The (1) git-remote-ext and (2) unspecified other remote helper programs in Git before 2.3.10, 2.4.x before 2.4.10, 2.5.x before 2.5.4, and 2.6.x before 2.6.1 do not properly restrict the allowed protocols, which might allow remote attackers to execute arbitrary code via a URL in a (a) .gitmodules file or (b) unknown other sources in a submodule.
The product receives input or data, but it does not validate or incorrectly validates that the input has the properties that are required to process the data safely and correctly.