# CVE-2026-56408: libexpat integer overflow in copyString

Source: https://junglewise.ai/threats/cve-2026-56408-libexpat-integer-overflow-in-copystring  
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-2026-56408 · Severity: medium · CVSS 6.9 · Published 2026-06-21

libexpat is a widely used software library for processing XML data. A flaw in how it handles memory allocation for strings could allow an attacker to cause a system crash or potentially gain unauthorized access to data. This affects any application that relies on libexpat to parse untrusted XML content.

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

## Quick answers

**Is CVE-2026-56408 exploited in the wild?** No. Junglewise Threat Intelligence has no report of CVE-2026-56408 being exploited in the wild.

**How severe is CVE-2026-56408?** 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-2026-56408?** No fixed version is recorded yet. Check the vendor's advisory in the references below. If you run it, Junglewise drafts a mitigation that fits your setup and applies it once you approve, with senior security engineers behind it.

**How is CVE-2026-56408 attacked?** It is attacked with local access, in a software library.

**Am I affected by CVE-2026-56408?** 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 copyString function within libexpat's xmlparse.c. The issue occurs when calculating the memory required for a string copy (charsRequired * sizeof(XML_Char)), which can overflow if the input string is sufficiently large. This can lead to an undersized memory allocation followed by a heap-based buffer overflow. An attacker could exploit this via a local vector to cause a denial of service or achieve arbitrary code execution, though the high attack complexity suggests specific conditions must be met. The vulnerability is addressed in version 2.8.2 by adding a check against SIZE_MAX.

## Affected products

- libexpat project libexpat before 2.8.2

## Timeline

- 2026-06-21: disclosed
- 2026-06-21: advisory

## References

- [https://github.com/libexpat/libexpat/commit/16e2efd867ea8567ffa012210b52ef5918e20817](https://github.com/libexpat/libexpat/commit/16e2efd867ea8567ffa012210b52ef5918e20817)

## Related threats

- [libexpat use-after-free in doCdataSection](https://junglewise.ai/threats/cve-2026-56412-libexpat-use-after-free-in-docdatasection)
- [libexpat integer overflow in xmlwf endDoctypeDecl](https://junglewise.ai/threats/cve-2026-56411-libexpat-integer-overflow-in-xmlwf-enddoctypedecl)
- [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 doProlog entity handling](https://junglewise.ai/threats/cve-2026-56407-libexpat-integer-overflow-in-doprolog-entity-handling)