Medium
unbound — security advisory
In NLnet Labs Unbound up to and including version 1.25.1, when 'unwanted-reply-threshold' is enabled (set to any value greater than zero), glue records of 0.0.0.0/::0 can short-circuit Unbound, on systems that can direct such traffic, by issuing DNS queries and receiving seemingly unwanted replies since the remote IP does not match the original source IP of 0.0.0.0/::0. This behavior keeps on looping for the glue records and pushing the counter to the configured 'unwanted-reply-threshold' that triggers a defensive cache clear. A malicious actor who controls a delegation that returns in-bailiwick glue of 0.0.0.0/::0 can drive the counter to the limit of 'unwanted-reply-threshold' to the threshold and trigger a cache clean of the message and rrset caches; at will, indefinitely, without sending a single spoofed packet. The iterator uses the 0.0.0.0/::0 glue, and a system that can route this (e.g., Linux kernel routes the datagram over loopback), Unbound's own listener answers from 127.0.0.1. Because of the mismatch of 0.0.0.0 and 127.0.0.1, in this example, Unbound accounts the reply as an unwanted (probably spoofed) answer. The counter resets to zero on every cache flush, so the attack loops forever.
- Products
- unbound
- Affected versions
- 1.13.1-1ubuntu1, 1.13.1-1ubuntu3, 1.13.1-1ubuntu5, 1.13.1-1ubuntu5.1, 1.13.1-1ubuntu5.10, 1.13.1-1ubuntu5.11, 1.13.1-1ubuntu5.12, 1.13.1-1ubuntu5.13, 1.13.1-1ubuntu5.14, 1.13.1-1ubuntu5.15, 1.13.1-1ubuntu5.16, 1.13.1-1ubuntu5.17, 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, 1.17.1-2, 1.18.0-2ubuntu1, 1.18.0-2ubuntu2, 1.19.1-1ubuntu1, 1.19.2-1ubuntu1, 1.19.2-1ubuntu3, 1.19.2-1ubuntu3.1, 1.19.2-1ubuntu3.10, 1.19.2-1ubuntu3.12, 1.19.2-1ubuntu3.2, 1.19.2-1ubuntu3.3, 1.19.2-1ubuntu3.4, 1.19.2-1ubuntu3.5, 1.19.2-1ubuntu3.6, 1.19.2-1ubuntu3.7, 1.19.2-1ubuntu3.8, 1.19.2-1ubuntu3.9, 1.22.0-2ubuntu2, 1.22.0-2ubuntu4, 1.24.2-1ubuntu1, 1.24.2-1ubuntu2, 1.24.2-1ubuntu2.1, 1.24.2-1ubuntu2.2, 1.24.2-1ubuntu2.3, 1.24.2-1ubuntu2.4, 1.25.1-1ubuntu1, 1.25.1-1ubuntu3, 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, 1.4.22-1ubuntu4.14.04.3+esm4, 1.4.22-1ubuntu4.14.04.3+esm5, 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, 1.5.8-1ubuntu1.1+esm4, 1.5.8-1ubuntu1.1+esm5, 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, 1.6.7-1ubuntu2.6+esm5, 1.6.7-1ubuntu2.6+esm6, 1.9.0-2ubuntu1, 1.9.0-2ubuntu2, 1.9.4-2, 1.9.4-2ubuntu1, 1.9.4-2ubuntu1.1, 1.9.4-2ubuntu1.11, 1.9.4-2ubuntu1.11+esm1, 1.9.4-2ubuntu1.11+esm2, 1.9.4-2ubuntu1.11+esm3, 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
- Fixed versions
- Not specified
- CVEs
- CVE-2026-50251
- Confirmed fixed CVEs
- Not specified
- Updated in this feed
- 31 Oct 2026, 14:34 UTC · Revision 1
Source evidence
In NLnet Labs Unbound up to and including version 1.25.1, when 'unwanted-reply-threshold' is enabled (set to any value greater than zero), glue records of 0.0.0.0/::0 can short-circuit Unbound, on systems that can direct such traffic, by issuing DNS queries and receiving seemingly unwanted replies since the remote IP does not match the original source IP of 0.0.0.0/::0. This behavior keeps on looping for the glue records and pushing the counter to the configured 'unwanted-reply-threshold' that triggers a defensive cache clear. A malicious actor who controls a delegation that returns in-bailiwick glue of 0.0.0.0/::0 can drive the counter to the limit of 'unwanted-reply-threshold' to the threshold and trigger a cache clean of the message and rrset caches; at will, indefinitely, without sending a single spoofed packet. The iterator uses the 0.0.0.0/::0 glue, and a system that can route this (e.g., Linux kernel routes the datagram over loopback), Unbound's own listener answers from 127.0.0.1. Because of the mismatch of 0.0.0.0 and 127.0.0.1, in this example, Unbound accounts the reply as an unwanted (probably spoofed) answer. The counter resets to zero on every cache flush, so the attack loops forever.
