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The Git Project announced yesterday a critical arbitrary code execution vulnerability in the Git command line client, Git Desktop, and Atom that could allow malicious repositories to remotely execute commands on a vulnerable machine.

This vulnerability has been assigned the CVE-2018-17456 ID and is similar to a previous CVE-2017-1000117 option injection vulnerability. Like the previous vulnerability, a malicious repository can create a .gitmodules file that contains an URL that starts with a dash.

By using a dash, when Git clones a repository using the --recurse-submodules argument, the command will interpret the URL as an option, which could then be used to perform remote code execution on the computer.

"When running "git clone --recurse-submodules", Git parses the supplied .gitmodules file for a URL field and blindly passes it as an argument to a "git clone" subprocess.  If the URL field is set to a string that begins with a dash, this "git clone" subprocess interprets the URL as an option.  This can lead to executing an arbitrary script shipped in the superproject as the user who ran "git clone"."

An example of a malicious .gitmodules file that was used in the previous vulnerability can be seen  below. Notice how the URL starts with a dash so that Git considers it an option, which then causes the "touch VULNERABLE/git@github.com:/timwr/test.git" command to be executed.

[submodule "test"]
	path = test
	url = ssh://-oProxyCommand=touch VULNERABLE/git@github.com:/timwr/test.git

This vulnerability has been fixed in Git v2.19.1 (with backports in v2.14.5, v2.15.3, v2.16.5, v2.17.2, and v2.18.1), GitHub Desktop 1.4.2, Github Desktop 1.4.3-beta0, Atom 1.31.2, and Atom 1.32.0-beta3.

The Git Project strongly recommends that all users upgrade to the latest version of the Git client, Github Desktop, or Atom in order to be protected from malicious repositories.

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