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.
Advisories

No advisories yet.

Fixes

Solution

This issue is fixed starting with version 1.26.1


Workaround

No workaround given by the vendor.

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'}


Projects

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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 : Received

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

Modified: 2026-09-16T16:17:16.157

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