CVE-2026-102633 in Libexpat
Summary
by MITRE • 09/29/2026
libexpat versions 2.7.2 through 2.8.5 contain an integer overflow vulnerability in expat_realloc() function on 32-bit platforms when computing allocation sizes. Attackers supplying malicious XML to applications parsing with vulnerable libexpat can cause heap buffer overflow, memory corruption, or denial of service.
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Analysis
by VulDB Data Team • 09/29/2026
The vulnerability identified in libexpat versions ranging from 2.7.2 through 2.8.5 represents a critical integer overflow flaw located within the expat_realloc function. This specific component is responsible for managing dynamic memory allocation during the parsing process, particularly when resizing buffers to accommodate expanding XML data structures. The root cause of this vulnerability lies in how the library calculates the size required for reallocation on 32-bit platforms. When processing certain inputs, the arithmetic operation used to determine the new buffer size fails to account for potential integer wraparound conditions. This miscalculation results in a significantly smaller allocated memory block than what is actually needed by the application logic that follows. Consequently, subsequent write operations into this undersized buffer exceed its boundaries, leading directly to heap buffer overflow and widespread memory corruption within the process address space.
From an operational perspective, this flaw poses severe risks to any software system relying on libexpat for XML parsing. An attacker can craft malicious XML documents designed specifically to trigger these integer arithmetic errors during the reallocation phase. Upon successful exploitation, the immediate consequence is heap buffer overflow, which allows for arbitrary memory writes. This capability enables attackers to overwrite adjacent heap metadata or function pointers, potentially leading to remote code execution with the privileges of the vulnerable application. In scenarios where exploitability for code execution is limited due to mitigations such as Address Space Layout Randomization or Data Execution Prevention, the vulnerability can still be leveraged to cause a denial of service by corrupting critical memory structures and crashing the parsing process. The impact is particularly acute on 32-bit architectures because their narrower address space makes integer overflow conditions more predictable and easier to exploit compared to modern 64-bit environments where such arithmetic errors often result in immediate allocation failures rather than silent corruption.
This vulnerability aligns with Common Weakness Enumeration category CWE-190, which describes Integer Overflow or Wraparound. The specific mechanism involves an incorrect calculation of memory size that leads to a buffer overflow, mapping closely to CWE-787: Out-of-bounds Write. In the context of the MITRE ATT&CK framework, this flaw facilitates initial access and privilege escalation techniques associated with Tactic TA0004 (Privilege Escalation) and specifically relates to exploitation patterns found in T1203: Exploitation for Client Execution if the vulnerable application is a client-side parser. The attack vector typically involves supplying crafted input via network services, file uploads, or local data processing pipelines that feed XML content into the affected library functions.
Mitigation strategies must prioritize immediate patching to versions of libexpat released after 2.8.5 where this arithmetic flaw has been corrected with proper bounds checking and overflow detection mechanisms. For organizations unable to upgrade immediately due to dependency constraints, implementing input validation at the application layer can provide a partial defense in depth by rejecting XML documents that exhibit patterns indicative of exploitation attempts, such as excessively large or deeply nested structures that trigger rapid memory reallocations. Additionally, deploying runtime protection technologies like heap hardening features available in modern compilers and operating systems can mitigate the impact of buffer overflows by detecting out-of-bounds writes before they lead to code execution. Regular security audits focusing on C/C++ memory management practices are also recommended to identify similar integer arithmetic vulnerabilities across other components within the software supply chain that rely on libexpat or similar parsing libraries.