CVE-2026-77119 in BIND
Summary
by MITRE • 09/17/2026
A validly signed NSEC3 from an unrelated sibling zone may be accepted as an insecurity proof, downgrading a secure delegation and letting a forged unsigned answer through. This issue affects BIND 9 versions 9.11.0 through 9.18.50, 9.20.0 through 9.20.27, 9.21.0 through 9.21.25, 9.11.3-S1 through 9.18.50-S1, and 9.20.9-S1 through 9.20.27-S1.
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Analysis
by VulDB Data Team • 09/18/2026
The vulnerability described represents a critical flaw in the DNSSEC validation logic within BIND 9 implementations, specifically affecting versions ranging from 9.11.0 to 9.20.27 and their respective Service Pack variants. This issue stems from an incorrect acceptance of NSEC3 records that are cryptographically valid but originate from an unrelated sibling zone rather than the authoritative source for the queried domain. In a properly functioning DNSSEC environment, validation relies on cryptographic proofs provided by the specific zones involved in the resolution chain to establish authenticity and integrity. However, this flaw allows an attacker or misconfigured resolver to exploit the loose verification logic by presenting a signed NSEC3 record from a sibling zone as proof of non-existence for a target domain. This manipulation effectively downgrades what should be a secure delegation into an insecure one, thereby bypassing the intended security guarantees of DNSSEC.
From a technical perspective, the core issue lies in how BIND 9 processes insecurity proofs during recursive resolution and validation. When a resolver encounters a query that does not match any existing record, it expects to receive NSEC or NSEC3 records from the closest parent zone to prove that the queried name indeed does not exist within its authoritative scope. The vulnerability allows an attacker to supply a validly signed NSEC3 response generated by a different, unrelated sibling zone. Because the signature itself is cryptographically correct for that sibling zone, BIND 9 incorrectly accepts it as a legitimate proof of non-existence for the target domain. This logical error undermines the chain of trust, allowing forged unsigned answers to be returned to the client as if they were authentic and verified by DNSSEC. The impact is severe because it enables cache poisoning attacks where an attacker can inject false DNS records into the resolver's cache, potentially redirecting users to malicious sites or disrupting critical services that rely on accurate domain name resolution.
The operational impact of this vulnerability extends beyond simple data forgery; it compromises the fundamental integrity and availability guarantees provided by DNSSEC. By allowing forged unsigned answers to pass validation checks, attackers can perform sophisticated phishing campaigns, intercept sensitive communications, or disrupt network infrastructure depending on which domains are targeted. The ability to downgrade secure delegations means that even organizations with robust DNSSEC deployments remain vulnerable if they run affected versions of BIND 9. This is particularly dangerous in environments where automated systems rely on DNS responses for service discovery and configuration updates. The flaw affects a wide range of production releases, indicating a systemic issue in the validation engine rather than an isolated edge case.
Mitigation strategies primarily involve updating to patched versions of BIND 9 that correct the logic governing NSEC3 acceptance criteria. Administrators should ensure their systems are running version 9.18.51 or later for the 9.18 branch, version 9.20.28 or later for the 9.20 branch, and corresponding updated Service Pack releases where applicable. Until patches can be applied, operators may consider implementing additional monitoring to detect anomalous DNSSEC validation failures or unexpected NSEC3 responses from non-authoritative sources. Furthermore, deploying response policy zones (RPZ) can help filter out potentially malicious queries before they trigger the vulnerable code path. It is also advisable to review network segmentation and access controls to limit exposure to external resolvers that might be exploited in this manner.
This vulnerability aligns with CWE-20 Improper Input Validation, as the resolver fails to adequately validate the origin of the NSEC3 record relative to the queried domain's zone cut. Additionally, it relates to ATT&CK technique T1498 Network Denial of Service and potentially T1567 Data Exfiltration over Web Protocol if used in conjunction with other attacks for redirecting traffic. The flaw highlights the importance of strict adherence to DNSSEC standards during validation processes, ensuring that proofs are not only cryptographically sound but also contextually appropriate for the specific zone being queried. Organizations must prioritize patch management and continuous monitoring to mitigate risks associated with this critical logic error in one of the most widely used DNS server software packages globally.