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Vulnerability · Published Jul 22, 2026 · Assigned by NLnet Labs

CVE-2026-55991Remote DNS-over-QUIC (DoQ) flow-control assertion failure in libngtcp2

Medium 5.9

In plain English

A medium-severity security flaw 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 0.4% (higher than 30% of all known vulnerabilities). Fixed versions are available - upgrade the affected package.

Severity (CVSS 3.1)5.9Medium
Exploit odds (30 days)0.4%top 69.5% of all CVEs
Exploited in the wildNo reportsCISA SSVC: none
Attack toolingNoneno public tooling known

Official description

In NLnet Labs Unbound 1.22.0 up to and including 1.25.1, a remote unauthenticated client can trigger a libngtcp2 assertion (if compiled with assertions on) and terminate the entire Unbound process using a single DNS-over-QUIC (DoQ) connection and one normal DNS query. This is caused by an erroneous error value passed to libngtcp2. When 'ngtcp2_conn_writev_stream()' returns 'NGTCP2_ERR_STREAM_DATA_BLOCKED', Unbound continues to call 'ngtcp2_ccerr_set_application_error()' with a '-1' error value. The 'int' literal '-1' is implicitly converted to the function's 'uint64_t error_code' parameter as '0xFFFFFFFFFFFFFFFF'. The follow-on 'ngtcp2_conn_write_connection_close()' serialises that value as a QUIC variable-length integer; because '2^64-1' exceeds the 62-bit varint ceiling, 'ngtcp2_put_uvarintlen()' fails 'assert(n < 4611686018427387904ULL)' and the whole resolver process aborts. A remote, unauthenticated DoQ client can trigger this deterministically with a single QUIC connection by advertising 'initial_max_stream_data_bidi_local = 1' in its transport parameters and sending one DoQ query without ever reading the stream.

Affected packages & fixed versions

EcosystemPackageFixed inAdvisory
Debian:12unboundno fix listedDEBIAN-CVE-2026-55991
Debian:13unbound1.26.1-0+deb13u1DEBIAN-CVE-2026-55991
Debian:14unbound1.25.2-1DEBIAN-CVE-2026-55991
Ubuntu:22.04:LTSunboundno fix listedUBUNTU-CVE-2026-55991
Ubuntu:24.04:LTSunboundno fix listedUBUNTU-CVE-2026-55991
Ubuntu:26.04:LTSunboundno fix listedUBUNTU-CVE-2026-55991
Ubuntu:26.10unboundno fix listedUBUNTU-CVE-2026-55991

Affected products (as reported)

VendorProductVersions
NLnet Labs Unbound 1.22.0 – before 1.25.2

Weakness: CWE-195 · Vector: CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:H

References