CVE-2026-77214 in Libexpat
Summary
by MITRE • 10/07/2026
libexpat before commit 13c5f63 contains a heap buffer over-read vulnerability in xmlparse.c. XML_ParseBuffer advances the parse buffer end with parser->m_bufferEnd += len using a caller-supplied length that is not validated against the allocated buffer size, so repeated XML_ParseBuffer calls move m_bufferEnd past the end of the heap allocation and subsequent parsing reads out of bounds. Reaching this path requires a parse buffer to already be present; otherwise XML_ParseBuffer returns XML_ERROR_NO_BUFFER. A buffer is present after a prior call to XML_GetBuffer, either directly (the common case) or indirectly through a prior XML_Parse call that allocates the buffer internally. The over-read discloses adjacent heap memory to the calling application, recovering heap pointers, libc function pointers, and code pointers sufficient to defeat ASLR and build further exploitation primitives.
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Analysis
by VulDB Data Team • 10/07/2026
The vulnerability identified in libexpat prior to commit 13c5f63 represents a critical heap buffer over-read flaw located within the xmlparse.c module. This issue stems from an improper validation of input parameters during the parsing process, specifically affecting how the parser manages its internal buffer boundaries. The core technical failure occurs when the XML_ParseBuffer function is invoked with a length parameter supplied by the caller. Instead of verifying that this requested length does not exceed the size of the currently allocated heap memory block, the implementation blindly advances the parser's m_bufferEnd pointer by adding the provided length to it. This lack of bounds checking allows an attacker to manipulate the internal state of the XML parser in a way that pushes the buffer end pointer beyond the limits of the actual heap allocation.
The operational mechanics of this vulnerability require specific conditions to be met for successful exploitation. The attack path necessitates that a parse buffer is already present within the parser context, as subsequent calls to XML_ParseBuffer will return an error if no buffer exists. A buffer becomes available either through direct invocation of XML_GetBuffer or indirectly via prior calls to XML_Parse which allocate and manage buffers internally. Once these conditions are satisfied, repeated invocations with carefully crafted length values allow the attacker to incrementally shift m_bufferEnd past the allocated memory region. This state corruption transforms a standard parsing operation into an out-of-bounds read scenario where the parser attempts to access memory locations that lie outside the intended heap segment boundaries.
The impact of this vulnerability is severe due to its potential for information disclosure and subsequent code execution. When the over-read condition is triggered, the application reads data from adjacent heap memory regions rather than discarding or erroring out as expected. This leakage exposes sensitive internal structures stored in contiguous memory spaces, including heap pointers, function pointers within the libc library, and other executable code addresses. The recovery of these pointers provides an attacker with critical intelligence about the runtime environment, effectively neutralizing address space layout randomization protections by revealing base addresses of libraries and application segments. With this information, it becomes feasible to construct further exploitation primitives aimed at achieving arbitrary code execution on the target system.
From a classification perspective, this vulnerability aligns with CWE-125, which describes out-of-bounds read errors resulting from insufficient validation of input lengths against buffer capacities. The attack vector leverages improper neutralization techniques typical of CWE-20 issues where inputs are not adequately sanitized before processing. In the context of the MITRE ATT&CK framework, this vulnerability facilitates reconnaissance and privilege escalation phases by allowing an attacker to gather system information through memory disclosure. This aligns with tactics such as Collection or Credential Access if sensitive data is present in adjacent heap regions, though its primary utility lies in bypassing security mitigations like ASLR to enable more direct exploitation techniques.
Mitigation strategies for this vulnerability focus on enforcing strict boundary checks within the parsing logic. Developers must ensure that any length parameter provided by callers during buffer operations is validated against the actual allocated size of the memory block before updating internal pointers like m_bufferEnd. Implementing defensive programming practices, such as using safe arithmetic functions that detect overflow and verifying array bounds prior to access, can prevent this class of errors. For users relying on libexpat, upgrading to a version released after commit 13c5f63 is the primary remediation step, as these later versions include patches that properly validate buffer lengths against allocated sizes. Additionally, employing memory-safe languages or static analysis tools during development can help identify similar logic flaws in other components of an application stack before deployment.