CVE-2026-78227: Nlnetlabs Unbound

Medium severity, CVSS 6.5. EPSS: 0.3% chance of exploitation in the next 30 days.

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 products

• Nlnetlabs Unbound: from 1.22.0, before 1.26.1 (fixed in 1.26.1)

Published 2026-09-16. Last modified 2026-09-23.