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.

(from https://nvd.nist.gov/vuln/detail/CVE-2022-25314) In Expat (aka libexpat) before 2.4.5, there is an integer overflow in copyString.

In copyString of xmlparse.c, there is a possible memory corruption due to an integer overflow. This could lead to local escalation of privilege with no additional execution privileges needed. User interaction is not needed for exploitation.

A flaw was found in expat. The vulnerability occurs due to large content in element type declarations when there is an element declaration handler present which leads to an integer overflow. This flaw allows an attacker to inject an unsigned integer, leading to a crash or a denial of service.

Expat (aka libexpat) before 2.4.4 has a signed integer overflow in XMLGetBuffer, for configurations with a nonzero XMLCONTEXTBYTES.

addBinding in xmlparse.c in Expat (aka libexpat) before 2.4.3 has an integer overflow.

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.

A use-after-free flaw was found in the Expat package, caused by destruction of a shared DTD in XMLExternalEntityParserCreate in out-of-memory situations. This may lead to availability disruptions.

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 6 October 2026

Last updated 6 October 2026

Last updated 21 September 2026

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

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.

Last updated 21 September 2026

Last updated 21 September 2026

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.

In libexpat before 2.2.8, crafted XML input could fool the parser into changing from DTD parsing to document parsing too early; a consecutive call to XMLGetCurrentLineNumber (or XMLGetCurrentColumnNumber) then resulted in a heap-based buffer over-read.

An out-of-bounds read flaw was found in the way Expat processed certain input. A remote attacker could send specially crafted XML that, when parsed by an application using the Expat library, would cause that application to crash or, possibly, execute arbitrary code with the permission of the user running the application.

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

libexpat. Multiple issues were addressed by updating to version 2.2.1

An integer overflow was found in expat. The issue occurs in storeRawNames() by abusing the mbuffer expansion logic to allow allocations very close to INTMAX and out-of-bounds heap writes. This flaw can cause a denial of service or potentially arbitrary code execution.

A flaw in XML parsing could have led to a use-after-free causing a potentially exploitable crash.In official releases of Firefox this vulnerability is mitigated by wasm sandboxing; versions managed by Linux distributions may have other settings.

A flaw was found in expat. A stack exhaustion in doctype parsing could be triggered by a file with a large number of opening braces, resulting in a denial of service.

defineAttribute in xmlparse.c in Expat (aka libexpat) before 2.4.3 has an integer overflow.

buildmodel in xmlparse.c in Expat (aka libexpat) before 2.4.3 has an integer overflow.

A flaw was found in expat. Passing one or more namespace separator characters in the "xmlns[:prefix]" attribute values made expat send malformed tag names to the XML processor on top of expat. This issue causes arbitrary code execution depending on how unexpected cases are handled inside the XML processor.

expat (libexpat) is susceptible to a software flaw that causes process interruption. When processing a large number of prefixed XML attributes on a single tag can libexpat can terminate unexpectedly due to integer overflow. The highest threat from this vulnerability is to availability, confidentiality and integrity.

expat (libexpat) is susceptible to a software flaw that causes process interruption. When processing a large number of prefixed XML attributes on a single tag can libexpat can terminate unexpectedly due to integer overflow. The highest threat from this vulnerability is to availability, confidentiality and integrity.

expat (libexpat) is susceptible to a software flaw that causes process interruption. When processing a large number of prefixed XML attributes on a single tag can libexpat can terminate unexpectedly due to integer overflow. The highest threat from this vulnerability is to availability, confidentiality and integrity.

A flaw was found in expat. Passing malformed 2- and 3-byte UTF-8 sequences (for example, from start tag names) to the XML processing application on top of expat can lead to arbitrary code execution. This issue is dependent on how invalid UTF-8 is handled inside the XML processor.