Dulwich is a pure-Python implementation of the Git file formats and protocols. Starting in version 0.24.0 and prior to version 1.2.5, Dulwich's "ProcessMergeDriver" substitutes the file path (from the git tree, controllable by an attacker via a malicious branch) into the merge driver command via the "%P" placeholder and executes it with "subprocess.run(..., shell=True)". An attacker who can cause a victim to merge an untrusted branch can achieve arbitrary command execution by crafting malicious file paths. Version 1.2.5 fixes the issue.