libexpat Heap Buffer Over-read in xmlparse.c via XML_ParseBuffer

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.

Source published

7 Oct 2026 14:35:42 UTC

Source modified

10 Oct 2026 02:26:09 UTC

Observed here

23 Oct 2026 15:48:27 UTC

Source terms

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CVSS 4.0 · 8.3 CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:H/VI:N/VA:L/SC:N/SI:N/SA:N

CVSS 3.1 · 8.2 CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:N/A:L