Medium severity · published Mar 25, 2026
CVE-2026-3591
A stack use-after-return flaw in SIG(0) handling code may enable ACL bypass
5.4
CVSS
0.4%
EPSS
No
On KEV
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.
CVSS vector
CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:L/I:L/A:N
Attack vector
Network
Complexity
Low
Privileges
Low
User interaction
None
Confidentiality
Low
Integrity
Low
Availability
None
Scope
Unchanged
Exploitation
Is it being used?
Exploit prediction (EPSS)0.4% · higher than 30% of CVEs
Exploited in the wildNo evidence yet
CISA KEVNot listed
Ransomware campaign useNot known
Affected products
Which versions
B9
BIND 9 · ISC
9.20.0 through 9.20.20; 9.21.0 through 9.21.19; 9.20.9-S1 through 9.20.20-S1 · source: vendor
BI
Bind · Isc
9.20.0 before 9.20.21; 9.21.0 before 9.21.20 · source: NVD
References
NVD
https://nvd.nist.gov/vuln/detail/CVE-2026-3591
CVE.org
https://www.cve.org/CVERecord?id=CVE-2026-3591
Patch
https://downloads.isc.org/isc/bind9/9.20.21
Patch
https://downloads.isc.org/isc/bind9/9.21.20
Vendor Advisory
https://kb.isc.org/docs/cve-2026-3591
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