# CVE-2026-56411: libexpat integer overflow in xmlwf endDoctypeDecl

Source: https://junglewise.ai/threats/cve-2026-56411-libexpat-integer-overflow-in-xmlwf-enddoctypedecl  
Publisher: Junglewise Threat Intelligence, https://junglewise.ai/threats  
Published: 2026-06-21. Updated: 2026-06-21  
License: CC BY 4.0 with a link back

CVE-2026-56411 · Severity: medium · CVSS 6.9 · Published 2026-06-21

A vulnerability exists in libexpat, a widely used library for parsing XML data. An attacker could provide a specially crafted XML file that causes the software to miscalculate memory requirements, potentially leading to a system crash or unauthorized data access. This affects applications that use the xmlwf tool to process untrusted XML documents.

Technologies: [Libexpat Project Expat](https://junglewise.ai/threats/technologies/expat). Vendors: [Libexpat Project](https://junglewise.ai/threats/vendors/libexpat-project).

## Quick answers

**Is CVE-2026-56411 exploited in the wild?** No. Junglewise Threat Intelligence has no report of CVE-2026-56411 being exploited in the wild.

**How severe is CVE-2026-56411?** Medium severity, with a CVSS score of 6.9, published 21 June 2026.

**Which products and versions are affected?** Libexpat Project Expat before 2.8.2.

**Is there a fix for CVE-2026-56411?** A patch was published on 2026-06-03: Fix merged into master branch Junglewise finds every place you run it, drafts the upgrade and applies it once you approve.

**How is CVE-2026-56411 attacked?** It is attacked with local access, in a software library.

**Am I affected by CVE-2026-56411?** Check whether you run Libexpat Project Expat at an affected version. Junglewise does that check for you around the clock: connect your code, cloud and domains, and it matches new vulnerabilities like this one against what you actually run and tells you when one reaches you.

## Technical details

An integer overflow vulnerability exists in the xmlwf utility of libexpat within the endDoctypeDecl() function. The root cause is the use of a plain integer to count NOTATION declarations from a Document Type Definition (DTD), followed by a memory allocation (malloc) that multiplies this count by the size of a pointer without overflow guards. On 32-bit systems, this multiplication can wrap around, leading to a heap-based buffer under-allocation. A subsequent loop that populates this buffer can then result in a heap buffer overflow. The vulnerability is addressed in version 2.8.2 by switching the counter to size_t and implementing proper overflow checks.

## Affected products

- libexpat project libexpat before 2.8.2

## Timeline

- 2026-06-02: disclosed: Vulnerability details shared in pull request
- 2026-06-03: patched: Fix merged into master branch
- 2026-06-21: advisory: CVE-2026-56411 published

## References

- [https://github.com/libexpat/libexpat/pull/1263](https://github.com/libexpat/libexpat/pull/1263)

## Related threats

- [libexpat use-after-free in doCdataSection](https://junglewise.ai/threats/cve-2026-56412-libexpat-use-after-free-in-docdatasection)
- [libexpat xmlwf integer overflow in resolveSystemId](https://junglewise.ai/threats/cve-2026-56410-libexpat-xmlwf-integer-overflow-in-resolvesystemid)
- [libexpat xmlwf integer overflow in output filename handling](https://junglewise.ai/threats/cve-2026-56409-libexpat-xmlwf-integer-overflow-in-output-filename-handling)
- [libexpat integer overflow in copyString](https://junglewise.ai/threats/cve-2026-56408-libexpat-integer-overflow-in-copystring)
- [libexpat integer overflow in doProlog entity handling](https://junglewise.ai/threats/cve-2026-56407-libexpat-integer-overflow-in-doprolog-entity-handling)