Medium
unbound — security advisory
In NLnet Labs Unbound 1.10.0 up to and including 1.25.1, when 'serve-expired: yes' is set together with a 'response-ip: redirect' /'response-ip-data: CNAME ' rule (or the RPZ 'rpz-cname-override' equivalent), a remote client who controls any delegated domain can crash the daemon. The serve-expired-client-timeout callback runs a two-pass loop to chase the respip-generated CNAME alias; on the second pass it resets 'alias_rrset' but not 'partial_rep'. Later, this inconsistency leads to a NULL pointer dereference and an eventual crash. A malicious actor can exploit the vulnerability by controlling any zone that replies with an A/AAAA record that falls inside the configured response-ip/rpz subnet. By delaying the answer when the previous record has expired, the vulnerable path of 'serve-expired-client-timeout' is taken leading to denial of service via the server crash.
- 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-2026-55717
- Confirmed fixed CVEs
- Not specified
- Updated in this feed
- 31 Oct 2026, 14:34 UTC · Revision 1
Source evidence
In NLnet Labs Unbound 1.10.0 up to and including 1.25.1, when 'serve-expired: yes' is set together with a 'response-ip: redirect' /'response-ip-data: CNAME ' rule (or the RPZ 'rpz-cname-override' equivalent), a remote client who controls any delegated domain can crash the daemon. The serve-expired-client-timeout callback runs a two-pass loop to chase the respip-generated CNAME alias; on the second pass it resets 'alias_rrset' but not 'partial_rep'. Later, this inconsistency leads to a NULL pointer dereference and an eventual crash. A malicious actor can exploit the vulnerability by controlling any zone that replies with an A/AAAA record that falls inside the configured response-ip/rpz subnet. By delaying the answer when the previous record has expired, the vulnerable path of 'serve-expired-client-timeout' is taken leading to denial of service via the server crash.
osv:UBUNTU-CVE-2026-55717
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": "e3c21ba808929fdb15904c95c4f8ef93db462421fffa2916e91af64937788925",
"url": "https://osv.dev/vulnerability/UBUNTU-CVE-2026-55717",
"method": "structured",
"payload": {
"kind": "advisory",
"title": "UBUNTU-CVE-2026-55717",
"summary": "In NLnet Labs Unbound 1.10.0 up to and including 1.25.1, when 'serve-expired: yes' is set together with a 'response-ip: redirect' /'response-ip-data: CNAME ' rule (or the RPZ 'rpz-cname-override' equivalent), a remote client who controls any delegated domain can crash the daemon. The serve-expired-client-timeout callback runs a two-pass loop to chase the respip-generated CNAME alias; on the second pass it resets 'alias_rrset' but not 'partial_rep'. Later, this inconsistency leads to a NULL pointer dereference and an eventual crash. A malicious actor can exploit the vulnerability by controlling any zone that replies with an A/AAAA record that falls inside the configured response-ip/rpz subnet. By delaying the answer when the previous record has expired, the vulnerable path of 'serve-expired-client-timeout' is taken leading to denial of service via the server crash.",
"evidence": "osv:UBUNTU-CVE-2026-55717",
"products": [
{
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},
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"affected_versions": [
{
"value": "1.4.20-1",
"evidence": "/affected/0/versions"
},
{
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"evidence": "/affected/0/versions"
},
{
"value": "1.4.21-1ubuntu1",
"evidence": "/affected/0/versions"
},
{
"value": "1.4.21-1ubuntu2",
"evidence": "/affected/0/versions"
},
{
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{
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"evidence": "/affected/0/versions"
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{
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"evidence": "/affected/0/versions"
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{
"value": "1.4.22-1ubuntu4",
"evidence": "/affected/0/versions"
},
{
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"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"
},
{
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"evidence": "/affected/0/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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"evidence": "/affected/1/versions"
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{
"value": "1.5.7-1ubuntu2",
"evidence": "/affected/1/versions"
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{
"value": "1.5.8-1ubuntu1",
"evidence": "/affected/1/versions"
},
{
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"evidence": "/affected/1/versions"
},
{
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"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": [],
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"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"
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{
"value": "1.6.7-1ubuntu2.1",
"evidence": "/affected/2/versions"
},
{
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"evidence": "/affected/2/versions"
},
{
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"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"
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{
"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": [],
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"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"
},
{
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},
{
"value": "1.9.4-2ubuntu1.5",
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{
"value": "1.9.4-2ubuntu1.6",
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},
{
"value": "1.9.4-2ubuntu1.8",
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},
{
"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",
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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/4/versions"
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{
"value": "1.13.1-1ubuntu5.1",
"evidence": "/affected/4/versions"
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{
"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",
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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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},
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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-2026-55717",
"aliases": [
"UBUNTU-CVE-2026-55717"
],
"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"
},
"versions": [
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"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-2026-55717.json"
},
"ecosystem_specific": {
"binaries": [
{
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},
{
"binary_name": "python-unbound",
"binary_version": "1.4.22-1ubuntu4.14.04.3+esm5"
},
{
"binary_name": "unbound",
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},
{
"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": [
{
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}
]
}
],
"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-2026-55717.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"
},
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"modified": "2026-10-21T16:31:00.198752736Z",
"provider": "osv",
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"references": [
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"https://www.cve.org/CVERecord?id=CVE-2026-55717",
"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": 394533,
"published_at": "2026-07-22T14:17:00+00:00"
}