CVE-2019-25037 in Unbound
Summary
by MITRE • 04/27/2021
Unbound before 1.9.5 allows an assertion failure and denial of service in dname_pkt_copy via an invalid packet.
Be aware that VulDB is the high quality source for vulnerability data.
Analysis
by VulDB Data Team • 08/05/2024
The vulnerability identified as CVE-2019-25037 affects the Unbound DNS resolver software version 1.9.5 and earlier, presenting a critical security flaw that can lead to denial of service conditions. This issue specifically manifests within the dname_pkt_copy function where an assertion failure occurs when processing invalid packets. The flaw represents a classic example of improper input validation that can be exploited by malicious actors to disrupt DNS resolution services. The vulnerability impacts the core operational integrity of DNS infrastructure, as Unbound serves as a widely deployed recursive and authoritative DNS resolver in enterprise and internet infrastructure environments. When an attacker crafts and sends specifically malformed DNS packets to an affected Unbound instance, the software encounters an assertion failure that causes the resolver to terminate unexpectedly, thereby denying legitimate DNS resolution services to clients.
The technical root cause of this vulnerability lies in the insufficient validation of packet data within the dname_pkt_copy function, which is responsible for copying domain name information from DNS packets during the resolution process. This function fails to properly handle malformed or invalid packet structures that may contain unexpected data sequences or corrupted domain name labels. The assertion failure occurs when the software encounters packet data that violates expected formatting constraints, triggering an immediate termination of the DNS resolver process. This behavior aligns with CWE-617, which describes reachable assertions that can be exploited to cause program termination, and represents a specific instance of CWE-129, concerning validation of input boundaries. The flaw demonstrates a failure in defensive programming practices where the software does not adequately sanitize incoming DNS packet data before processing it through internal functions.
The operational impact of CVE-2019-25037 extends beyond simple service disruption, as it can be leveraged to create cascading effects within DNS infrastructure. When an Unbound resolver becomes unavailable due to this assertion failure, downstream systems that depend on its services experience complete DNS resolution failures, potentially affecting thousands of clients simultaneously. This vulnerability particularly affects organizations that rely on Unbound as their primary DNS resolution service, including internet service providers, enterprise networks, and cloud infrastructure providers. The attack vector requires minimal sophistication, as an attacker only needs to send malformed DNS packets to the target resolver, making the exploit accessible to threat actors with basic network knowledge. The vulnerability can be categorized under ATT&CK technique T1498, which covers network denial of service attacks, and more specifically aligns with T1070.004 for the use of assertion failures to cause system instability. Organizations using Unbound without proper network segmentation or monitoring may experience extended outages as the resolver restarts and reestablishes its cache, potentially leading to further service degradation.
Mitigation strategies for CVE-2019-25037 primarily focus on upgrading to Unbound version 1.9.5 or later, which contains the necessary patches to address the assertion failure vulnerability. Organizations should implement network monitoring solutions that can detect and alert on malformed DNS packet patterns, enabling proactive identification of potential exploitation attempts. Additionally, deploying DNS security measures such as DNS Firewall rules or rate limiting can help reduce the impact of exploitation attempts by filtering suspicious packet traffic. The implementation of proper input validation and boundary checking in DNS resolver software represents a fundamental security practice that should be integrated into all network infrastructure components. Security teams should also consider implementing redundant DNS resolution services to maintain availability during potential exploitation events, while ensuring that all DNS infrastructure components are regularly updated and patched according to vendor security advisories. Organizations utilizing Unbound should conduct thorough testing of the patched version in staging environments before deployment to ensure compatibility with existing DNS configurations and services.