CVE-2017-9438 in YARA
Summary
by MITRE
libyara/re.c in the regexp module in YARA 3.5.0 allows remote attackers to cause a denial of service (stack consumption) via a crafted rule (involving hex strings) that is mishandled in the _yr_re_emit function, a different vulnerability than CVE-2017-9304.
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Analysis
by VulDB Data Team • 12/07/2022
The vulnerability identified as CVE-2017-9438 represents a critical denial of service flaw within the YARA threat detection engine version 3.5.0. This issue specifically affects the regexp module's handling of regular expressions, particularly when processing crafted rules that involve hex strings. The vulnerability stems from improper memory management during the compilation and execution of regular expression patterns, creating a scenario where malicious input can trigger excessive stack consumption. YARA, widely used for identifying and classifying malware samples through pattern matching, becomes vulnerable to this attack vector when processing specially crafted rules that exploit the underlying regular expression engine.
The technical flaw manifests in the _yr_re_emit function within the libyara/re.c module, where the implementation fails to properly validate or limit the recursive depth of certain hex string patterns during regular expression compilation. When attackers submit maliciously constructed rules containing specific hex string combinations, the function enters an exponential recursion pattern that rapidly consumes stack memory. This behavior creates a stack overflow condition that ultimately leads to process termination and system unavailability. The vulnerability is distinct from CVE-2017-9304, indicating a separate code path within the same module that exhibits similar denial of service characteristics but through different execution patterns. The issue demonstrates poor input validation and lacks proper recursion depth limiting mechanisms, making it susceptible to exploitation by remote attackers without authentication requirements.
The operational impact of CVE-2017-9438 extends beyond simple service disruption, as it can affect any system running YARA 3.5.0 that processes untrusted rule sets. This includes security operations centers, malware analysis environments, and automated threat detection systems that rely on YARA for pattern matching. Attackers can exploit this vulnerability to cause denial of service against security tools, potentially disrupting incident response capabilities or malware analysis workflows. The vulnerability is particularly concerning because it requires no privileged access or complex exploitation techniques, making it accessible to attackers with basic knowledge of YARA rule syntax. The stack consumption pattern can be triggered through simple rule submissions, making it an attractive target for attackers seeking to disable security infrastructure or disrupt operations.
Mitigation strategies for CVE-2017-9438 primarily focus on updating to patched versions of YARA where the recursion limits have been properly implemented. Organizations should immediately upgrade to YARA versions that address this vulnerability, typically those released after the initial patching of similar issues in the same codebase. Additionally, implementing proper input validation and rate limiting for rule processing can help reduce the attack surface, though these measures are secondary to the core fix. Security teams should also consider deploying monitoring solutions that can detect unusual stack consumption patterns or rule processing behavior that might indicate exploitation attempts. The vulnerability aligns with CWE-674, which describes "Uncontrolled Recursion" in software systems, and can be categorized under ATT&CK technique T1499.1 for "Endpoint Denial of Service" as it targets the availability of security tools rather than the confidentiality or integrity of data. System administrators should also implement proper network segmentation and access controls to limit exposure of YARA processing systems to untrusted inputs while maintaining operational effectiveness for legitimate rule processing activities.