CVE-2021-25219 in BIND
Summary
by MITRE • 10/28/2021
In BIND 9.3.0 -> 9.11.35, 9.12.0 -> 9.16.21, and versions 9.9.3-S1 -> 9.11.35-S1 and 9.16.8-S1 -> 9.16.21-S1 of BIND Supported Preview Edition, as well as release versions 9.17.0 -> 9.17.18 of the BIND 9.17 development branch, exploitation of broken authoritative servers using a flaw in response processing can cause degradation in BIND resolver performance. The way the lame cache is currently designed makes it possible for its internal data structures to grow almost infinitely, which may cause significant delays in client query processing.
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Analysis
by VulDB Data Team • 11/01/2021
The vulnerability identified as CVE-2021-25219 represents a critical performance degradation issue within the Berkeley Internet Name Domain (BIND) software ecosystem affecting multiple version ranges including 9.3.0 through 9.11.35, 9.12.0 through 9.16.21, and specific preview editions with similar version patterns. This flaw specifically targets the resolver component of BIND rather than the authoritative server functionality, creating a scenario where malicious or compromised authoritative servers can manipulate the resolver's behavior to cause significant operational impacts. The vulnerability manifests through improper handling of response processing in the lame cache mechanism, which serves as a temporary storage for information about servers that have been identified as potentially problematic or "lame" in their authoritative responses.
The technical root cause of this vulnerability lies within the design of BIND's lame cache data structures which are intended to store information about authoritative servers that have been deemed unreliable or non-compliant with DNS standards. When a resolver encounters a response from an authoritative server that triggers a lame cache entry, the system should properly manage the lifecycle of these entries to prevent uncontrolled growth. However, the flaw allows for continuous expansion of these internal data structures without adequate bounds checking or memory management controls, potentially leading to infinite growth of cache entries. This design weakness enables an attacker to craft malicious DNS responses that will cause the resolver to continuously add new entries to the lame cache, creating a resource exhaustion condition that fundamentally degrades the resolver's ability to process legitimate queries efficiently.
The operational impact of this vulnerability extends beyond simple performance degradation to encompass potential service disruption and resource exhaustion across affected systems. When the lame cache grows without bounds, the resolver experiences increasingly significant delays in query processing as it must traverse through exponentially growing data structures to find relevant information. This degradation directly affects the availability and responsiveness of DNS resolution services, potentially causing cascading failures in network infrastructure that relies heavily on timely DNS resolution. The vulnerability affects systems that operate as DNS resolvers rather than authoritative servers, making it particularly concerning for caching servers, recursive resolvers, and any system that processes DNS queries from external sources. Organizations running affected versions of BIND may experience complete service paralysis during periods of high query volume or when subjected to sustained attacks exploiting this vulnerability.
Mitigation strategies for CVE-2021-25219 should focus on immediate version upgrades to patched releases of BIND 9.11.36, 9.16.22, or 9.17.19, which contain the necessary code modifications to prevent unlimited growth of the lame cache data structures. System administrators should prioritize patching all resolver instances within their infrastructure, particularly those serving high-volume or mission-critical environments. Additionally, implementing monitoring solutions that track cache size metrics and memory utilization patterns can help detect early signs of exploitation attempts. Network-level controls such as rate limiting and query filtering can provide additional defense-in-depth measures, though these are secondary to the primary remediation of upgrading to patched versions. The vulnerability aligns with CWE-400, which addresses unchecked resource consumption, and represents a specific implementation weakness that could be categorized under ATT&CK technique T1499.004 for network denial of service attacks. Organizations should also consider implementing DNS security measures including DNSSEC validation and proper access controls to limit exposure to potentially malicious authoritative servers that could exploit this vulnerability.