CVE-2021-40083 in Knot Resolver
Summary
by MITRE • 08/25/2021
Knot Resolver before 5.3.2 is prone to an assertion failure, triggerable by a remote attacker in an edge case (NSEC3 with too many iterations used for a positive wildcard proof).
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Analysis
by VulDB Data Team • 08/27/2021
The vulnerability identified as CVE-2021-40083 affects Knot Resolver versions prior to 5.3.2 and represents a critical assertion failure that can be remotely exploited under specific edge case conditions. This flaw manifests when the DNS resolver encounters NSEC3 records with an excessive number of iterations during the processing of positive wildcard proofs, creating a scenario where the assertion mechanism within the resolver's codebase fails to properly validate the cryptographic parameters. The issue stems from inadequate input validation and boundary checking within the DNSSEC validation logic, particularly when handling the iterative hashing process that occurs during NSEC3 proof generation and verification.
The technical exploitation of this vulnerability occurs through carefully crafted DNS responses that contain NSEC3 records with malformed or excessively high iteration counts. When Knot Resolver processes these records during normal DNS resolution operations, the assertion failure causes the resolver to terminate abruptly or enter an unstable state, potentially leading to denial of service conditions for legitimate users. This assertion failure specifically impacts the resolver's ability to properly validate DNSSEC signatures when wildcard records are involved, as the system cannot correctly handle the computational overhead required for excessive iteration counts in the NSEC3 hashing algorithm. The flaw operates at the protocol validation layer of the resolver, affecting the core DNSSEC implementation and potentially allowing attackers to disrupt normal DNS resolution services.
From an operational impact perspective, this vulnerability creates significant risks for organizations relying on Knot Resolver for DNS infrastructure services, particularly those operating in environments where DNSSEC validation is enabled. The remote exploitability means that attackers can potentially disrupt DNS resolution services without requiring local access or authentication, making this a particularly dangerous vulnerability for public-facing DNS servers. The assertion failure can result in complete service outages or inconsistent resolution behavior, affecting both internal network services and external DNS resolution capabilities. Organizations using older versions of Knot Resolver are vulnerable to attacks that could cause widespread disruption of DNS services, particularly in scenarios where wildcard records are frequently used or when DNSSEC validation is actively enforced.
Mitigation strategies for CVE-2021-40083 primarily involve upgrading to Knot Resolver version 5.3.2 or later, which includes patches addressing the assertion failure in NSEC3 processing. System administrators should also implement monitoring for unusual DNS resolution patterns or service disruptions that could indicate exploitation attempts. Network segmentation and access controls should be reviewed to limit exposure of vulnerable DNS resolver instances, while implementing proper DNSSEC validation policies that can help detect and prevent malformed responses from reaching the resolver. Organizations should also consider implementing DNS firewalling rules and rate limiting to reduce the impact of potential exploitation attempts, though the most effective solution remains the application of the official security patch. This vulnerability aligns with CWE-611 (Improper Restriction of XML External Entity Reference) and follows ATT&CK techniques related to service disruption and denial of service operations, highlighting the importance of proper input validation and assertion handling in security-critical systems.