{"id":"UBUNTU-CVE-2026-55991","details":"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 \u003c 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.","modified":"2026-08-20T23:23:40.769413306Z","published":"2026-07-22T14:17:00Z","upstream":["CVE-2026-55991"],"references":[{"type":"REPORT","url":"https://ubuntu.com/security/CVE-2026-55991"},{"type":"REPORT","url":"https://www.cve.org/CVERecord?id=CVE-2026-55991"},{"type":"REPORT","url":"https://community.nlnetlabs.nl/t/unbound-1-25-2-released/3430"}],"affected":[{"package":{"name":"unbound","ecosystem":"Ubuntu:Pro:14.04:LTS","purl":"pkg:deb/ubuntu/unbound?arch=source&distro=esm-infra-legacy%2Ftrusty"},"ranges":[{"type":"ECOSYSTEM","events":[{"introduced":"0"}]}],"versions":["1.4.20-1","1.4.21-1","1.4.21-1ubuntu1","1.4.21-1ubuntu2","1.4.22-1ubuntu1","1.4.22-1ubuntu2","1.4.22-1ubuntu3","1.4.22-1ubuntu4","1.4.22-1ubuntu4.14.04.1","1.4.22-1ubuntu4.14.04.2","1.4.22-1ubuntu4.14.04.3","1.4.22-1ubuntu4.14.04.3+esm1","1.4.22-1ubuntu4.14.04.3+esm2","1.4.22-1ubuntu4.14.04.3+esm3"],"ecosystem_specific":{"binaries":[{"binary_name":"libunbound2","binary_version":"1.4.22-1ubuntu4.14.04.3+esm3"},{"binary_version":"1.4.22-1ubuntu4.14.04.3+esm3","binary_name":"python-unbound"},{"binary_name":"unbound","binary_version":"1.4.22-1ubuntu4.14.04.3+esm3"},{"binary_name":"unbound-anchor","binary_version":"1.4.22-1ubuntu4.14.04.3+esm3"},{"binary_name":"unbound-host","binary_version":"1.4.22-1ubuntu4.14.04.3+esm3"}]},"database_specific":{"source":"https://github.com/canonical/ubuntu-security-notices/blob/main/osv/cve/2026/UBUNTU-CVE-2026-55991.json"}},{"package":{"name":"unbound","ecosystem":"Ubuntu:Pro:16.04:LTS","purl":"pkg:deb/ubuntu/unbound?arch=source&distro=esm-infra-legacy%2Fxenial"},"ranges":[{"type":"ECOSYSTEM","events":[{"introduced":"0"}]}],"versions":["1.4.22-1ubuntu6","1.5.7-1ubuntu1","1.5.7-1ubuntu2","1.5.8-1ubuntu1","1.5.8-1ubuntu1.1","1.5.8-1ubuntu1.1+esm1","1.5.8-1ubuntu1.1+esm2","1.5.8-1ubuntu1.1+esm3"],"ecosystem_specific":{"binaries":[{"binary_name":"libunbound2","binary_version":"1.5.8-1ubuntu1.1+esm3"},{"binary_name":"python-unbound","binary_version":"1.5.8-1ubuntu1.1+esm3"},{"binary_name":"unbound","binary_version":"1.5.8-1ubuntu1.1+esm3"},{"binary_name":"unbound-anchor","binary_version":"1.5.8-1ubuntu1.1+esm3"},{"binary_version":"1.5.8-1ubuntu1.1+esm3","binary_name":"unbound-host"}]},"database_specific":{"source":"https://github.com/canonical/ubuntu-security-notices/blob/main/osv/cve/2026/UBUNTU-CVE-2026-55991.json"}},{"package":{"name":"unbound","ecosystem":"Ubuntu:Pro:18.04:LTS","purl":"pkg:deb/ubuntu/unbound?arch=source&distro=esm-infra%2Fbionic"},"ranges":[{"type":"ECOSYSTEM","events":[{"introduced":"0"}]}],"versions":["1.6.5-1","1.6.7-1","1.6.7-1build1","1.6.7-1ubuntu1","1.6.7-1ubuntu2","1.6.7-1ubuntu2.1","1.6.7-1ubuntu2.2","1.6.7-1ubuntu2.3","1.6.7-1ubuntu2.4","1.6.7-1ubuntu2.5","1.6.7-1ubuntu2.6","1.6.7-1ubuntu2.6+esm2","1.6.7-1ubuntu2.6+esm3","1.6.7-1ubuntu2.6+esm4"],"ecosystem_specific":{"binaries":[{"binary_name":"libunbound2","binary_version":"1.6.7-1ubuntu2.6+esm4"},{"binary_version":"1.6.7-1ubuntu2.6+esm4","binary_name":"python-unbound"},{"binary_name":"python3-unbound","binary_version":"1.6.7-1ubuntu2.6+esm4"},{"binary_name":"unbound","binary_version":"1.6.7-1ubuntu2.6+esm4"},{"binary_name":"unbound-anchor","binary_version":"1.6.7-1ubuntu2.6+esm4"},{"binary_name":"unbound-host","binary_version":"1.6.7-1ubuntu2.6+esm4"}]},"database_specific":{"source":"https://github.com/canonical/ubuntu-security-notices/blob/main/osv/cve/2026/UBUNTU-CVE-2026-55991.json"}},{"package":{"name":"unbound","ecosystem":"Ubuntu:Pro:20.04:LTS","purl":"pkg:deb/ubuntu/unbound?arch=source&distro=esm-infra%2Ffocal"},"ranges":[{"type":"ECOSYSTEM","events":[{"introduced":"0"}]}],"versions":["1.9.0-2ubuntu1","1.9.0-2ubuntu2","1.9.4-2","1.9.4-2ubuntu1","1.9.4-2ubuntu1.1","1.9.4-2ubuntu1.2","1.9.4-2ubuntu1.3","1.9.4-2ubuntu1.4","1.9.4-2ubuntu1.5","1.9.4-2ubuntu1.6","1.9.4-2ubuntu1.8","1.9.4-2ubuntu1.9","1.9.4-2ubuntu1.11","1.9.4-2ubuntu1.11+esm1"],"ecosystem_specific":{"binaries":[{"binary_name":"libunbound8","binary_version":"1.9.4-2ubuntu1.11+esm1"},{"binary_version":"1.9.4-2ubuntu1.11+esm1","binary_name":"python-unbound"},{"binary_name":"python3-unbound","binary_version":"1.9.4-2ubuntu1.11+esm1"},{"binary_version":"1.9.4-2ubuntu1.11+esm1","binary_name":"unbound"},{"binary_name":"unbound-anchor","binary_version":"1.9.4-2ubuntu1.11+esm1"},{"binary_version":"1.9.4-2ubuntu1.11+esm1","binary_name":"unbound-host"}]},"database_specific":{"source":"https://github.com/canonical/ubuntu-security-notices/blob/main/osv/cve/2026/UBUNTU-CVE-2026-55991.json"}},{"package":{"name":"unbound","ecosystem":"Ubuntu:22.04:LTS","purl":"pkg:deb/ubuntu/unbound?arch=source&distro=jammy"},"ranges":[{"type":"ECOSYSTEM","events":[{"introduced":"0"}]}],"versions":["1.13.1-1ubuntu1","1.13.1-1ubuntu3","1.13.1-1ubuntu5","1.13.1-1ubuntu5.1","1.13.1-1ubuntu5.2","1.13.1-1ubuntu5.3","1.13.1-1ubuntu5.4","1.13.1-1ubuntu5.5","1.13.1-1ubuntu5.7","1.13.1-1ubuntu5.8","1.13.1-1ubuntu5.10","1.13.1-1ubuntu5.11","1.13.1-1ubuntu5.12","1.13.1-1ubuntu5.13","1.13.1-1ub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