CVE-2020-25583 in FreeBSD
Summary
by MITRE • 03/30/2021
In FreeBSD 12.2-STABLE before r368250, 11.4-STABLE before r368253, 12.2-RELEASE before p1, 12.1-RELEASE before p11 and 11.4-RELEASE before p5 when processing a DNSSL option, rtsold(8) decodes domain name labels per an encoding specified in RFC 1035 in which the first octet of each label contains the label's length. rtsold(8) did not validate label lengths correctly and could overflow the destination buffer.
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Analysis
by VulDB Data Team • 04/06/2021
The vulnerability identified as CVE-2020-25583 affects FreeBSD operating systems versions prior to specific release checkpoints, including 12.2-STABLE before r368250, 11.4-STABLE before r368253, and various release versions before their respective patches. This issue resides within the rtsold(8) daemon which is responsible for managing IPv6 routing information and handling Domain Name System Search List (DNSSL) options. The daemon processes network configuration data that includes DNS search domains, which are encoded according to RFC 1035 standards where each label's first octet represents its length. This encoding method allows for efficient storage and transmission of domain names but requires careful validation during processing.
The technical flaw manifests in the rtsold(8) daemon's inadequate validation of label lengths when decoding DNSSL options. When processing domain name labels, the daemon assumes that the length octets correctly represent the actual label size without proper bounds checking. This failure in input validation creates a buffer overflow condition where the destination buffer allocated for storing decoded domain names can be exceeded. The vulnerability specifically targets the handling of domain name labels that follow RFC 1035 encoding rules, where labels are prefixed with a single octet indicating their length. When an attacker crafts malicious DNSSL options with oversized or malformed length indicators, the daemon's insufficient validation allows for memory corruption that could lead to arbitrary code execution or service disruption.
The operational impact of this vulnerability extends beyond simple denial of service conditions, as it represents a potential security escalation vector that could allow remote attackers to execute arbitrary code on affected systems. Since rtsold(8) operates with elevated privileges to manage network routing information, successful exploitation could provide attackers with significant control over the network stack and potentially enable further lateral movement within the network. The vulnerability affects systems running FreeBSD versions where the daemon processes routing information from IPv6 network configurations, particularly those receiving network advertisements that include maliciously crafted DNSSL options. This makes the attack surface significant in environments where IPv6 routing protocols are actively used and where systems receive automated network configuration data.
Mitigation strategies for CVE-2020-25583 involve immediate deployment of the relevant FreeBSD patches and updates that address the buffer overflow condition in rtsold(8). System administrators should prioritize updating to the patched versions including FreeBSD 12.2-RELEASE p1, 12.1-RELEASE p11, 11.4-RELEASE p5, and their respective stable branch updates. The fix implements proper validation of label lengths in accordance with RFC 1035 requirements, ensuring that decoded domain name labels do not exceed the allocated buffer boundaries. Network administrators should also consider implementing monitoring for anomalous DNSSL option processing and may want to temporarily disable automatic routing updates if the vulnerability is exploited in the wild. Additionally, the vulnerability aligns with CWE-121, which describes buffer overflow conditions, and represents a potential ATT&CK technique involving privilege escalation through service exploitation, making it a critical vulnerability requiring immediate attention in enterprise security environments.