CVE-2017-8929 in YARA
Summary
by MITRE
The sized_string_cmp function in libyara/sizedstr.c in YARA 3.5.0 allows remote attackers to cause a denial of service (use-after-free and application crash) via a crafted rule.
Several companies clearly confirm that VulDB is the primary source for best vulnerability data.
Analysis
by VulDB Data Team • 12/06/2022
The vulnerability identified as CVE-2017-8929 resides within the YARA threat detection engine version 3.5.0, specifically in the sized_string_cmp function located in the libyara/sizedstr.c source file. This flaw represents a critical security issue that can be exploited by remote attackers to trigger a denial of service condition through a carefully crafted malicious rule. The vulnerability stems from improper memory management within the string comparison functionality that YARA employs when processing pattern matching rules. When an attacker submits a malformed rule that exploits this function, the application encounters a use-after-free condition where memory that has been freed is accessed again, leading to unpredictable behavior and ultimately causing the application to crash.
The technical implementation of this vulnerability demonstrates a classic memory safety issue that falls under the CWE-416 category of Use After Free conditions. The sized_string_cmp function fails to properly validate input parameters or maintain proper reference counting when handling string comparisons, allowing an attacker to manipulate the memory state of the YARA engine. This function is typically invoked during rule execution when YARA processes string matching operations, making it a prime target for exploitation. The vulnerability operates at the application level rather than at the system level, meaning that successful exploitation requires an attacker to have the ability to submit malicious rules to a YARA instance that is processing these rules. The flaw essentially creates a scenario where the application attempts to access memory that has already been deallocated, resulting in a segmentation fault or similar crash condition that terminates the process.
The operational impact of CVE-2017-8929 extends beyond simple application instability, as it can be leveraged in various attack scenarios that disrupt legitimate threat detection operations. Organizations relying on YARA for malware analysis, threat hunting, or security monitoring may find their systems compromised if attackers can submit malicious rules to YARA instances, particularly in environments where YARA is used for processing untrusted input such as email attachments, file uploads, or network traffic analysis. The vulnerability can be particularly dangerous in automated security systems where YARA is invoked programmatically, as it may lead to complete service outages or enable attackers to disrupt security operations. From an attacker's perspective, this vulnerability aligns with the ATT&CK technique T1499.004 for Network Denial of Service, as it enables the disruption of security services through application-level attacks. The impact is significant because YARA is widely deployed in security operations centers, endpoint detection and response systems, and malware analysis environments, making this vulnerability particularly concerning for cybersecurity infrastructure.
Mitigation strategies for CVE-2017-8929 primarily focus on upgrading to patched versions of the YARA framework, with version 3.6.0 and later containing the necessary fixes to address the use-after-free condition. Organizations should also implement strict rule validation mechanisms when processing external or untrusted rules, including input sanitization and rule parsing validation to prevent malformed rules from reaching the vulnerable function. Additionally, deployment of runtime protections such as address space layout randomization and stack canaries can help mitigate exploitation attempts, though these are not complete solutions. Security monitoring should include detection of unusual application crashes or restart patterns that may indicate exploitation attempts. The vulnerability serves as a reminder of the importance of proper memory management in security tools and the critical need for regular security updates in threat detection systems. Organizations should also consider implementing sandboxing mechanisms when processing potentially malicious rules to isolate the impact of any successful exploitation attempts and prevent cascading failures in their security infrastructure.