The vulnerability (CVE-2026-76219) exists in GitPython's IndexFile.from_tree, reset, and merge_tree methods, which call git read-tree with positional treeish arguments that are influenced by the caller. These methods do not implement any checks for unsafe options nor use a '--' separator before positional arguments. This allows an attacker to inject the '--index-output=<file>' option, which causes git read-tree to write its index output to an arbitrary file path controlled by the attacker. The last occurrence of --index-output wins, enabling overwriting of arbitrary files with a valid git-index blob. This can lead to arbitrary file overwrite or destruction at the privileges of the host process. The vulnerability was not fixed in the commit that guarded other git commands and remains present in GitPython versions up to 3.25.0. The suggested fix is to add a check_unsafe_options guard and/or insert a '--' separator before positional arguments to block unsafe options like --index-output.

An attacker able to control the treeish argument to these methods can overwrite arbitrary files on the host filesystem with a valid git-index blob. While the content is constrained and does not allow remote code execution, the arbitrary file overwrite can corrupt or destroy important files such as configuration files or other writable files, leading to integrity and availability impacts. The attack executes with the privileges of the host process running the vulnerable code.

A patch is available to address this vulnerability by adding a check_unsafe_options guard with an allow_unsafe_options parameter to the affected methods and/or by inserting a '--' separator before positional treeish arguments to block injection of unsafe options like --index-output. Users should apply the official fix from GitPython to versions >=3.4.1 <3.25.0. Until patched, avoid passing untrusted input to these methods.