# CVE-2026-124317: libexpat out-of-bounds read in unknown encoding handlers

Source: https://junglewise.ai/threats/cve-2026-124317-libexpat-through-2-10-0-contains-an-out-of-bounds-read  
Publisher: Junglewise Threat Intelligence, https://junglewise.ai/threats  
Published: 2026-11-07. Updated: 2026-11-07  
License: CC BY 4.0 with a link back

CVE-2026-124317 · Severity: medium · CVSS 4.8 · Published 2026-11-07

libexpat is a widely-used XML parsing library used in many applications to process XML documents. This vulnerability allows attackers to read sensitive data from memory adjacent to the parse buffer or crash applications by supplying specially crafted XML documents with truncated multibyte sequences in application-registered custom encodings.

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

## Quick answers

**Is CVE-2026-124317 exploited in the wild?** No. Junglewise Threat Intelligence has no report of CVE-2026-124317 being exploited in the wild.

**How severe is CVE-2026-124317?** Medium severity, with a CVSS score of 4.8, published 7 November 2026.

**Which products and versions are affected?** Libexpat through 2.10.0.

**Is there a fix for CVE-2026-124317?** 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-2026-124317 attacked?** It is attacked over the network, in a software library.

**Am I affected by CVE-2026-124317?** Check whether you run Libexpat 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 out-of-bounds read vulnerability exists in the unknown_toUtf8() and unknown_toUtf16() functions in xmltok.c when processing application-registered unknown encodings. When a document ends with a truncated multibyte sequence, the conversion callback reads past the parse buffer boundary. This requires an application to have registered custom unknown encodings and to parse untrusted XML input.

## Affected products

- libexpat libexpat through 2.10.0

## Timeline

- 2026-11-07: disclosed

## References

- [https://github.com/libexpat/libexpat](https://github.com/libexpat/libexpat)
- [https://github.com/libexpat/libexpat/blob/a70841d9ffa64ddde29ab824ed6305035ebe4c47/expat/lib/xmltok.c](https://github.com/libexpat/libexpat/blob/a70841d9ffa64ddde29ab824ed6305035ebe4c47/expat/lib/xmltok.c)

## Related threats

- [libexpat NULL pointer dereference in external entity parsing](https://junglewise.ai/threats/cve-2026-117610-libexpat-through-2-9-0-contains-a-null-pointer-dereference)
- [libexpat denial of service in billion laughs protection](https://junglewise.ai/threats/cve-2026-115506-libexpat-through-2-9-0-contains-a-denial-of-service)
- [libexpat denial of service via XML_ParserReset byte count inconsistency](https://junglewise.ai/threats/cve-2026-113351-libexpat-2-7-2-through-2-9-0-contains-a-denial-of-service)