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.

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.

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.

Accessibility. A privacy issue was addressed by removing sensitive data.

Last updated 21 September 2026

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.

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.

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 21 September 2026

Last updated 21 September 2026

Last updated 24 September 2026

Last updated 24 September 2026

Last updated 21 September 2026

Last updated 24 September 2026

Last updated 21 September 2026

Last updated 24 September 2026

Last updated 24 September 2026

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.

Last updated 21 September 2026

Last updated 21 September 2026

libexpat versions 2.7.2 through 2.8.5 contain an integer overflow vulnerability in expatrealloc() 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.

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).

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

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 2.6.4. There is a crash within the XMLResumeParser function because XMLStopParser can stop/suspend an unstarted parser.

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.

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.

The Expat XML parser mishandles certain kinds of malformed input documents, resulting in buffer overflows during processing and error reporting. The overflows can manifest as a segmentation fault or as memory corruption during a parse operation. The bugs allow for a denial of service attack in many applications by an unauthenticated attacker, and could conceivably result in remote code execution.

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