Summary

gix-path executes git to find the path of a configuration file that belongs to the git installation itself, but mistakenly treats the local repository's configuration as system-wide if no higher scoped configuration is found. In rare cases, this causes a less trusted repository to be treated as more trusted, or leaks sensitive information from one repository to another, such as sending credentials to another repository's remote.

Details

In gix_path::env, the underlying implementation of the installation_config and installation_config_prefix functions calls git config -l --show-origin and parses the first line of the output to extract the path to the configuration file holding the configuration variable of highest scope:

https://github.com/Byron/gitoxide/blob/12251eb052df30105538fa831e641eea557f13d8/gix-path/src/env/git/mod.rs#L91

https://github.com/Byron/gitoxide/blob/12251eb052df30105538fa831e641eea557f13d8/gix-path/src/env/git/mod.rs#L112

While the configuration variable of highest scope is not usually in the local scope, there are practical situations where this occurs:

• A configuration file truly associated with the installation is not present on all systems and can occasionally be empty. Likewise, there may be no variables in the global scope.
• Configuration files associated with those higher scopes may be deliberately skipped by setting the GIT_CONFIG_SYSTEM and GIT_CONFIG_GLOBAL environment variables to /dev/null (or to NUL on Windows). This prevents gix-path from finding the path of configuration files for those scopes, while not preventing downstream components such as the function in gix-config from reporting a local path as being associated with the installation.
• The GIT_CONFIG_NOSYSTEM environment variable can be used to disable configuration associated with the installation. (GIT_CONFIG_NOSYSTEM=1 is more powerful than GIT_CONFIG_SYSTEM=/dev/null on systems where an additional "unknown" scope is associated with the installation, as occurs on macOS with Apple Git.) This will cause the local scope to be the highest nonempty scope under even more situations, though in practice it is less dangerous because most, though possibly not all, downstream components would disregard the value.

A user may use either or both of the latter two techniques to turn off an undesired configuration or to create a more replicable environment. Such a user would expect that this results in a more controlled configuration.

Often, when located inside one repository, a user performs operations on that repository or that are not specific to any repository. In such use, local configuration is typically desired or at least acceptable, and mistaking it as coming from another scope is not typically harmful.

However, sometimes a user is in one repository and operates on another repository. A major case where this occurs is cloning one repository while located in another. This can be done in an ad-hoc fashion, including when cloning the repository outside of the one we are inside. It may also potentially be automated by an application for purposes such as submodule handling. Two kinds of problems are anticipated:

• A less secure configuration may be set for a specific repository where it is judged acceptable, even though it would not be wanted for other repositories, such as to enable a protocol or set up debugging.
• More likely, a configuration that supplies secrets for use in one repository's remote can be used to send those secrets to another repository's remote.

PoC

In this example, we send mock Authorization: Basic ... credentials meant for one repository's remote to another remote, by running gix while inside the first repository to clone the second repository.

These instructions are written for a Unix shell, but they will work in other shells, including in PowerShell on Windows if the method of setting environment variables is adapted and /dev/null is replaced with NUL. This procedure is likely to demonstrate the problem on all systems except macOS. This is due to the high-scoped "unknown" configuration that usually accompanies Apple Git, and reflects that gix-path is in practice much less vulnerable on macOS (though still potentially vulnerable).

1. Install dummyhttp to serve as a local HTTP server for the demonstration.

2. Obtain a build of gitoxide with the max feature set enabled. While this vulnerability affects other builds, this example requires max for http.extraHeader support.

   Running cargo install gitoxide will install such a build though it may build against a patched version of gix-path. Cloning the repository (12251eb and earlier are affected) and building with cargo build or cargo install --path . are also sufficient. In contrast, installing from published binaries with binstall or quickinstall does not provide the max feature, as of this writing.

