CVE-2020-8617 in BIND
Summary
by MITRE
Using a specially-crafted message, an attacker may potentially cause a BIND server to reach an inconsistent state if the attacker knows (or successfully guesses) the name of a TSIG key used by the server. Since BIND, by default, configures a local session key even on servers whose configuration does not otherwise make use of it, almost all current BIND servers are vulnerable. In releases of BIND dating from March 2018 and after, an assertion check in tsig.c detects this inconsistent state and deliberately exits. Prior to the introduction of the check the server would continue operating in an inconsistent state, with potentially harmful results.
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Analysis
by VulDB Data Team • 05/21/2025
The vulnerability described in CVE-2020-8617 represents a critical inconsistency issue within the Berkeley Internet Name Domain (BIND) DNS server implementation that stems from improper handling of TSIG (Transaction Signature) key validation. This weakness exists in the tsig.c component of BIND software and affects systems where TSIG key names are predictable or known to attackers, creating a pathway for state corruption within the DNS server. The vulnerability is particularly concerning because BIND servers are configured with a local session key by default, even when the server configuration does not explicitly require TSIG functionality, making nearly all current BIND installations susceptible to this attack vector.
The technical flaw manifests when an attacker crafts a specially crafted DNS message that exploits the TSIG key name guessing capability to manipulate the server's internal state management. When the server processes such a message, it enters an inconsistent operational state where the server's internal data structures become corrupted or misaligned with the actual processing conditions. This inconsistency occurs because the TSIG validation mechanism fails to properly validate the message against the expected key state, allowing the server to continue processing in a corrupted condition rather than rejecting the malformed input. The vulnerability is classified under CWE-248 as an "Uncaught Exception" and represents a failure in proper error handling within the DNS server's transaction signature validation subsystem.
The operational impact of this vulnerability extends beyond simple service disruption to potentially enable more sophisticated attacks including cache poisoning, denial of service conditions, and unauthorized modifications to DNS records. When the server continues operating in an inconsistent state, it may process subsequent legitimate requests incorrectly, leading to malformed responses or complete service failure. The server's continued operation in this corrupted state can also provide attackers with opportunities to escalate their privileges or manipulate DNS resolution results. According to ATT&CK framework category T1499, this vulnerability enables adversaries to disrupt services and potentially gain unauthorized access to network resources through manipulation of core infrastructure components.
Mitigation strategies for CVE-2020-8617 primarily focus on updating BIND to versions that include the assertion check introduced in March 2018 or later releases that properly handle TSIG state validation. Organizations should also implement strict TSIG key management practices including random key generation, regular key rotation, and proper key distribution mechanisms to prevent attackers from guessing or discovering valid key names. Network segmentation and access controls should be implemented to limit exposure of BIND servers to untrusted networks, while monitoring systems should be configured to detect unusual patterns in DNS query processing that might indicate exploitation attempts. Additionally, administrators should review and minimize unnecessary TSIG configurations on DNS servers to reduce the attack surface, as the vulnerability is most prevalent when TSIG functionality is enabled but not properly secured.