CVE-2019-20394 in libyang
Summary
by MITRE
A double-free is present in libyang before v1.0-r3 in the function yyparse() when a type statement in used in a notification statement. Applications that use libyang to parse untrusted input yang files may be vulnerable to this flaw, which would cause a crash or potentially code execution.
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Analysis
by VulDB Data Team • 03/25/2024
The vulnerability identified as CVE-2019-20394 represents a critical double-free error in the libyang library version prior to v1.0-r3, specifically within the yyparse() function. This flaw occurs when processing yang files containing type statements within notification statements, creating a scenario where memory allocated for parsing operations is freed twice, leading to potential system instability and security risks. The issue stems from improper memory management practices within the parser's handling of complex yang syntax constructs, particularly when notifications contain type definitions that trigger the vulnerable code path.
The technical implementation of this vulnerability demonstrates a classic memory corruption flaw that operates through the parser's state management during yang file processing. When libyang encounters a notification statement containing a type statement, the yyparse() function executes code paths that fail to properly track memory allocation and deallocation sequences. This double-free condition arises from the parser's inability to correctly handle reference counting or memory state tracking for dynamically allocated structures used in the parsing context. The vulnerability is particularly dangerous because it can be triggered through untrusted input files, making it an attractive target for remote exploitation scenarios where attackers can craft malicious yang files to exploit the flaw.
From an operational impact perspective, this vulnerability creates significant risks for applications that rely on libyang for yang file validation and processing. Systems using libyang to parse configuration data, network management information, or API schema definitions become susceptible to denial of service attacks that can crash processes or potentially enable remote code execution. The vulnerability's exploitation potential increases when applications process yang files from untrusted sources, such as user uploads, network protocols, or third-party integrations. Security researchers have classified this as a high-severity issue due to its potential for both service disruption and arbitrary code execution, making it particularly concerning for network infrastructure devices, configuration management systems, and any application that parses external yang schema definitions.
The vulnerability aligns with CWE-415 which specifically addresses double free conditions in memory management, representing a fundamental flaw in how the parser handles memory allocation and deallocation. From an attack framework perspective, this vulnerability maps to ATT&CK technique T1203 which involves exploitation of software vulnerabilities to gain access to systems, and T1499 which covers network disruption through service availability attacks. The remediation strategy involves upgrading to libyang version 1.0-r3 or later where the double-free condition has been addressed through proper memory management implementation. Organizations should implement input validation measures and consider sandboxed parsing environments to mitigate potential exploitation attempts while awaiting official patches. Additionally, monitoring for unusual parsing behavior or process crashes in systems utilizing libyang can serve as an early warning mechanism for potential exploitation attempts.