Dulwich versions 0.23.1 through 1.2.7 contain a path traversal flaw in CommitFilter._apply_index_filter that permits attackers to create symbolic links that, when filter_branch is run, write files outside the intended working directory. This ability to write arbitrary files, including executable hooks in .git/hooks, can lead to remote code execution on the system hosting the repository. The weakness is a classic directory traversal (CWE-22 and CWE-59) and threatens confidentiality, integrity, and availability by enabling unauthorized file manipulation and script execution within the Git repository environment.

The vulnerability affects the Dulwich project library released under the jelmer:dulwich vendor, including all versions from 0.23.1 up to but excluding 1.2.8. Any deployment that uses these versions in a development or production environment is potentially impacted. This includes self-hosted Git services or applications embedding Dulwich for repository manipulation.

The CVSS score of 7.3 classifies the issue as high severity, while the EPSS score of less than 1% indicates a very low but present likelihood of exploitation. The flaw is not listed in the CISA KEV catalog. Attackers can exploit the weakness by crafting a malicious repository that contains a symlink in an ancestor commit pointing to a location outside the working directory, which is then followed during a filter_branch operation. The attack vector is inferred to be local or remote repository manipulation, typically within environments where filter_branch is executed on untrusted content.