Quick answers
- Is CVE-2026-55991 exploited in the wild?
- No. Junglewise Threat Intelligence has no report of CVE-2026-55991 being exploited in the wild.
- How severe is CVE-2026-55991?
- Medium severity, with a CVSS score of 5.9, published 22 July 2026.
- Which products and versions are affected?
- NLnet Labs Unbound 1.22.0 to 1.25.1.
- Is there a fix for CVE-2026-55991?
- A patch was published on 2026-07-22. Junglewise finds every place you run it, drafts the upgrade and applies it once you approve.
- How is CVE-2026-55991 attacked?
- It is attacked over the network, in a hosted service.
- Am I affected by CVE-2026-55991?
- Check whether you run NLnet Labs Unbound at an affected version. Junglewise does that check for you around the clock: connect your code, cloud and domains, and it matches new vulnerabilities like this one against what you actually run and tells you when one reaches you.
Technical details
A signed-to-unsigned conversion error (CWE-195) exists in Unbound's DNS-over-QUIC (DoQ) implementation when interacting with the libngtcp2 library. When 'ngtcp2_conn_writev_stream()' returns a 'DATA_BLOCKED' error, Unbound incorrectly passes a '-1' integer literal to 'ngtcp2_ccerr_set_application_error()', which is implicitly converted to a 64-bit unsigned integer (0xFFFFFFFFFFFFFFFF). During serialization as a QUIC variable-length integer, this value exceeds the 62-bit maximum, triggering an assertion failure in libngtcp2 and aborting the Unbound process. An unauthenticated remote attacker can trigger this deterministically by manipulating QUIC transport parameters and sending a single query. The issue is fixed in Unbound version 1.25.2.
Affected products
- NLnet Labs Unbound 1.22.0 to 1.25.1
Timeline
- 2026-07-22: advisory: NLnet Labs published the security advisory and released version 1.25.2.
- 2026-07-22: patched