A stack overflow vulnerability exists in the libexpat library due to the way it handles recursive entity expansion in XML documents. When parsing an XML document with deeply nested entity references, libexpat can be forced to recurse indefinitely, exhausting the stack space and causing a crash. This issue could lead to denial of service (DoS) or, in some cases, exploitable memory corruption, depending on the environment and library usage.

In libexpat before 2.8.1, the computational complexity of attribute name collision checks allows a denial of service via moderately sized crafted XML input.

Last updated 21 September 2026

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libexpat before commit 13c5f63 contains a heap buffer over-read vulnerability in xmlparse.c. XMLParseBuffer advances the parse buffer end with parser->mbufferEnd += len using a caller-supplied length that is not validated against the allocated buffer size, so repeated XMLParseBuffer calls move mbufferEnd 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 XMLParseBuffer returns XMLERRORNOBUFFER. A buffer is present after a prior call to XMLGetBuffer, either directly (the common case) or indirectly through a prior XMLParse 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.

libexpat 2.7.2 through 2.8.5 Integer Overflow in expatrealloc

Expat before 2.8.5 fails to validate that a high surrogate in UTF-16 input is followed by a low surrogate, allowing malformed UTF-16 sequences to be accepted. Attackers can supply UTF-16 encoded XML containing lone high surrogates that consume the following code unit, causing Expat to pass unpaired surrogates to applications built with XMLUNICODE and to silently replace input characters in other builds.

Hello oss-security,

just a quick note that libexpat 2.9.0 (or "Expat 2.9.0") released today is fixing two vulnerabilities:

- CVE-2026-77214 - CVE-2026-102633

The related part of the change log is this:

#1392 CVE-2026-102633 -- Integer overflow in function expatrealloc on 32bit platforms #1393 CVE-2026-77214 -- Validate parameter len against available buffer capacity in XMLParseBuffer

Some key links are:

- The blog post about it https://blog.hartwork.org/posts/expat-2-9-0-released/

- The full change log of release 2.9.0 https://github.com/libexpat/libexpat/blob/R290/expat/Changes

- The fixing pull requests - https://github.com/libexpat/libexpat/pull/1392 - https://github.com/libexpat/libexpat/pull/1393

- The NVD CVE metadata - https://nvd.nist.gov/vuln/detail/cve-2026-77214 - https://nvd.nist.gov/vuln/detail/cve-2026-102633

Best

Sebastian

xml.parsers.expat and xml.etree.ElementTree use insufficient entropy for Expat hash-flooding protection, which allows a crafted XML document to trigger hash flooding.\r\n\r\nFully mitigating this vulnerability requires both updating libexpat to 2.8.0 or later and applying this patch.

Expat through 2.8.3 contains an out-of-bounds read vulnerability that allows attackers to trigger memory corruption by processing XML with external entity parsers created via XMLExternalEntityParserCreate. A struct size mismatch between ELEMENTTYPE members causes storeAtts to read the attIndex member past allocated memory boundaries, resulting in failure to normalize whitespace in non-CDATA attributes or a wild pointer dereference causing a segfault. This vulnerability was introduced by the fix for CVE-2026-66046.

Hello oss-security,

just a quick note that libexpat 2.8.5 (or "Expat 2.8.5") released today is fixing CVE-2026-93990:

Reject high surrogates not followed by a low surrogate during UTF-16 decoding; previously, malformed UTF-16 could be smuggled into the application using Expat and could cause arbitrary damage there, depending on how malformed UTF-16 was handled inside the application; validation was not their job but Expat's. This is similar to past vulnerability CVE-2022-25235. Upstream CVSS 3.1 vector: AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H (CVSS score: 9.8)

Some key links are:

- The blog post about it https://blog.hartwork.org/posts/expat-2-8-5-released/

- The change log of release 2.8.5 https://github.com/libexpat/libexpat/blob/R285/expat/Changes

- The fixing pull request https://github.com/libexpat/libexpat/pull/1282

- The NVD CVE metadata https://nvd.nist.gov/vuln/detail/CVE-2026-93990 (with a different CVSS vector)

Best

Sebastian

In libexpat before 2.8.1, the computational complexity of attribute name collision checks allows a denial of service via moderately sized crafted XML input.

libexpat in Expat before 2.7.2 allows attackers to trigger large dynamic memory allocations via a small document that is submitted for parsing.

An issue was discovered in libexpat before 2.6.3. dtdCopy in xmlparse.c can have an integer overflow for nDefaultAtts on 32-bit platforms (where UINTMAX equals SIZEMAX).

An issue was discovered in libexpat before 2.6.3. xmlparse.c does not reject a negative length for XMLParseBuffer.

The libexpat library is vulnerable to a stack overflow due to uncontrolled recursion when processing deeply nested XML entities. This can cause the application to crash, resulting in a denial of service (DoS) or potentially leading to memory corruption, depending on the user's environment and how the library is used. The issue is triggered by supplying a specially crafted XML document designed to create a long chain of recursive entities.

It was found that original patch for issues CVE-2015-1283 and CVE-2015-2716 used overflow checks that could be optimized out by some compilers applying certain optimization settings, which can cause the vulnerability to remain even after applying the patch.

One pattern in the fix for CVE-2015-1283/CVE-2015-2716 is:

/ bufferSize is positive here / do { bufferSize = 2; } while (bufferSize < neededSize && bufferSize > 0); if (bufferSize <= 0) { errorCode = XMLERRORNOMEMORY; return NULL;

Any of the modern optimizing compiler, IF able to infer that bufferSize is initially positive (which is true but not obvious to see through local reasoning), will eliminate bufferSize > 0 as always true when the execution is defined, and bufferSize <= 0 as always false when the execution is defined.

Without knowing that bufferSize starts positive, an optimizing compiler could also move the test bufferSize > 0 out of the loop, that is, compile the code as if it had been written:

if (bufferSize <= 0) errorCode = XMLERRORNOMEMORY; return NULL; else { do { bufferSize = 2; } while (bufferSize < neededSize); }

Both cases leads to not eliminating the vulnerability.

Upstream patch:

https://sourceforge.net/p/expat/codegit/ci/f0bec73b018caa07d3e75ec8dd967f3785d71bde/tree/expat/lib/xmlparse.c?diff=a238d7ea7a715ef3850c4cbdd86aeda7077b6bbc

Last updated 21 September 2026

Last updated 21 September 2026

Last updated 6 October 2026

Last updated 24 September 2026

Last updated 21 September 2026

Last updated 24 September 2026

In libexpat before 2.8.2, there is a heap-based buffer overflow in doProlog in xmlparse.c because scaffold backing array reallocation is mishandled when there is data-structure sharing across parsers.

Last updated 6 October 2026

libexpat before 2.8.2 lacks handler call depth tracking for calls to XMLResumeParser from within handlers in cases of a policy violation. Thus, a use-after-free can occur (similar to the CVE-2026-50219 situation).

When an Expat parser with a registered ElementDeclHandler parses an inline document type definition containing a deeply nested content model a C stack overflow occurs.

Last updated 24 September 2026

Last updated 24 September 2026

Last updated 24 September 2026