CVE-2026-76957 in Libexpat
Summary
by MITRE • 08/20/2026
libexpat before 2.8.4 lacks handler call depth tracking with custom encoding callbacks. Thus, a use-after-free can occur. NOTE: this is similar to CVE-2026-50219, CVE-2026-56131 and CVE-2026-56412.
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Analysis
by VulDB Data Team • 08/20/2026
The vulnerability identified in libexpat prior to version 2.8.4 represents a critical memory safety flaw rooted in the library's handling of custom encoding callbacks during XML parsing operations. Libexpat is widely utilized across numerous software ecosystems for its high-performance, standards-compliant processing of Extensible Markup Language data. The core issue arises when developers implement custom encoding handlers to process non-standard or legacy character encodings that are not natively supported by the library's default parser logic. In these specific scenarios, the internal state management mechanisms fail to adequately track the depth of recursive handler calls initiated during the decoding process. This lack of depth tracking creates a scenario where the stack frame associated with the encoding callback can grow uncontrollably or persist beyond its intended lifecycle if the parsing context is modified or destroyed while callbacks are still active.
This architectural oversight directly leads to a use-after-free condition, which is classified under CWE-416 in the Common Weakness Enumeration framework. A use-after-free vulnerability occurs when a program continues to use memory that has already been freed by another part of the application. In the context of libexpat, if an encoding callback triggers further parsing or state changes that result in the deallocation of internal buffers while those same buffers are still referenced by subsequent execution paths within the untracked call stack, the integrity of the heap is compromised. An attacker who can influence the input XML structure and trigger these specific custom encoding pathways could exploit this race condition to execute arbitrary code with the privileges of the vulnerable application or cause a denial of service through segmentation faults and crashes.
The operational impact of this vulnerability extends beyond simple instability. Because libexpat is often embedded in security-sensitive applications such as web browsers, document processors, and API gateways, exploiting this flaw could allow for remote code execution if an attacker can supply maliciously crafted XML data that forces the parser into a state where custom encoding callbacks are invoked repeatedly or recursively without proper depth limits. This aligns with attack patterns described in the MITRE ATT&CK framework under techniques involving memory corruption and exploitation of binary vulnerabilities, specifically those targeting parsers to bypass security controls. The similarity noted to CVE-2026-50219, CVE-2026-56131, and CVE-2026-56412 indicates a systemic issue in how the library manages state transitions during complex encoding operations, suggesting that these vulnerabilities share common root causes related to reference counting or stack management errors.
Mitigation strategies must prioritize immediate upgrading of libexpat to version 2.8.4 or later, where the developers have implemented robust depth tracking mechanisms for custom encoding callbacks. This update ensures that recursive calls are monitored and limited, preventing the stack overflow conditions that lead to memory corruption. For organizations unable to upgrade immediately due to dependency constraints, defensive coding practices should be employed. Developers implementing custom encoding handlers must ensure they do not trigger nested parsing operations or modify parser state in ways that could invalidate existing pointers during callback execution. Additionally, integrating fuzzing tools into the continuous integration pipeline can help identify edge cases where deep nesting of XML elements combined with specific encoding requirements might expose similar latent defects before deployment to production environments.