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Vulnerability · Published Sep 16, 2026 · Assigned by NLnet Labs

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

Medium 6.5

In plain English

A medium-severity use-after-free in Unbound from NLnet Labs. There are no confirmed reports of exploitation so far. The chance it is exploited in the next 30 days is estimated at 0.3% (higher than 28% of all known vulnerabilities). Fixed versions are available - upgrade the affected package.

Severity (CVSS 3.1)6.5Medium
Exploit odds (30 days)0.3%top 72.4% of all CVEs
Exploited in the wildNo reportsCISA SSVC: none
Attack toolingNoneno public tooling known

Official 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.

Affected packages & fixed versions

EcosystemPackageFixed inAdvisory
Debian:12unboundno fix listedDEBIAN-CVE-2026-78227
Debian:13unbound1.26.1-0+deb13u1DEBIAN-CVE-2026-78227
Debian:14unbound1.26.1-1DEBIAN-CVE-2026-78227
Ubuntu:22.04:LTSunboundno fix listedUBUNTU-CVE-2026-78227
Ubuntu:24.04:LTSunboundno fix listedUBUNTU-CVE-2026-78227
Ubuntu:26.04:LTSunboundno fix listedUBUNTU-CVE-2026-78227
Ubuntu:26.10unboundno fix listedUBUNTU-CVE-2026-78227

Affected products (as reported)

VendorProductVersions
NLnet Labs Unbound 1.22.0 – before 1.26.1

Weakness: CWE-416 · Vector: CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:N/A:L

References