CVE-2020-9308: 
NixOS vulnerability analysis and mitigation

CVE-2020-9308 affects libarchive versions before 3.4.2, specifically in the archive_read_support_format_rar5.c component. The vulnerability was discovered in February 2020 and involves the improper handling of RAR5 files with invalid or corrupted headers, such as those with a header size of zero (NVD, CVE).

The vulnerability occurs when the libarchive attempts to unpack a RAR5 file with an invalid or corrupted header. The minimum header size for RAR5 base blocks should be 7 bytes (4 bytes for CRC and 3 bytes for the rest), but the vulnerability allows processing of files with zero-byte headers. This can lead to a SIGSEGV (segmentation fault) or other unspecified impacts. The vulnerability has a CVSS v3.1 base score of 8.8 (HIGH) with the vector string CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:H (NVD, GitHub PR).

The vulnerability can lead to multiple severe consequences including segmentation faults, potential arbitrary code execution, or other unspecified impacts when processing maliciously crafted RAR5 files. The high CVSS score indicates potential impacts on confidentiality, integrity, and availability of the affected systems (Gentoo Security, Ubuntu Security).

The vulnerability was fixed in libarchive version 3.4.2 by adding validation checks for header sizes. The fix specifically rejects files that declare invalid header flags, including those with zero-byte headers. Users are advised to upgrade to libarchive version 3.4.2 or later. Various Linux distributions have also released patched versions through their package management systems (GitHub Patch, Ubuntu Security).