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CVE-2026-3591 is a stack use-after-return vulnerability in the BIND 9 named server’s handling of DNS queries signed with SIG(0). A specially crafted DNS request can trigger the flaw and cause an access control list (ACL) check to improperly match an IP address. As a result, ACL enforcement may fail in some configurations. ISC states that in default-allow ACL configurations, where only specific IP addresses are denied, the flaw may permit unauthorized access by causing a denied source to be treated as allowed. The 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.
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What an attacker gets, and what they’ve been doing with it.
named deployments. The practical impact is unauthorized access to functionality or query paths that administrators intended to restrict by source IP address. According to the advisory, the risk is most relevant for default-allow ACLs that deny only specific IPs; in that model, an improper ACL match can convert an intended deny into an allow. Default-deny ACLs are expected to fail secure, reducing or eliminating the bypass impact in those configurations.If you can’t patch tonight, do this now.
named service using upstream packet filters, host firewalls, or segmentation so untrusted sources cannot reach ACL-protected interfaces or functions. These are compensating controls only; patching is the primary mitigation.Patch, then assume compromise.
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No public exploit code observed for this vulnerability.
Products and vendors Mallory has correlated with this vulnerability. Open in Mallory to drill down to specific CPE configurations and version ranges.
Vendor-confirmed product mapping. Mallory continuously reconciles this list against your asset inventory.
12 sources tracked across advisories, community write-ups, and news. New activity surfaces here as Mallory finds it.
Query your assets running an affected version, and investigate the blast radius.
Every observed campaign linking this CVE to a named adversary.
Malware families riding this exploit, with evidence and IOCs.
YARA, Sigma, Snort, and vendor rules, auto-deployed to your SIEM.
Cross-references every affected SKU, including bundled OEM variants.
Community discussion across Reddit, Mastodon, and other social sources.