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ISC BIND 9 CVE-2026-12617

| EUVDEUVD-2026-47711 HIGH
Reachable Assertion (CWE-617)
2026-07-22 isc GHSA-cc7q-gqfw-hrm4
7.5
CVSS 3.1 · Vendor: isc
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Severity by source

Vendor (isc) PRIMARY
7.5 HIGH
AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
vuln.today AI
7.5 HIGH

Unauthenticated network-reachable crash of a recursive resolver with availability-only impact; attacker controlling the authoritative server can reliably drive the response ordering, supporting AC:L.

3.1 AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
4.0 AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N
SUSE
HIGH
qualitative
Red Hat
7.5 HIGH
qualitative

Primary rating from Vendor (isc).

CVSS VectorVendor: isc

Attack Vector
Network
Attack Complexity
Low
Privileges Required
None
User Interaction
None
Scope
Unchanged
Confidentiality
None
Integrity
None
Availability
High

Lifecycle Timeline

2
Analysis Generated
Jul 22, 2026 - 15:18 vuln.today
CVE Published
Jul 22, 2026 - 14:14 cve.org
HIGH 7.5

DescriptionCVE.org

The issue is unexpected program termination based on ordering and/or specific content in responses to queries for CNAME or DNAME, and A records. Specifically, if a client queries for a DNAME and A record below the DNAME to the resolver, and the authoritative server responds positively to the A query but delays the DNAME response and later responds negatively, named may quit unexpectedly. Or, if a client queries for a CNAME and A record for the same name to the resolver, and the authoritative server responds positively to the A query but delays the CNAME response and later responds with a self-referential CNAME, the same failure may occur. This issue affects BIND 9 versions 9.18.0 through 9.18.50, 9.20.0 through 9.20.24, 9.18.11-S1 through 9.18.50-S1, and 9.20.9-S1 through 9.20.24-S1.

AnalysisAI

Remote denial of service in ISC BIND 9 lets an off-path attacker crash a recursive resolver by controlling the timing and content of authoritative responses to paired queries. When a client asks named to resolve a DNAME plus an A record beneath it (or a CNAME plus an A record for the same name) and the attacker-controlled authoritative server answers the A query positively but delays the DNAME/CNAME response before returning a negative or self-referential answer, named hits a fatal assertion and terminates. The flaw is unauthenticated and network-reachable (CVSS 7.5, availability-only); there is no public exploit identified at time of analysis and it is not on CISA KEV.

Technical ContextAI

BIND 9's named is the reference DNS server implementation, here operating in its recursive resolver role. The bug is CWE-617 (Reachable Assertion): a REQUIRE/INSIST-style consistency check inside the resolver's answer-processing logic can be violated by a legal-but-unusual sequence of authoritative answers. DNAME records redirect an entire subtree of the namespace and are normally synthesized into CNAMEs, while CNAME records alias a single name; both interact with the caching and chain-following code that stitches together A-record answers. By ordering the positive A answer ahead of a delayed, then negative or self-referential (CNAME pointing to itself) redirect answer, the attacker drives the resolver into a state its internal invariants did not anticipate, tripping the assertion. The CPE cpe:2.3:a:isc:bind_9 confirms the affected component is the BIND 9 package rather than any downstream distribution wrapper.

RemediationAI

Vendor-released patch: upgrade the 9.20 branch to 9.20.26 (https://downloads.isc.org/isc/bind9/9.20.26); operators on the 9.18 branch or on the -S1 Supported editions should consult the ISC advisory at https://kb.isc.org/docs/cve-2026-12617 for the corresponding fixed release, as the exact 9.18/-S1 fix version is not stated in the available data and should be confirmed with ISC before deployment. If patching cannot be immediate, reduce exposure by restricting recursion to trusted client networks (allow-recursion/allow-query-cache ACLs) so untrusted clients cannot steer the resolver toward attacker-controlled zones - the trade-off is that open or broadly-serving resolvers lose availability to legitimate external clients. Because triggering depends on the resolver querying a malicious authoritative server, response-policy or DNS firewall controls and monitoring named for unexpected assertion-failure exits with automatic restart (process supervision) further limit downtime while you schedule the upgrade.

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Vendor StatusVendor

SUSE

Severity: Important
Product Status
SUSE Linux Enterprise Desktop 15 SP7 SUSE Linux Enterprise Module for Basesystem 15 SP7 Fixed
SUSE Linux Enterprise High Performance Computing 15 SP7 SUSE Linux Enterprise Server 15 SP7 SUSE Linux Enterprise Server for SAP Applications 15 SP7 Fixed
SUSE Linux Enterprise Module for Server Applications 15 SP7 Fixed
openSUSE Leap 16.0 Fixed
openSUSE Tumbleweed Fixed

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CVE-2026-12617 vulnerability details – vuln.today

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