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VulnerabilityPublished

CVE-2026-3591: ISC BIND 9 · Return of Stack Variable Address

Affects ISC BIND 9Source CVE Database V5

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Not known to be exploited · CVSS 5.4 Medium : upgrade to 9.21.20 or 9.20.21 when convenient.

Data as of 30 Oct 2026, 11:52 UTC

CVSS 3.1 base score
5.4Medium
How severe the flaw is on the 0–10 CVSS scale.
EPSS
0.3% chance of exploitation in 30 days – higher than 30% of CVEsExploit Prediction Scoring System: the likelihood attackers will use this flaw soon.
Patch
Available – fixed in 9.21.20, 9.20.21
CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:L/I:L/A:N

Exploitation evidence

  1. Disclosed: evidence found.

    CVE published 25 Mar 2026

  2. Detectable: no evidence on record.

    No public nuclei template indexed

  3. Weaponized: no evidence on record.

    No public exploit code indexed in Metasploit or Exploit-DB

  4. Exploited in the wild: no evidence on record.

    Not in CISA KEV and no source reports exploitation

  5. Observed by OffSeq Mirage: no evidence on record.

    No attack traffic recorded by OffSeq Mirage sensors

A missing step is not evidence of safety. Sources: cve.org (opens in a new tab), the Metasploit, Exploit-DB and nuclei-templates index as of 30 Oct 2026.

What to do

Upgrade isc/bind9 to 9.21.20 or 9.20.21.

Affected software

ProductAffectedFixed in
ISCBIND 9Affected
  • >=9.20.0 <=9.20.20
  • >=9.21.0 <=9.21.19
  • >=9.20.9-S1 <=9.20.20-S1
Fixed in–
GitHubisc/bind9pkg:github/isc/bind9Other threatsAffected
  • <9.20.21
  • <9.21.20
Fixed in
  • 9.20.21
  • 9.21.20

Check your own hosts for these packages with threat-finder (opens in a new tab), our free open-source scanner.

AI-suggested

Organizations should upgrade affected BIND 9 versions to patched releases once available from ISC. In the interim, administrators should audit and modify ACL configurations to use default-deny policies rather than default-allow, minimizing the risk of unauthorized access due to ACL mis-matches. Additionally, disabling SIG(0) signing on DNS queries if not required can reduce exposure. Network segmentation and firewall rules should restrict access to DNS servers to trusted IP ranges only. Monitoring DNS server logs for unusual ACL matches or unexpected query patterns may help detect exploitation attempts. Applying secure coding practices and memory safety checks in future BIND releases is recommended by ISC. Finally, organizations should maintain up-to-date inventories of DNS server versions and configurations to quickly identify and remediate vulnerable instances.

Description

A use-after-return vulnerability exists in the `named` server when handling DNS queries signed with SIG(0). Using a specially-crafted DNS request, an attacker may be able to cause an ACL to improperly (mis)match an IP address. In a default-allow ACL (denying only specific IP addresses), this may lead to unauthorized access. Default-deny ACLs should fail-secure. This issue affects BIND 9 versions 9.20.0 through 9.20.20, 9.21.0 through 9.21.19, and 9.20.9-S1 through 9.20.20-S1. BIND 9 versions 9.18.0 through 9.18.46 and 9.18.11-S1 through 9.18.46-S1 are NOT affected.

References

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