CVE-2024-1931 in Unbound
Summary
by MITRE • 03/07/2024
NLnet Labs Unbound version 1.18.0 up to and including version 1.19.1 contain a vulnerability that can cause denial of service by a certain code path that can lead to an infinite loop. Unbound 1.18.0 introduced a feature that removes EDE records from responses with size higher than the client's advertised buffer size. Before removing all the EDE records however, it would try to see if trimming the extra text fields on those records would result in an acceptable size while still retaining the EDE codes. Due to an unchecked condition, the code that trims the text of the EDE records could loop indefinitely. This happens when Unbound would reply with attached EDE information on a positive reply and the client's buffer size is smaller than the needed space to include EDE records. The vulnerability can only be triggered when the 'ede: yes' option is used; non default configuration. From version 1.19.2 on, the code is fixed to avoid looping indefinitely.
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Analysis
by VulDB Data Team • 12/17/2024
The vulnerability identified as CVE-2024-1931 affects NLnet Labs Unbound DNS resolver versions 1.18.0 through 1.19.1, representing a critical denial of service condition that can be exploited through an infinite loop in the EDE (Extended DNS Error) record processing logic. This flaw specifically manifests when Unbound attempts to handle DNS responses that exceed the client's advertised buffer size, triggering a code path designed to trim EDE records to fit within acceptable limits. The vulnerability stems from an unchecked condition in the text trimming algorithm that processes EDE record fields, creating a scenario where the loop condition never properly terminates, resulting in indefinite execution.
The technical implementation of this vulnerability occurs within Unbound's EDE handling mechanism that was introduced in version 1.18.0, where the resolver attempts to optimize responses by removing EDE records when they exceed client buffer capacity. However, the code path responsible for trimming text fields within EDE records lacks proper boundary checking, causing it to enter an infinite loop when processing responses containing EDE information that must be reduced in size. This condition specifically requires the 'ede: yes' configuration option to be enabled, making it a non-default setting that must be explicitly configured by administrators. The vulnerability is particularly insidious because it only triggers under specific conditions where positive DNS replies contain EDE information and the client's buffer size is insufficient to accommodate the full EDE records, creating a precise attack vector.
The operational impact of this vulnerability extends beyond simple service disruption, as it can cause complete denial of service for the affected Unbound resolver instance, potentially affecting thousands of DNS queries and applications dependent on the resolver's functionality. Attackers can exploit this vulnerability by crafting DNS queries that will trigger the specific code path leading to the infinite loop, effectively consuming system resources and preventing legitimate DNS resolution operations. The vulnerability demonstrates a classic software engineering flaw where defensive programming practices were insufficient, as the code does not properly validate loop termination conditions when processing variable-length text fields within EDE records. This type of vulnerability aligns with CWE-835, which describes infinite loops in software implementations, and represents a failure in proper input validation and boundary condition handling.
Security practitioners should consider this vulnerability in the context of the ATT&CK framework, particularly under the T1499.004 technique for network denial of service attacks, where adversaries exploit software flaws to consume system resources and disrupt operations. The remediation approach requires immediate deployment of Unbound version 1.19.2 or later, which contains the fixed code implementation that properly handles the EDE record trimming logic with appropriate loop termination conditions. Organizations should also implement monitoring for unusual CPU consumption patterns that might indicate exploitation attempts, and consider disabling the 'ede: yes' option if it's not actively required for their DNS infrastructure. Additionally, the vulnerability highlights the importance of thorough code review processes for new features, particularly those involving dynamic content trimming and size calculations, as the fix demonstrates that proper boundary checking and loop validation are essential for preventing such denial of service conditions in network services.