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.
Published: 2026-09-16
Score: 6.5 Medium
EPSS: < 1% Very Low
KEV: No
Impact: Denial of Service via use‑after‑free
Action: Immediate Patch
AI Analysis

Impact

The vulnerability surfaces when Unbound 1.22.0 through 1.26.1 is built with DNS‑over‑QUIC support ('--with-libngtcp2'). Each QUIC stream holds an output buffer containing the DNS response. The ngtcp2 library keeps a shallow reference to this buffer for retransmissions. If a client issues a RESET_STREAM after the buffer has been freed, ngtcp2 still retains the pointer. When a path‑timeout occurs, the library attempts to re‑encode the frame and reads from the freed memory, leading to a use‑after‑free fault. An attacker that can send DoQ queries and trigger RESET_STREAM frames can repeatedly provoke this behavior, eventually crashing Unbound and denying service to legitimate clients.

Affected Systems

NLnet Labs Unbound versions 1.22.0 through 1.26.1 are affected, but only when compiled with the '--with-libngtcp2' option that enables DNS‑over‑QUIC. The product is a recursive DNS resolver widely used in both public and private networks.

Risk and Exploitability

The CVSS score of 6.5 denotes moderate severity. The EPSS score is below 1 %, indicating low but non‑zero exploitation probability, and the flaw is not in the CISA KEV catalog. The likely attack vector is network‑based: an adversary who can initiate DoQ queries and send RESET_STREAM frames can trigger the flaw. Because the bug does not privilege‑escalate, the primary impact is service disruption on a single Unbound instance via repeated retransmissions and crash.

Generated by OpenCVE AI on September 16, 2026 at 14:29 UTC.

Remediation

Vendor Solution

This issue is fixed starting with version 1.26.1

OpenCVE Recommended Actions

  • Upgrade Unbound to version 1.26.1 or later, which contains the fix for the use‑after‑free bug.
  • If an upgrade is not immediately available, recompile Unbound without the '--with-libngtcp2' option or disable DoQ support in the server configuration to eliminate the vulnerable code path.
  • Deploy network controls to limit or monitor DoQ traffic – for example, rate‑limit QUIC connections or restrict access to the DoQ port to trusted hosts to reduce the chance of RESET_STREAM abuse.

Generated by OpenCVE AI on September 16, 2026 at 14:29 UTC.

Tracking

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History

Wed, 16 Sep 2026 16:30:00 +0000

Type Values Removed Values Added
Metrics ssvc

{'options': {'Automatable': 'no', 'Exploitation': 'none', 'Technical Impact': 'partial'}, 'version': '2.0.3'}

Wed, 16 Sep 2026 12:15:00 +0000

Type Values Removed Values Added
Weaknesses CWE-825
References
Metrics threat_severity

None

 threat_severity

Moderate

Wed, 16 Sep 2026 08:45:00 +0000

Type Values Removed Values Added
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.
Title Use-after-free in DoQ stream output buffer on reset re-transmission
Weaknesses CWE-416
References
Metrics cvssV3_1

{'score': 6.5, 'vector': 'CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:N/A:L'}

Subscriptions

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cve-icon MITRE

Status: PUBLISHED

Assigner: NLnet Labs

Published:

Updated: 2026-09-16T15:33:54.267Z

Reserved: 2026-09-07T14:06:21.963Z

Link: CVE-2026-78227

cve-icon Vulnrichment

Updated: 2026-09-16T15:33:44.461Z

cve-icon NVD

Status : Awaiting Analysis

Published: 2026-09-16T09:17:05.987

Modified: 2026-09-16T19:41:10.423

Link: CVE-2026-78227

cve-icon Redhat

Severity : Moderate

Publid Date: 2026-09-16T08:29:52Z

Links: CVE-2026-78227 - Bugzilla

cve-icon OpenCVE Enrichment

Updated: 2026-09-16T14:30:10Z

Weaknesses