Type conversion in Unbound - CVE-2026-55991
Published: July 23, 2026
Vulnerability identifier: #VU139220
CSH Severity: Medium
CVSS v4.0: CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N/E:U/U:Green
CVE-ID: CVE-2026-55991
CWE-ID: CWE-704
Exploitation vector: Remote access
Exploit availability:
No public exploit available
Vendor: NLnet Labs
Affected software:
Unbound
Unbound
Detailed vulnerability description
The vulnerability allows a remote attacker to cause a denial of service.
The vulnerability exists due to improper type conversion in libngtcp2 error handling in the DNS-over-QUIC handling path when processing a DNS-over-QUIC connection and query. A remote attacker can send a specially crafted DNS-over-QUIC connection and query to cause a denial of service.
Exploitation requires assertions to be enabled and can be triggered by advertising an initial_max_stream_data_bidi_local value of 1 and sending a query without reading the stream.
How to mitigate CVE-2026-55991
Install security update from vendor's website.