CVE-2019-25040 in Unbound
Summary
by MITRE • 04/27/2021
Unbound before 1.9.5 allows an infinite loop via a compressed name in dname_pkt_copy.
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Analysis
by VulDB Data Team • 08/05/2024
The vulnerability identified as CVE-2019-25040 affects the Unbound DNS resolver version 1.9.5 and earlier, presenting a critical denial of service risk through an infinite loop condition. This flaw manifests when processing compressed domain names within DNS packet structures, specifically during the dname_pkt_copy function execution. The issue stems from improper handling of compressed name structures that can create circular references or malformed compression pointers, leading to recursive processing that never terminates.
The technical root cause resides in the DNS name compression mechanism implementation within Unbound's packet parsing logic. When a DNS packet contains a compressed name where the compression pointer references a location that creates a loop or circular dependency, the dname_pkt_copy function enters an infinite recursive loop. This occurs because the function does not properly validate the integrity of compression pointers or detect circular references in the name structure. The flaw affects the core DNS packet processing functionality and can be triggered by sending a specially crafted DNS query containing malformed compressed names to an affected Unbound resolver instance.
The operational impact of this vulnerability is severe as it allows remote attackers to cause a denial of service condition on affected DNS resolver systems. An attacker can craft malicious DNS packets with compressed names that trigger the infinite loop, consuming excessive CPU resources and potentially causing the resolver to become unresponsive. This can result in complete service disruption for legitimate DNS queries, affecting all clients relying on the vulnerable resolver. The vulnerability is particularly dangerous in network environments where Unbound serves as a critical DNS infrastructure component, as it can be exploited without authentication and requires minimal effort to implement.
Mitigation strategies include upgrading to Unbound version 1.9.5 or later, which contains the necessary patches to prevent the infinite loop condition. Organizations should also implement network-level protections such as rate limiting and packet filtering to reduce the impact of potential attacks. The vulnerability aligns with CWE-835, which covers infinite loops or iterations, and relates to ATT&CK technique T1499.004 for network denial of service attacks. Additionally, implementing proper input validation and boundary checking mechanisms within DNS parsing components would prevent similar issues in other implementations. Network administrators should monitor for unusual CPU utilization patterns that might indicate exploitation attempts and ensure proper patch management procedures are in place for all DNS infrastructure components.