osv:UBUNTU-CVE-2026-50251
unbound
Fixed versions: Not specified
unbound
Fixed versions: Not specified
unbound
Fixed versions: Not specified
unbound
Fixed versions: Not specified
unbound
Fixed versions: Not specified
unbound
Fixed versions: Not specified
unbound
Fixed versions: Not specified
unbound
Fixed versions: Not specified
Complete source record
{
"id": "6409c3e5da5b654b3ca909a276fe1c5eab06ab6227432fbb69c4178ebae4dfba",
"url": "https://osv.dev/vulnerability/UBUNTU-CVE-2026-50251",
"method": "structured",
"payload": {
"kind": "advisory",
"title": "UBUNTU-CVE-2026-50251",
"summary": "In NLnet Labs Unbound up to and including version 1.25.1, when 'unwanted-reply-threshold' is enabled (set to any value greater than zero), glue records of 0.0.0.0/::0 can short-circuit Unbound, on systems that can direct such traffic, by issuing DNS queries and receiving seemingly unwanted replies since the remote IP does not match the original source IP of 0.0.0.0/::0. This behavior keeps on looping for the glue records and pushing the counter to the configured 'unwanted-reply-threshold' that triggers a defensive cache clear. A malicious actor who controls a delegation that returns in-bailiwick glue of 0.0.0.0/::0 can drive the counter to the limit of 'unwanted-reply-threshold' to the threshold and trigger a cache clean of the message and rrset caches; at will, indefinitely, without sending a single spoofed packet. The iterator uses the 0.0.0.0/::0 glue, and a system that can route this (e.g., Linux kernel routes the datagram over loopback), Unbound's own listener answers from 127.0.0.1. Because of the mismatch of 0.0.0.0 and 127.0.0.1, in this example, Unbound accounts the reply as an unwanted (probably spoofed) answer. The counter resets to zero on every cache flush, so the attack loops forever.",
"evidence": "osv:UBUNTU-CVE-2026-50251",
"products": [
{
"name": {
"value": "unbound",
"evidence": "/affected/0/package/name"
},
"prerequisites": [],
"fixed_versions": [],
"affected_versions": [
{
"value": "1.4.20-1",
"evidence": "/affected/0/versions"
},
{
"value": "1.4.21-1",
"evidence": "/affected/0/versions"
},
{
"value": "1.4.21-1ubuntu1",
"evidence": "/affected/0/versions"
},
{
"value": "1.4.21-1ubuntu2",
"evidence": "/affected/0/versions"
},
{
"value": "1.4.22-1ubuntu1",
"evidence": "/affected/0/versions"
},
{
"value": "1.4.22-1ubuntu2",
"evidence": "/affected/0/versions"
},
{
"value": "1.4.22-1ubuntu3",
"evidence": "/affected/0/versions"
},
{
"value": "1.4.22-1ubuntu4",
"evidence": "/affected/0/versions"
},
{
"value": "1.4.22-1ubuntu4.14.04.1",
"evidence": "/affected/0/versions"
},
{
"value": "1.4.22-1ubuntu4.14.04.2",
"evidence": "/affected/0/versions"
},
{
"value": "1.4.22-1ubuntu4.14.04.3",
"evidence": "/affected/0/versions"
},
{
"value": "1.4.22-1ubuntu4.14.04.3+esm1",
"evidence": "/affected/0/versions"
},
{
"value": "1.4.22-1ubuntu4.14.04.3+esm2",
"evidence": "/affected/0/versions"
},
{
"value": "1.4.22-1ubuntu4.14.04.3+esm3",
"evidence": "/affected/0/versions"
},
{
"value": "1.4.22-1ubuntu4.14.04.3+esm4",
"evidence": "/affected/0/versions"
},
{
