings before processing, monitoring for unusual pattern matching behavior, and deploying intrusion detection systems that can identify potential exploitation attempts. Organizations should also consider implementing network segmentation and access controls to limit exposure of systems that may be vulnerable to this type of denial of service attack. The fix in glibc 2.22 addresses the core parsing logic and adds proper recursion depth limits and boundary checking to prevent the exploitable conditions that lead to system crashes.