CVE-2026-5950 — Unbounded resend loop in BIND 9 resolver
An unbounded resend loop vulnerability exists in the BIND 9 resolver state machine during bad-server handling, enabling a remote unauthenticated attacker to cause severe resource exhaustion by sending queries that trigger specific retry conditions. This issue affects BIND 9 versions 9.18.36 through 9.18.48, 9.20.8 through 9.20.22, 9.21.7 through 9.21.21, 9.18.36-S1 through 9.18.48-S1, and 9.20.9-S1 through 9.20.22-S1.
Who exploits it and how urgent it is
A 0–100 score combining confirmed exploitation, ransomware and APT use, public exploits, EPSS and severity.
How the score was calculated
- EPSS 0.8% (30-day exploitation probability)+8
- Technical severity (medium)+3
- Automatable exploitation (SSVC)+3
The score orders the patch queue; it does not replace your asset context (internet exposure, criticality, compensating controls).
Triage
Is it exploited, how likely is exploitation, what does it touch, and how severe do the sources call it.
CISA SSVC assessment CISA
Three decision points CISA publishes for the CVEs it assesses (SSVC 2.0.3). A stakeholder decision, not a severity score.
Remediation
Fixes and mitigations documented by the sources — linked to the original advisory.
- Documented patch — downloads.isc.org (NIST National Vulnerability Database)
- Documented patch — downloads.isc.org (NIST National Vulnerability Database)
- Documented patch — downloads.isc.org (NIST National Vulnerability Database)
First 24 hours
Ordered from the record’s own fields — exposure first, because you cannot patch what you have not found. A rule-generated starting point.
- Identify exposed assets running the affected vendor/product/version combinations (Isc Bind, ISC BIND 9).
- The attack is network-based and needs no privileges: restrict exposure (ACLs, VPN, WAF) until patched.
- Apply the fixed version or the patch documented under Remediation.
- Search available logs for exploit probes, errors and authentication anomalies.
Affected scope
Vendor, product and versions as the sources word them.
| Vendor | Product | Versions | Status | Source |
|---|---|---|---|---|
| Isc | Bind | ≥ 9.18.36 and < 9.18.49≥ 9.20.8 and < 9.20.23≥ 9.21.7 and < 9.21.21 | Vulnerable | NIST National Vulnerability Database |
| ISC | BIND 9 | ≥ 9.18.36 and ≤ 9.18.48≥ 9.20.8 and ≤ 9.20.22≥ 9.21.7 and ≤ 9.21.21≥ 9.18.36-S1 and ≤ 9.18.48-S1≥ 9.20.9-S1 and ≤ 9.20.22-S1 | Vulnerable | CVE Program (cve.org) e CISA ADP |
Attack characteristics
The CVSS vector, decoded. It describes the attack, not your exposure to it.
Every base score collected
Sources score independently and may disagree; each row says who scored it and under which version.
| Score | Version | Severity | Expl. | Impact | Scored by |
|---|---|---|---|---|---|
| 5.3 | CVSS 3.1 | Medium | 3.9 | 1.4 | security-officer@isc.org via NIST National Vulnerability Database |
| 5.3 | CVSS 3.1 | Medium | — | — | isc via CVE Program (cve.org) e CISA ADP |
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L (score shown at the top: security-officer@isc.org) Weakness MITRE
Weakness class according to MITRE’s CWE catalog.
EPSS over time FIRST
Oct 13, 2026 → Oct 14, 2026 · 2 reading(s) · max 0.80%
Detection
Read off the CVSS vector and the weakness class. Starting points, not tested rules.
- Prioritize edge telemetry for the network-reachable component (proxy, WAF, IDS/IPS).
Timeline
What happened to this CVE, newest first.
- 2026
- Affected products added · isc/bind 9
- SSVC assessment updated
- Record published
- Fix documented
- CVSS changed · 5.3
- Source status changed · → Analyzed
- Record created in this database
- CVE published
Source activity
Repeated readings are counted rather than listed.
- 1× Official description updated · Oct 13, 2026 – Oct 13, 2026
References
4 on the record
Elsewhere on this site
Related CVEs (same products)
- High CVE-2026-13321 DNSSEC Validation Bypass via Out-of-Zone NSEC Next Field Jul 22, 2026
- High CVE-2026-13204 Unexpected exit in certain situations with NSEC and NSEC3 both present Jul 22, 2026
- High CVE-2026-11622 Potential memory usage beyond configured limits Jul 22, 2026
- High CVE-2026-12617 Record ordering based unexpected exit with CNAME or DNAME Jul 22, 2026
- High CVE-2026-11721 Cache poisoning possible with label count discrepancy, RRSIG, and wildcards Jul 22, 2026
- High CVE-2026-11331 Potential wildcard CNAME RPZ policy bypass Jul 22, 2026
Not in any source we poll
Listed rather than left blank: an empty field and an unmeasured one look identical on screen, and only one is a reason to look elsewhere.
- No confirmed IOCs, IP addresses, domains, file hashes, or malware artifacts supplied.
- No organization-specific asset inventory, compensating-control status, or patch deployment evidence supplied.
- No exploit packet captures, log samples, or incident case IDs supplied.
Answered from this record
Text assembled only from the fields above — no additional claims.
What should defenders know first?
CVE-2026-5950 is "Unbounded resend loop in BIND 9 resolver", rated Medium. It affects Bind (Isc); versions are in the scope table. It is not in CISA KEV, which does not rule out exploitation. CVSS 5.3 describes technical severity. EPSS 0.80% estimates the probability of exploitation in 30 days; neither replaces your asset context. First verify whether exposed or business-critical assets run the affected versions, apply vendor fixes or mitigations, and preserve logs for review.
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