Medium
unbound — security advisory
In NLnet Labs Unbound 1.20.0 up to and including 1.25.1, when Unbound is configured with 'serve-expired: yes' and 'serve-expired-client-timeout > discard-timeout > 0' (contrary to the suggested values), the discard-timeout branch during the serve expired logic drops an aged client reply without performing the correct accounting for the number of reply addresses for the query. Other identical branches outside of serve expired perform the correct decrement. Since the counter is never decremented in such scenario, it can reach the maximum limit and new clients for duplicate in-flight queries are silently dropped resulting in degradation of resolution service. A malicious actor can exploit the vulnerability by querying the resolver for a client-controlled slow-on-demand authoritative zone that can drive the counter past the threshold. Shipped defaults for 'serve-expired-client-timeout: 1800' and 'discard-timeout: 1900' make the branch unreachable.
- 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-2026-56444
- Confirmed fixed CVEs
- Not specified
- Updated in this feed
- 31 Oct 2026, 14:34 UTC · Revision 1
Source evidence
In NLnet Labs Unbound 1.20.0 up to and including 1.25.1, when Unbound is configured with 'serve-expired: yes' and 'serve-expired-client-timeout > discard-timeout > 0' (contrary to the suggested values), the discard-timeout branch during the serve expired logic drops an aged client reply without performing the correct accounting for the number of reply addresses for the query. Other identical branches outside of serve expired perform the correct decrement. Since the counter is never decremented in such scenario, it can reach the maximum limit and new clients for duplicate in-flight queries are silently dropped resulting in degradation of resolution service. A malicious actor can exploit the vulnerability by querying the resolver for a client-controlled slow-on-demand authoritative zone that can drive the counter past the threshold. Shipped defaults for 'serve-expired-client-timeout: 1800' and 'discard-timeout: 1900' make the branch unreachable.
osv:UBUNTU-CVE-2026-56444
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": "7cc2d6a0d8a45f4049eafd2d75940c2d71a9332fa7c0c22cf2c446aafcd55fec",
"url": "https://osv.dev/vulnerability/UBUNTU-CVE-2026-56444",
"method": "structured",
"payload": {
"kind": "advisory",
"title": "UBUNTU-CVE-2026-56444",
"summary": "In NLnet Labs Unbound 1.20.0 up to and including 1.25.1, when Unbound is configured with 'serve-expired: yes' and 'serve-expired-client-timeout > discard-timeout > 0' (contrary to the suggested values), the discard-timeout branch during the serve expired logic drops an aged client reply without performing the correct accounting for the number of reply addresses for the query. Other identical branches outside of serve expired perform the correct decrement. Since the counter is never decremented in such scenario, it can reach the maximum limit and new clients for duplicate in-flight queries are silently dropped resulting in degradation of resolution service. A malicious actor can exploit the vulnerability by querying the resolver for a client-controlled slow-on-demand authoritative zone that can drive the counter past the threshold. Shipped defaults for 'serve-expired-client-timeout: 1800' and 'discard-timeout: 1900' make the branch unreachable.",
"evidence": "osv:UBUNTU-CVE-2026-56444",
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{
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{
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{
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{
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{
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{
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{
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},
{
"name": {
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{
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{
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{
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{
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{
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{
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{
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{
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{
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{
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{
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{
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}
]
},
{
"name": {
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"evidence": "/affected/3/package/name"
},
"prerequisites": [],
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"affected_versions": [
{
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"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"
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{
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},
{
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{
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{
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{
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{
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{
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{
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{
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"evidence": "/affected/3/versions"
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{
"value": "1.9.4-2ubuntu1.11+esm1",
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{
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"evidence": "/affected/3/versions"
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{
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{
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{
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"evidence": "/affected/4/versions"
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{
"value": "1.13.1-1ubuntu5.4",
"evidence": "/affected/4/versions"
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{
"value": "1.13.1-1ubuntu5.5",
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{
"value": "1.13.1-1ubuntu5.7",
"evidence": "/affected/4/versions"
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{
"value": "1.13.1-1ubuntu5.8",
"evidence": "/affected/4/versions"
},
{
"value": "1.13.1-1ubuntu5.10",
"evidence": "/affected/4/versions"
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{
"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",
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},
{
"value": "1.13.1-1ubuntu5.15",
"evidence": "/affected/4/versions"
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{
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]
},
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{
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{
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{
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{
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{
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{
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],
"severity": "medium",
"structured": {
"id": "UBUNTU-CVE-2026-56444",
"aliases": [
"UBUNTU-CVE-2026-56444"
],
"metrics": [
{
"type": "CVSS_V3",
"score": "CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:H"
},
{
"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"
},
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],
"database_specific": {
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},
"ecosystem_specific": {
"binaries": [
{
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},
{
"binary_name": "python-unbound",
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},
{
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},
{
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},
{
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}
]
}
},
{
"ranges": [
{
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{
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}
]
}
],
"package": {
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"ecosystem": "Ubuntu:Pro:16.04:LTS"
},
"versions": [
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"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-2026-56444.json"
},
"ecosystem_specific": {
"binaries": [
{
"binary_name": "libunbound2",
"binary_version": "1.5.8-1ubuntu1.1+esm5"
},
{
"binary_name": "python-unbound",
"binary_version": "1.5.8-1ubuntu1.1+esm5"
},
{
"binary_name": "unbound",
"binary_version": "1.5.8-1ubuntu1.1+esm5"
},
{
"binary_name": "unbound-anchor",
"binary_version": "1.5.8-1ubuntu1.1+esm5"
},
{
"binary_name": "unbound-host",
"binary_version": "1.5.8-1ubuntu1.1+esm5"
}
]
}
},
{
"ranges": [
{
"type": "ECOSYSTEM",
"events": [
{
"introduced": "0"
}
]
}
],
"package": {
"name": "unbound",
"purl": "pkg:deb/ubuntu/unbound?arch=source&distro=esm-infra%2Fbionic",
"ecosystem": "Ubuntu:Pro:18.04:LTS"
},
"versions": [
"1.6.5-1",
"1.6.7-1",
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"modified": "2026-10-21T16:31:03.970954145Z",
"provider": "osv",
"upstream": [
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],
"references": [
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"https://www.cve.org/CVERecord?id=CVE-2026-56444",
"https://community.nlnetlabs.nl/t/unbound-1-25-2-released/3430"
]
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"evidence": "/upstream"
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],
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"severity_evidence": "/severity/0/score"
},
"source_id": "core-osv",
"document_id": 394534,
"published_at": "2026-07-22T14:17:00+00:00"
}