Carriage-return character in remote URL allows malicious repository to leak credentials in Git Credential Manager

Summary

Git Credential Manager (GCM) is a secure Git credential helper built on .NET that runs on Windows, macOS, and Linux. 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 keys or values. When Git reads from standard input, it considers both LF and CRLF 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. When a user clones or otherwise interacts 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. This issue has been patched in version 2.6.1 and all users are advised to upgrade. Users unable to upgrade should only interact with trusted remote repositories, and not clone with `--recursive` to allow inspection of any submodule URLs before cloning those submodules.