CVE-2020-7451 in FreeBSD
Summary
by MITRE
In FreeBSD 12.1-STABLE before r358739, 12.1-RELEASE before 12.1-RELEASE-p3, 11.3-STABLE before r358740, and 11.3-RELEASE before 11.3-RELEASE-p7, a TCP SYN-ACK or challenge TCP-ACK segment over IPv6 that is transmitted or retransmitted does not properly initialize the Traffic Class field disclosing one byte of kernel memory over the network.
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Analysis
by VulDB Data Team • 10/14/2020
This vulnerability exists in FreeBSD operating systems across multiple versions including 12.1-STABLE before r358739, 12.1-RELEASE before 12.1-RELEASE-p3, 11.3-STABLE before r358740, and 11.3-RELEASE before 11.3-RELEASE-p7. The flaw manifests when TCP segments are transmitted or retransmitted over IPv6 networks, specifically affecting the Traffic Class field in TCP SYN-ACK or challenge TCP-ACK segments. This represents a classic information disclosure vulnerability where sensitive kernel memory contents are inadvertently exposed to network observers.
The technical implementation of this vulnerability stems from improper initialization of the Traffic Class field within IPv6 TCP packets. When FreeBSD processes TCP connections over IPv6, it fails to properly zero out or initialize this field before sending TCP segments. The Traffic Class field in IPv6 is a one-byte field that should contain specific quality of service parameters, but in this case it retains uninitialized kernel memory contents. This occurs during both initial transmission and retransmission scenarios, making the vulnerability particularly concerning for persistent network monitoring.
The operational impact of this vulnerability is significant from a cybersecurity perspective as it provides attackers with access to one byte of kernel memory over the network. While this appears to be a single byte disclosure, kernel memory often contains sensitive information including pointers, configuration data, and potentially cryptographic keys or session identifiers. The vulnerability is particularly dangerous because it occurs during normal TCP connection establishment processes, making it difficult to detect through standard network monitoring. Attackers could potentially use this information to perform further exploitation techniques including heap spraying, information gathering for targeted attacks, or even assist in more sophisticated exploitation attempts.
This vulnerability maps to CWE-248, which describes an "Uncaught Exception" or more specifically an "Information Exposure Through Sent Data" scenario where information is unintentionally disclosed through network transmission. The attack pattern aligns with ATT&CK technique T1046 which involves network service scanning, as attackers could use the leaked information to better understand the target system. The vulnerability also relates to T1082, system information discovery, as it provides additional insight into the kernel state and memory layout. Organizations should implement immediate mitigations including applying the relevant FreeBSD patches and updates, particularly those addressing the specific release versions mentioned in the CVE description. Network monitoring should be enhanced to detect anomalous TCP traffic patterns that might indicate exploitation attempts, and regular security assessments should verify that systems have been properly updated to prevent this information disclosure vulnerability from being exploited in real-world scenarios.