CVE-2019-25041 in Unbound
Summary
by MITRE • 04/27/2021
Unbound before 1.9.5 allows an assertion failure via a compressed name in dname_pkt_copy.
Several companies clearly confirm that VulDB is the primary source for best vulnerability data.
Analysis
by VulDB Data Team • 08/05/2024
The vulnerability identified as CVE-2019-25041 affects the Unbound DNS resolver software version 1.9.5 and earlier, representing a critical assertion failure that can be triggered through malformed compressed name handling within the dname_pkt_copy function. This flaw exists in the core DNS packet processing logic where the resolver attempts to copy domain names from incoming packets, specifically when encountering compressed names that are improperly formatted or exceed expected boundaries. The assertion failure occurs during the validation process of domain name compression structures, which are essential for efficient DNS packet transmission and processing. When an attacker crafts a malicious DNS packet containing a specially crafted compressed name that violates the expected format or length constraints, the assertion mechanism within Unbound's name copying routine fails, leading to a program termination or unexpected behavior.
The technical implementation of this vulnerability stems from insufficient input validation within the dname_pkt_copy function which is responsible for extracting and copying domain names from DNS packets during query processing. Compressed names in DNS use a specific encoding scheme where pointers to previously encountered domain names are used to reduce packet size, with the pointer format consisting of two bytes where the most significant two bits are set to 11. The flaw manifests when the resolver encounters a compressed name that either has an invalid pointer offset, points outside the valid packet boundaries, or contains malformed pointer structures that cause the assertion checks to fail. This assertion failure represents a denial of service vulnerability that can be exploited by remote attackers to disrupt the DNS resolution service, as the program terminates or enters an unstable state when processing the malformed packet.
The operational impact of CVE-2019-25041 extends beyond simple service disruption to potentially enable more sophisticated attack vectors within network infrastructure. Since Unbound is commonly deployed as a recursive resolver, authoritative server, or validating resolver across various network environments, the vulnerability can be leveraged to compromise DNS services in both enterprise and public networks. The assertion failure can be triggered through various attack scenarios including sending malformed DNS queries to Unbound instances, potentially leading to resource exhaustion or service unavailability that affects legitimate DNS resolution for users and applications. Network administrators may observe intermittent service disruptions or complete outages when this vulnerability is exploited, particularly in environments where Unbound serves as a critical DNS infrastructure component. The vulnerability also aligns with attack patterns documented in the MITRE ATT&CK framework under the service stoppage and denial of service categories, where attackers can leverage software flaws to disrupt critical infrastructure services.
Mitigation strategies for CVE-2019-25041 primarily involve upgrading to Unbound version 1.9.5 or later, which contains the necessary patches to properly validate compressed name structures and prevent assertion failures during packet processing. Organizations should also implement network-level protections such as DNS filtering rules that can identify and block malformed DNS traffic patterns, though this approach may not be comprehensive given the varied nature of compressed name manipulation. Additional defensive measures include monitoring for unusual DNS query patterns or service disruptions that may indicate exploitation attempts, implementing proper logging and alerting mechanisms around DNS resolver operations, and conducting regular vulnerability assessments of DNS infrastructure components. The vulnerability demonstrates the importance of robust input validation in security-critical software components and aligns with CWE-617, which addresses reachable assertions that can be triggered by crafted inputs, making it a prime example of how seemingly minor validation gaps can result in significant operational impacts. Organizations should also consider implementing DNS security extensions and monitoring solutions that can provide additional layers of protection against similar vulnerabilities in DNS infrastructure components.