"value": "1.4.22-1ubuntu4.14.04.3+esm5",
"evidence": "/affected/0/versions"
}
]
},
{
"name": {
"value": "unbound",
"evidence": "/affected/1/package/name"
},
"prerequisites": [],
"fixed_versions": [],
"affected_versions": [
{
"value": "1.4.22-1ubuntu6",
"evidence": "/affected/1/versions"
},
{
"value": "1.5.7-1ubuntu1",
"evidence": "/affected/1/versions"
},
{
"value": "1.5.7-1ubuntu2",
"evidence": "/affected/1/versions"
},
{
"value": "1.5.8-1ubuntu1",
"evidence": "/affected/1/versions"
},
{
"value": "1.5.8-1ubuntu1.1",
"evidence": "/affected/1/versions"
},
{
"value": "1.5.8-1ubuntu1.1+esm1",
"evidence": "/affected/1/versions"
},
{
"value": "1.5.8-1ubuntu1.1+esm2",
"evidence": "/affected/1/versions"
},
{
"value": "1.5.8-1ubuntu1.1+esm3",
"evidence": "/affected/1/versions"
},
{
"value": "1.5.8-1ubuntu1.1+esm4",
"evidence": "/affected/1/versions"
},
{
"value": "1.5.8-1ubuntu1.1+esm5",
"evidence": "/affected/1/versions"
}
]
},
{
"name": {
"value": "unbound",
"evidence": "/affected/2/package/name"
},
"prerequisites": [],
"fixed_versions": [],
"affected_versions": [
{
"value": "1.6.5-1",
"evidence": "/affected/2/versions"
},
{
"value": "1.6.7-1",
"evidence": "/affected/2/versions"
},
{
"value": "1.6.7-1build1",
"evidence": "/affected/2/versions"
},
{
"value": "1.6.7-1ubuntu1",
"evidence": "/affected/2/versions"
},
{
"value": "1.6.7-1ubuntu2",
"evidence": "/affected/2/versions"
},
{
"value": "1.6.7-1ubuntu2.1",
"evidence": "/affected/2/versions"
},
{
"value": "1.6.7-1ubuntu2.2",
"evidence": "/affected/2/versions"
},
{
"value": "1.6.7-1ubuntu2.3",
"evidence": "/affected/2/versions"
},
{
"value": "1.6.7-1ubuntu2.4",
"evidence": "/affected/2/versions"
},
{
"value": "1.6.7-1ubuntu2.5",
"evidence": "/affected/2/versions"
},
{
"value": "1.6.7-1ubuntu2.6",
"evidence": "/affected/2/versions"
},
{
"value": "1.6.7-1ubuntu2.6+esm2",
"evidence": "/affected/2/versions"
},
{
"value": "1.6.7-1ubuntu2.6+esm3",
"evidence": "/affected/2/versions"
},
{
"value": "1.6.7-1ubuntu2.6+esm4",
"evidence": "/affected/2/versions"
},
{
"value": "1.6.7-1ubuntu2.6+esm5",
"evidence": "/affected/2/versions"
},
{
"value": "1.6.7-1ubuntu2.6+esm6",
"evidence": "/affected/2/versions"
}
]
},
{
"name": {
"value": "unbound",
"evidence": "/affected/3/package/name"
},
"prerequisites": [],
"fixed_versions": [],
"affected_versions": [
{
"value": "1.9.0-2ubuntu1",
"evidence": "/affected/3/versions"
},
{
"value": "1.9.0-2ubuntu2",
"evidence": "/affected/3/versions"
},
{
"value": "1.9.4-2",
"evidence": "/affected/3/versions"
},
{
"value": "1.9.4-2ubuntu1",
"evidence": "/affected/3/versions"
},
{
"value": "1.9.4-2ubuntu1.1",
"evidence": "/affected/3/versions"
},
{
"value": "1.9.4-2ubuntu1.2",
"evidence": "/affected/3/versions"
},
{
"value": "1.9.4-2ubuntu1.3",
"evidence": "/affected/3/versions"
},
{
"value": "1.9.4-2ubuntu1.4",
"evidence": "/affected/3/versions"
},
{
"value": "1.9.4-2ubuntu1.5",
"evidence": "/affected/3/versions"
},
{
"value": "1.9.4-2ubuntu1.6",
"evidence": "/affected/3/versions"
},
{
"value": "1.9.4-2ubuntu1.8",
"evidence": "/affected/3/versions"
},
{
"value": "1.9.4-2ubuntu1.9",
"evidence": "/affected/3/versions"
