# CVE-2026-78227: Use-after-free in DoQ stream output buffer on reset re-transmission

CVE-2026-78227 is fixed in unbound on 1 platform, first by DEBIAN-CVE-2026-78227, released September 19, 2026. It is not reported as exploited. Patch Radar priority: Routine.

- Web page: https://patchblox.com/security/cve/CVE-2026-78227
- Priority: Routine
- CVSS: 6.5 (Moderate)
- EPSS: 0.0034
- CISA KEV: not listed
- Exploited (vendor report): no
- Publicly disclosed: no
- Published: Sep 16, 2026
- Weakness: Use After Free
- Vendor advisory: https://msrc.microsoft.com/update-guide/vulnerability/CVE-2026-78227

## Description

NLnet Labs Unbound 1.22.0 up to and including 1.26.1, has a use-after-free vulnerability when compiled for DNS-over-QUIC support with '--with-libngtcp2'. Each DoQ stream owns an output buffer that holds the DNS response. ngtcp2's retransmission buffer keeps a shallow pointer into the output buffer for as long as a STREAM frame may be resent. On a client RESET_STREAM, the output buffer is freed but ngtcp2 still holds the matching retransmission entries. The next PTO timeout makes ngtcp2 re-encode the STREAM frame and copy from the freed buffer. A malicious actor that can query Unbound over DoQ and that withholds ACKs, sends RESET_STREAM, and waits for PTO, reaches this use-after-free with no privilege. This leads to retransmissions against freed memory and eventually an abnormal server exit under a 20-query spray.

## Where it's fixed

| Platform | Product | Fixed in | Fix | Released |
|---|---|---|---|---|
| Linux | unbound Debian 13.5 (trixie) | 1.26.1-0+deb13u1 | [DEBIAN-CVE-2026-78227](https://patchblox.com/security/data/fix/linux/debian-cve-2026-78227.md) | Sep 19, 2026 |
| Linux | unbound Debian 14 | 1.26.1-1 | [DEBIAN-CVE-2026-78227](https://patchblox.com/security/data/fix/linux/debian-cve-2026-78227.md) | Sep 19, 2026 |
| Linux | unbound opensuse-leap 16.0 | 1.26.1-160000.1.1 | [OPENSUSE-SU-2026:22082-1](https://patchblox.com/security/data/fix/linux/opensuse-su-2026-22082-1.md) | Nov 7, 2026 |
| Linux | unbound sles 15.6 | 1.26.1-150600.23.24.1 | [SUSE-SU-2026:4620-1](https://patchblox.com/security/data/fix/linux/suse-su-2026-4620-1.md) | Nov 7, 2026 |
| Linux | unbound sles 15.7 | 1.26.1-150600.23.24.1 | [SUSE-SU-2026:4620-1](https://patchblox.com/security/data/fix/linux/suse-su-2026-4620-1.md) | Nov 7, 2026 |
| Linux | unbound sles 16.0 | 1.26.1-160000.1.1 | [SUSE-SU-2026:24226-1](https://patchblox.com/security/data/fix/linux/suse-su-2026-24226-1.md) | Nov 7, 2026 |

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Data as of 2026-11-08T19:12:23Z UTC. Source: Patchblox Patch Radar (https://patchblox.com/security/patch-radar). Patchblox finds out-of-date software on Windows, macOS and Linux and updates it through release rings: https://patchblox.com/demo

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