3. Run: dummyhttp -i 127.0.0.1 -c 403 -v

4. In a separate terminal, create a new local repository and set up a mock remote and http.extraHeader configuration:

   git init myrepo
   cd myrepo
   git remote add origin http://127.0.0.1:8080/mygit.git
   git config --local http.extraHeader 'Authorization: Basic abcde'
   

5. Make sure the testing setup is working by running gix fetch in the repository and checking that it fails in the expected way. In the terminal where that is run, a message should be shown indicating an HTTP 403 error. The more interesting output is in the terminal where dummyhttp is running, which should look like this:

   2024-30-30 03:30:16 127.0.0.1:55689 GET /myrepo.git/info/refs?service=git-upload-pack HTTP/1.1
   ┌─Incoming request
   │ GET /myrepo.git/info/refs?service=git-upload-pack HTTP/1.1
   │ Accept: */*
   │ Authorization: Basic abcde
   │ Git-Protocol: version=2
   │ Host: 127.0.0.1:8080
   │ User-Agent: git/oxide-0.42.2
   ┌─Outgoing response
   │ HTTP/1.1 403 Forbidden
   │ Content-Length: 9
   │ Content-Type: text/plain; charset=utf-8
   │ Date: Fri, 30 Aug 2024 03:30:16 -0400
   

   Some details may differ, especially dates and times. But Authorization: Basic abcde should be shown.

6. Now, in the terminal where you ran gix fetch, try cloning a separate repository:

   gix clone http://127.0.0.1:8080/other.git
   

   Check the output appended in the terminal where dummyhttp is running. This is to observe that Authorization: Basic abcde was rightly not sent.

   Alternatively, if it does appear, then your system may be in one of the uncommon configurations that is vulnerable without further action.

7. Now rerun that command, but with a modified environment, to cause gix-path to wrongly treat configuration from the local scope as being associated with the git installation:

   env GIT_CONFIG_SYSTEM=/dev/null GIT_CONFIG_GLOBAL=/dev/null gix clone http://127.0.0.1:8080/other.git
   

   Check the output appended in the terminal where dummyhttp is running. Observe that Authorization: Basic abcde was wrongly sent.

While this procedure uses the same remote host for both repositories, this is not a required element. If the second repository had a different, untrusted host, the extra header would still be sent.

Impact

It is believed to be very difficult to exploit this vulnerability deliberately, due to the need either to anticipate a situation in which higher-scoped configuration variables would be absent, or to arrange for this to happen. Although any operating system may be affected, users running Apple Git on macOS are much less likely to be affected.

In the example shown above, more secure general practices would avoid it: using a credential manager, or even using http.<url>.extraHeader with as specific a <url> as possible, rather than the more general http.extraHeader. Many scenarios are analogous: if each repository's configuration is as secure as possible for how the repository is used, and secrets are stored securely and separately, then the circumstances under which an unacceptably unsecure configuration is used, or under which a leak of credentials would occur, become unlikely.

Summary

gix-path runs git to find the path of a configuration file associated with the git installation, but improperly resolves paths containing unusual or non-ASCII characters, in rare cases enabling a local attacker to inject configuration leading to code execution.

Details

In gix_path::env, the underlying implementation of the installation_config and installation_config_prefix functions calls git config -l --show-origin to find the path of a file to treat as belonging to the git installation.

Affected versions of gix-path do not pass -z/--null to cause git to report literal paths (650a1b5). Instead, to cover the occasional case that git outputs a quoted path, they attempt to parse the path by stripping the quotation marks:

https://github.com/Byron/gitoxide/blob/1cfe577d461293879e91538dbc4bbfe01722e1e8/gix-path/src/env/git/mod.rs#L138-L142

The problem is that, when a path is quoted, it may change in substantial ways beyond the concatenation of quotation marks. If not reversed, these changes can result in another valid path that is not equivalent to the original.

This is not limited to paths with unusual characters such as quotation marks or newlines. Unless git is explicitly configured with core.quotePath set to false, it also happens when the path contains most non-ASCII characters, including accented or non-English letters. For example, é is transformed to \303\251, with literal backslashes. (This is an octal representation of the bytes in its UTF-8 encoding. This behavior is not limited to systems that encode paths with UTF-8 on disk.)

Rarely, the configuration file gix-path wrongly attempts to open can be created by an attacker who has a limited user account on the system. The attacker would often need to request an account username tailored to carrying out the attack.