},
{
"value": "1.9.4-2ubuntu1.11",
"evidence": "/affected/3/versions"
},
{
"value": "1.9.4-2ubuntu1.11+esm1",
"evidence": "/affected/3/versions"
},
{
"value": "1.9.4-2ubuntu1.11+esm2",
"evidence": "/affected/3/versions"
},
{
"value": "1.9.4-2ubuntu1.11+esm3",
"evidence": "/affected/3/versions"
}
]
},
{
"name": {
"value": "unbound",
"evidence": "/affected/4/package/name"
},
"prerequisites": [],
"fixed_versions": [],
"affected_versions": [
{
"value": "1.13.1-1ubuntu1",
"evidence": "/affected/4/versions"
},
{
"value": "1.13.1-1ubuntu3",
"evidence": "/affected/4/versions"
},
{
"value": "1.13.1-1ubuntu5",
"evidence": "/affected/4/versions"
},
{
"value": "1.13.1-1ubuntu5.1",
"evidence": "/affected/4/versions"
},
{
"value": "1.13.1-1ubuntu5.2",
"evidence": "/affected/4/versions"
},
{
"value": "1.13.1-1ubuntu5.3",
"evidence": "/affected/4/versions"
},
{
"value": "1.13.1-1ubuntu5.4",
"evidence": "/affected/4/versions"
},
{
"value": "1.13.1-1ubuntu5.5",
"evidence": "/affected/4/versions"
},
{
"value": "1.13.1-1ubuntu5.7",
"evidence": "/affected/4/versions"
},
{
"value": "1.13.1-1ubuntu5.8",
"evidence": "/affected/4/versions"
},
{
"value": "1.13.1-1ubuntu5.10",
"evidence": "/affected/4/versions"
},
{
"value": "1.13.1-1ubuntu5.11",
"evidence": "/affected/4/versions"
},
{
"value": "1.13.1-1ubuntu5.12",
"evidence": "/affected/4/versions"
},
{
"value": "1.13.1-1ubuntu5.13",
"evidence": "/affected/4/versions"
},
{
"value": "1.13.1-1ubuntu5.14",
"evidence": "/affected/4/versions"
},
{
"value": "1.13.1-1ubuntu5.15",
"evidence": "/affected/4/versions"
},
{
"value": "1.13.1-1ubuntu5.16",
"evidence": "/affected/4/versions"
},
{
"value": "1.13.1-1ubuntu5.17",
"evidence": "/affected/4/versions"
}
]
},
{
"name": {
"value": "unbound",
"evidence": "/affected/5/package/name"
},
"prerequisites": [],
"fixed_versions": [],
"affected_versions": [
{
"value": "1.17.1-2",
"evidence": "/affected/5/versions"
},
{
"value": "1.18.0-2ubuntu1",
"evidence": "/affected/5/versions"
},
{
"value": "1.18.0-2ubuntu2",
"evidence": "/affected/5/versions"
},
{
"value": "1.19.1-1ubuntu1",
"evidence": "/affected/5/versions"
},
{
"value": "1.19.2-1ubuntu1",
"evidence": "/affected/5/versions"
},
{
"value": "1.19.2-1ubuntu3",
"evidence": "/affected/5/versions"
},
{
"value": "1.19.2-1ubuntu3.1",
"evidence": "/affected/5/versions"
},
{
"value": "1.19.2-1ubuntu3.2",
"evidence": "/affected/5/versions"
},
{
"value": "1.19.2-1ubuntu3.3",
"evidence": "/affected/5/versions"
},
{
"value": "1.19.2-1ubuntu3.4",
"evidence": "/affected/5/versions"
},
{
"value": "1.19.2-1ubuntu3.5",
"evidence": "/affected/5/versions"
},
{
"value": "1.19.2-1ubuntu3.6",
"evidence": "/affected/5/versions"
},
{
"value": "1.19.2-1ubuntu3.7",
"evidence": "/affected/5/versions"
},
{
"value": "1.19.2-1ubuntu3.8",
"evidence": "/affected/5/versions"
},
{
"value": "1.19.2-1ubuntu3.9",
"evidence": "/affected/5/versions"
},
{
"value": "1.19.2-1ubuntu3.10",
"evidence": "/affected/5/versions"
},
{
"value": "1.19.2-1ubuntu3.12",
"evidence": "/affected/5/versions"
}
]
},
{
"name": {
"value": "unbound",
"evidence": "/affected/6/package/name"
},
