CVE-2019-25035 in Unbound
Summary
by MITRE • 04/27/2021
Unbound before 1.9.5 allows an out-of-bounds write in sldns_bget_token_par.
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Analysis
by VulDB Data Team • 08/05/2024
The vulnerability identified as CVE-2019-25035 affects the Unbound DNS resolver software version 1.9.5 and earlier, representing a critical out-of-bounds write flaw in the sldns_bget_token_par function. This issue arises within the DNS resolver's token parsing mechanism, which is responsible for processing DNS messages and extracting various components for further processing. The flaw occurs when the software encounters malformed or specially crafted DNS packets that trigger improper memory handling during token extraction operations.
The technical nature of this vulnerability stems from insufficient bounds checking within the sldns_bget_token_par function, which processes DNS message tokens during parsing operations. When processing certain malformed input data, the function fails to validate array boundaries before writing data to memory locations, allowing an attacker to write beyond the allocated buffer space. This memory corruption vulnerability can be exploited through carefully crafted DNS responses or queries that cause the resolver to process malformed data structures during normal DNS resolution operations.
The operational impact of this vulnerability extends beyond simple memory corruption, as it creates opportunities for remote code execution and system compromise. An attacker who can influence DNS resolution behavior or intercept DNS traffic can potentially trigger this vulnerability by sending specially crafted DNS responses that cause the Unbound resolver to parse malformed tokens. The out-of-bounds write can overwrite adjacent memory locations, potentially corrupting critical data structures, function pointers, or control information within the resolver process, leading to unpredictable behavior including crashes, arbitrary code execution, or complete system compromise.
This vulnerability aligns with CWE-787, which specifically addresses out-of-bounds write conditions in software implementations. The flaw demonstrates poor input validation practices and inadequate memory management within the DNS parsing subsystem. From an adversarial perspective, this vulnerability maps to several ATT&CK techniques including T1059 for command and control through DNS resolution, and T1218 for execution through legitimate system processes. The attack surface is particularly concerning for organizations relying on Unbound as a recursive or authoritative DNS resolver, as it could enable attackers to compromise DNS infrastructure and potentially redirect traffic to malicious destinations.
Mitigation strategies for CVE-2019-25035 primarily involve upgrading to Unbound version 1.9.5 or later, where the out-of-bounds write issue has been resolved through proper bounds checking and input validation. Organizations should also implement network-level protections such as DNS sinkholing, DNSSEC validation, and monitoring for anomalous DNS traffic patterns that might indicate exploitation attempts. Additionally, deploying intrusion detection systems capable of identifying malformed DNS responses and implementing proper access controls to limit DNS resolver exposure can help reduce the risk of exploitation. The vulnerability underscores the importance of regular security updates and proper input validation in network infrastructure components, particularly those handling external data parsing operations.