libexpat through 2.9.0 contains a signed integer overflow vulnerability in storeAtts() in lib/xmlparse.c that allows attackers to trigger out-of-bounds writes via oversized attribute counts. Attackers can supply a multi-gigabyte document with a start tag carrying over 2^30 attributes to overflow attIndex and nPrefixes, crashing applications or corrupting heap memory.

libexpat through 2.9.0 contains a NULL pointer dereference vulnerability in externalEntityInitProcessor2() in lib/xmlparse.c that mishandles the XMLTOKNONE token from empty input. Attackers who control stream chunking can force an empty non-final first parse on an external entity subparser, causing a later parse to crash the application.

libexpat through 2.9.0 contains a denial of service vulnerability because billion laughs protection in accountingDiffTolerated() in xmlparse.c detects entity amplification too late. Attackers can supply a moderately sized XML document with a large DTD and nested entity expansions to consume tens of seconds of CPU per parse.

libexpat 2.7.2 through 2.9.0 contains a denial of service vulnerability because XMLParserReset zeroes the direct byte count but never offsets tracked heap allocations. Attackers can submit a large XML document so later documents on the same reset parser fail with XMLERRORNOMEMORY until it is recreated.

Last updated 24 September 2026

In libexpat 2.8.2 and 2.8.3 before 2.8.4, misinterpretation of getentropy's return code leads to insufficient entropy, which results in being vulnerable to hash flooding attacks, causing a denial of service via crafted XML content.

Expat Denial of Service via storeAtts() Quadratic Complexity

Last updated 21 September 2026

Last updated 24 September 2026

Last updated 24 September 2026

Last updated 24 September 2026

Last updated 21 September 2026

Last updated 24 September 2026

Last updated 24 September 2026

Last updated 6 October 2026

Last updated 6 October 2026

Last updated 21 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.

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).

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.

Last updated 21 September 2026

Last updated 21 September 2026

Last updated 21 September 2026

libexpat before 2.7.5 allows a NULL pointer dereference with empty external parameter entity content.

In libexpat before 2.7.4, the doContent function does not properly determine the buffer size bufSize because there is no integer overflow check for tag buffer reallocation.

In libexpat before 2.7.4, XMLExternalEntityParserCreate does not copy unknown encoding handler user data.

In libexpat through 2.7.3, a crafted file with an approximate size of 2 MiB can lead to dozens of seconds of processing time.

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

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.