"prerequisites": [],
"fixed_versions": [],
"affected_versions": [
{
"value": "1.24.2-1ubuntu2",
"evidence": "/affected/6/versions"
},
{
"value": "1.25.1-1ubuntu1",
"evidence": "/affected/6/versions"
},
{
"value": "1.25.1-1ubuntu3",
"evidence": "/affected/6/versions"
}
]
},
{
"name": {
"value": "unbound",
"evidence": "/affected/7/package/name"
},
"prerequisites": [],
"fixed_versions": [],
"affected_versions": [
{
"value": "1.22.0-2ubuntu2",
"evidence": "/affected/7/versions"
},
{
"value": "1.22.0-2ubuntu4",
"evidence": "/affected/7/versions"
},
{
"value": "1.24.2-1ubuntu1",
"evidence": "/affected/7/versions"
},
{
"value": "1.24.2-1ubuntu2",
"evidence": "/affected/7/versions"
},
{
"value": "1.24.2-1ubuntu2.1",
"evidence": "/affected/7/versions"
},
{
"value": "1.24.2-1ubuntu2.2",
"evidence": "/affected/7/versions"
},
{
"value": "1.24.2-1ubuntu2.3",
"evidence": "/affected/7/versions"
},
{
"value": "1.24.2-1ubuntu2.4",
"evidence": "/affected/7/versions"
}
]
}
],
"severity": "medium",
"structured": {
"id": "UBUNTU-CVE-2026-50251",
"aliases": [
"UBUNTU-CVE-2026-50251"
],
"metrics": [
{
"type": "CVSS_V3",
"score": "CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L"
},
{
"type": "Ubuntu",
"score": "medium"
}
],
"affected": [
{
"ranges": [
{
"type": "ECOSYSTEM",
"events": [
{
"introduced": "0"
}
]
}
],
"package": {
"name": "unbound",
"purl": "pkg:deb/ubuntu/unbound?arch=source&distro=esm-infra-legacy%2Ftrusty",
"ecosystem": "Ubuntu:Pro:14.04:LTS"
},
"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",
"1.4.22-1ubuntu4.14.04.3+esm4",
"1.4.22-1ubuntu4.14.04.3+esm5"
],
"database_specific": {
"source": "https://github.com/canonical/ubuntu-security-notices/blob/main/osv/cve/2026/UBUNTU-CVE-2026-50251.json"
},
"ecosystem_specific": {
"binaries": [
{
"binary_name": "libunbound2",
"binary_version": "1.4.22-1ubuntu4.14.04.3+esm5"
},
{
"binary_name": "python-unbound",
"binary_version": "1.4.22-1ubuntu4.14.04.3+esm5"
},
{
"binary_name": "unbound",
"binary_version": "1.4.22-1ubuntu4.14.04.3+esm5"
},
{
"binary_name": "unbound-anchor",
"binary_version": "1.4.22-1ubuntu4.14.04.3+esm5"
},
{
"binary_name": "unbound-host",
"binary_version": "1.4.22-1ubuntu4.14.04.3+esm5"
}
]
}
},
{
"ranges": [
{
"type": "ECOSYSTEM",
"events": [
{
"introduced": "0"
}
]
}
],
"package": {
"name": "unbound",
"purl": "pkg:deb/ubuntu/unbound?arch=source&distro=esm-infra-legacy%2Fxenial",
"ecosystem": "Ubuntu:Pro:16.04:LTS"
},
"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",
"1.5.8-1ubuntu1.1+esm4",
"1.5.8-1ubuntu1.1+esm5"
],
"database_specific": {
"source": "https://github.com/canonical/ubuntu-security-notices/blob/main/osv/cve/2026/UBUNTU-CVE-2026-50251.json"
},
"ecosystem_specific": {
"binaries": [
{
"binary_name": "libunbound2",
"binary_version": "1.5.8-1ubuntu1.1+esm5"
},
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"modified": "2026-10-21T16:30:58.907446416Z",
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"https://community.nlnetlabs.nl/t/unbound-1-25-2-released/3430"
]
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"published_at": "2026-07-22T14:17:00+00:00"
}