---
id: CVE-2026-55991
title: Remote DNS-over-QUIC (DoQ) flow-control assertion failure in libngtcp2
summary: >-
  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) conne…
severity: medium
cvss: 5.9
cvssVector: 'CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:H'
cvssSource: cna
cwe:
  - CWE-195
vendor: NLnet Labs
product: Unbound
affected:
  - Unbound >= 1.22.0 < 1.25.2
ssvc:
  exploitation: none
  automatable: 'no'
  technicalImpact: partial
  timestamp: '2026-07-22T14:01:59.779165Z'
published: '2026-07-22'
updated: '2026-09-16'
sourceUpdated: '2026-09-16T14:24:57.805Z'
source: CVEORG
sourceUrl: 'https://www.cve.org/CVERecord?id=CVE-2026-55991'
references:
  - url: 'https://www.nlnetlabs.nl/downloads/unbound/CVE-2026-55991.txt'
tags:
  - cve.org
epss: 0.00365
epssPercentile: 0.27668
ingestedAt: '2026-09-16T14:57:28.053Z'
---

## Overview

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

- `Unbound >= 1.22.0 < 1.25.2`

## Remediation

This issue is fixed starting with version 1.25.2