CVE-2026-56406

libexpat 2.7.2 through 2.9.0 contains a denial of service vulnerability because XML_ParserReset 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 XML_ERROR_NO_MEMORY until it is recreated.

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

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

The writeRandomBytes_RtlGenRandom function in xmlparse.c in libexpat versions 2.2.1 and 2.2.2 on Windows allows local users to gain privileges via a Trojan horse ADVAPI32.DLL in the current working directory due to an untrusted search path, known as DLL hijacking.

An issue was discovered in libexpat before version 2.6.3, which can lead to an integer overflow in the nextScaffoldPart function in xmlparse.c on 32-bit platforms.

An issue was discovered in libexpat before version 2.6.3 related to integer overflow in the dtdCopy function in xmlparse.c. This issue occurs on 32-bit platforms where UINT_MAX equals SIZE_MAX.

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 XML_GetCurrentLineNumber (or XML_GetCurrentColumnNumber) then resulted in a heap-based buffer over-read.

XML External Entity vulnerability in libexpat 2.2.0 and earlier allows attackers to put the parser in an infinite loop using a malformed external entity definition from an external DTD.

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 XML_TOK_NONE 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.