GHSA-86c2-4x57-wc8g

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

### Description The [Git credential protocol](https://git-scm.com/docs/git-credential#IOFMT) 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`](https://github.com/git/git/blob/6a11438f43469f3815f2f0fc997bd45792ff04c0/credential.c#L311) 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`](https://learn.microsoft.com/en-us/dotnet/api/system.io.streamreader?view=net-8.0) class to [read the standard input stream line-by-line](https://github.com/git-ecosystem/git-credential-manager/blob/ae009e11a0fbef804ad9f78816d84a0bc7e052fe/src/shared/Core/StreamExtensions.cs#L138-L141) and parse the `key=value` credential protocol format. The [implementation of the `ReadLineAsync` method](https://github.com/dotnet/runtime/blob/e476b43b5cb42eb44ce23b1c7b793aa361624cf6/src/libraries/System.Private.CoreLib/src/System/IO/StreamReader.cs#L926) 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](https://github.com/git-ecosystem/git-credential-manager/releases/tag/v2.6.1). [^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.