CVE-2011-4869 in Unbound
Summary
by MITRE
validator/val_nsec3.c in Unbound before 1.4.13p2 does not properly perform proof processing for NSEC3-signed zones, which allows remote DNS servers to cause a denial of service (daemon crash) via a malformed response that lacks expected NSEC3 records, a different vulnerability than CVE-2011-4528.
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Analysis
by VulDB Data Team • 12/03/2024
The vulnerability identified as CVE-2011-4869 affects the Unbound DNS resolver software, specifically within the validator component responsible for processing NSEC3 records in DNSSEC-signed zones. This issue represents a critical denial of service weakness that can be exploited by remote attackers to crash the DNS resolver daemon. The vulnerability exists in the validator/val_nsec3.c module of Unbound versions prior to 1.4.13p2, where the software fails to properly handle proof processing for NSEC3-signed zones under specific conditions.
The technical flaw stems from inadequate validation of NSEC3 record processing within the DNSSEC validation logic. When Unbound encounters a malformed DNS response that lacks expected NSEC3 records, the validator component does not properly handle this exceptional condition, leading to a daemon crash rather than graceful error handling or rejection of the invalid response. This improper error handling represents a classic buffer over-read or null pointer dereference scenario, where the software attempts to process data structures that do not contain the expected information. The vulnerability is distinct from CVE-2011-4528, indicating a separate code path or validation routine that requires different mitigation approaches. According to CWE classification, this vulnerability aligns with CWE-20: Improper Input Validation and CWE-121: Stack-based Buffer Overflow, though the specific manifestation appears to be more related to improper state handling during validation.
The operational impact of CVE-2011-4869 is significant for organizations relying on Unbound DNS resolvers, as it can result in complete service disruption. Attackers can craft malicious DNS responses containing malformed NSEC3 records that trigger the daemon crash, effectively creating a persistent denial of service condition. This vulnerability is particularly dangerous in environments where DNS resolution is critical for network operations, as it can be exploited without authentication and can be amplified through various attack vectors. The crash occurs during normal DNS resolution operations when the resolver processes responses from authoritative servers that may be compromised or misconfigured. Network administrators may experience intermittent service outages or complete DNS resolution failures until the daemon is manually restarted, potentially affecting multiple services that depend on DNS resolution.
Mitigation strategies for CVE-2011-4869 should focus on immediate software updates to Unbound versions 1.4.13p2 or later, which contain the necessary patches to properly handle NSEC3 record validation. Organizations should also implement network-level protections such as DNS response validation, rate limiting, and monitoring for unusual DNS traffic patterns that may indicate exploitation attempts. The implementation of proper input validation and error handling within the DNS resolver can help prevent similar issues in other components. Security teams should consider deploying intrusion detection systems that can identify malformed DNS responses that match the vulnerability characteristics, and establish automated monitoring for daemon crashes or restarts that could indicate exploitation. Additionally, organizations should review their DNSSEC configurations to ensure proper validation of NSEC3 records and consider implementing redundant DNS resolution services to maintain availability during potential exploitation attempts. The ATT&CK framework categorizes this vulnerability under T1499.004: Endpoint Denial of Service, as it targets the availability of DNS resolution services through daemon crashes.