PoC

Quick demonstration on Unix

On a Unix-like system in which Git supports no higher scope than system for configuration variables (i.e., not on macOS with Apple Git), in a locale that supports UTF-8, with gitoxide installed, run:

mkdir myrepo
cd myrepo
git init
printf '[real]\n\trealvar = realval\n' > 'é'
printf '[fake]\n\tfakevar = fakeval\n' > '\303\251'
GIT_CONFIG_SYSTEM='é' gix config

If the above conditions are satisfied and the gix command was built against an affected version of gix-path, then the last command's output looks something like this:

# From '\303\251' (GitInstallation)
[fake]
        fakevar = fakeval

# From 'é' (System)
[real]
        realvar = realval

# From '/home/ubuntu/.gitconfig' (User)
[init]
        defaultBranch = main

# From './.git/config' (Local)
[core]
        repositoryformatversion = 0
        filemode = true
        bare = false
        logallrefupdates = true

Demonstration across user accounts on Windows

On a test system running Windows on which Git for Windows is not installed system-wide—resembling a scenario in which users who wish to use Git are expected to install it themselves for their accounts—create two accounts, with these usernames:

• Renée, the target of the attack. This user may be a limited user or an administrator. Its user profile directory is assumed to be C:\Users\Renée.
• Ren, the user who carries out the attack. This user should be a limited user, since an administrator would not need to exploit this vulnerability to inject configuration. Its user profile directory is assumed to be C:\Users\Ren.

As Ren, run these commands in PowerShell:

$d = "$HOME\303\251e\AppData\Local\Programs\Git\etc"
mkdir $d
git config --file $d\gitconfig core.sshCommand calc.exe
icacls $HOME\303 /grant 'Renée:(RX)' /T

(The gitconfig file can instead be written manually, in which case Ren need not have git.)

As Renée:

1. Install Git for Windows in the default location for non-systemwide installations, which for that user account is inside C:\Users\Renée\AppData\Local\Programs. For a non-administrative installation, Git for Windows will pick this location automatically. Allow the installer to place the directory containing git in the user's PATH, as it does by default.

   (The scenario can be modified for any location the attacker can predict. So, for example, Renée can install Git for Windows with scoop, and Ren could carry out the attack with correspondingly modified path components in place of AppData\Local\Programs\Git.)

2. Install gitoxide using any common technique, such as by installing Rust and then running cargo install gitoxide.

3. Open a PowerShell window and run a gix command that attempts to run the SSH client for transport. For example:

   gix clone ssh://localhost/myrepo.git
   

   At least one, and usually two, instances of the Windows calculator will pop up. This happens because calc.exe was configured in the fake configuration file the user Ren was able to cause to be used, by placing it at the location gix-path wrongly resolved the path of Renée's own configuration file to.

The gitconfig file written by the attacker can be adjusted with an arbitrary choice of payload, or to set other configuration variables.

Impact

On a single-user system, it is not possible to exploit this, unless GIT_CONFIG_SYSTEM and GIT_CONFIG_GLOBAL have been set to unusual values or Git has been installed in an unusual way. Such a scenario is not expected.

Exploitation is unlikely even on a multi-user system, though it is plausible in some uncommon configurations or use cases. It is especially unlikely with Apple Git on macOS, due to its very high scoped configuration in /Library or /Applications that would be detected instead, as in CVE-2024-45305.

The likelihood of exploitation may be higher on Windows, where attacks such as those shown in the Windows proof-of-concept above can be performed due to the status of \ as a directory separator, and where there is no restriction on usernames containing accented or non-English letters (though the latter is also permitted on some other systems). Even then, complex user interaction is required. In most cases, a system administrator would have to approve an innocuous-seeming username, and then the targeted user (who could be the same or a different user) would have to use an application that uses gix-path.

In general, exploitation is more likely to succeed if at least one of the following applies:

• Users are expected to install git themselves, and are likely to do so in predictable locations.
• Locations where git is installed, whether due to usernames in their paths or otherwise, contain characters that git quotes by default in paths, such as non-English letters and accented letters.
• A custom system-scope configuration file is specified with the GIT_CONFIG_SYSTEM environment variable, and its path is in an unusual location or has strangely named components.
• A system-scope configuration file is absent, empty, or suppressed by means other than GIT_CONFIG_NOSYSTEM. Currently, gix-path can treat a global-scope configuration file as belonging to the installation if no higher scope configuration file is available. This increases the likelihood of exploitation even on a system where git is installed system-wide in an ordinary way.

However, exploitation is expected to be very difficult even under any combination of those factors.

Although the effect here is similar to CVE-2022-24765 once exploited, a greater degree of user interaction would usually be required, and the attack complexity here is much higher because the necessary conditions are uncommon and challenging to predict.