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

CVE-2026-55991 is fixed in unbound on 1 platform, first by DEBIAN-CVE-2026-55991, released July 28, 2026. It is not reported as exploited. Patch Radar priority: Routine.

- Web page: https://patchblox.com/security/cve/CVE-2026-55991
- Priority: Routine
- CVSS: 5.9 (Moderate)
- EPSS: 0.0037
- CISA KEV: not listed
- Exploited (vendor report): no
- Publicly disclosed: no
- Published: Jul 22, 2026
- Weakness: Signed to Unsigned Conversion Error
- Vendor advisory: https://msrc.microsoft.com/update-guide/vulnerability/CVE-2026-55991

## 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.

## Where it's fixed

| Platform | Product | Fixed in | Fix | Released |
|---|---|---|---|---|
| Linux | unbound Debian 13.5 (trixie) | 1.26.1-0+deb13u1 | [DEBIAN-CVE-2026-55991](https://patchblox.com/security/data/fix/linux/debian-cve-2026-55991.md) | Jul 28, 2026 |
| Linux | unbound Debian 14 | 1.25.2-1 | [DEBIAN-CVE-2026-55991](https://patchblox.com/security/data/fix/linux/debian-cve-2026-55991.md) | Jul 28, 2026 |
| Linux | unbound opensuse-leap 16.0 | 1.25.2-160000.1.1 | [OPENSUSE-SU-2026:21550-1](https://patchblox.com/security/data/fix/linux/opensuse-su-2026-21550-1.md) | Aug 10, 2026 |
| Linux | unbound sles 16.0 | 1.25.2-160000.1.1 | [SUSE-SU-2026:23226-1](https://patchblox.com/security/data/fix/linux/suse-su-2026-23226-1.md) | Aug 10, 2026 |

---

Data as of 2026-11-09T13:31:49Z UTC. Source: Patchblox Patch Radar (https://patchblox.com/security/patch-radar). Patchblox finds out-of-date software on Windows, macOS and Linux and updates it through release rings: https://patchblox.com/demo

Free to use with attribution: show "Data: Patchblox Patch Radar" with a link to https://patchblox.com/security/patch-radar. Terms, including what needs a written agreement: https://patchblox.com/security/patch-radar/use-the-data

Product names, logos and trademarks shown on Patch Radar belong to their respective owners. They are used only to identify the software each security fix applies to and to help readers find their platform. Their use does not imply any affiliation with, or endorsement by, those owners, and Patchblox is not affiliated with any of them.