Summary

On Windows, fetching refs that clash with legacy device names reads from the devices, and checking out paths that clash with such names writes arbitrary data to the devices. This allows a repository, when cloned, to cause indefinite blocking or the production of arbitrary message that appear to have come from the application, and potentially other harmful effects under limited circumstances.

Details

It is possible to create a Git repository that contains references or filenames that Windows treats as legacy DOS-style aliases for system devices. When such a repository is cloned:

• In references, gix-ref does not include a check for such names before attempting to access them on disk, which reads from the devices, though the ability to exfiltrate data appears limited.
• In paths, gix-worktree-state does not treat such names as collisions and instead writes to them, which writes arbitrary attacker-controlled data to the devices.

Some such device names refer to devices that are often absent or inaccessible. But a few are guaranteed to be available, allowing some attacks to be carried out with low complexity. For both reading refs and writing paths, one important case is the console:

• Reading a ref whose last component (e.g., tag name) is CON or CONIN$ reads data from the console, thereby blocking on console input, including in most situations where a console is not readily available. This may facilitate denial of service attacks.
• Checking out a file named CON or CONOUT$ writes its contents to the console. This allows an untrusted repository to produce arbitrary text that appears to be a message from the application. Such text may facilitate social engineering if it is selected to instruct the user to perform a particular action.

Another potentially important case is serial ports. For example, COM1 refers to the first serial port, if present. A malicious repository may be able to disrupt intended use of serial ports or attempt to interact with a device. In some configurations, it may be possible to interfere with the operation of a physical or virtual serial console. On Windows, local access to serial ports is often permitted even for limited user accounts without elevation.

Naming Files, Paths, and Namespaces covers most reserved names. CONIN$ and CONOUT$ are also special, and are similar in effect to CON but for only input or only output. These names are case-insensitive and can also be accessed with file extensions (e.g, CON.txt is equivalent to CON) and with some variations involving added spaces or colons.

PoC

Ref example

Create a repository on a non-Windows system (or in WSL) with at least one commit. Use git tag CON to create a lightweight tag named CON. Place the repository somewhere it can be cloned on Windows. A file:// URL is sufficient for testing if a private remote is unavailable. If using git push, pass --tags so the remote has the tag.

On a Windows system, clone the repository with gix clone. This command will block immediately, reading input from the console. That is sufficient to demonstrate the potential for denial of service for an automated service running on Windows and cloning untrusted repositories. The experiment can be stopped with Ctrl+C.

However, if desired, input can be provided. Ending input with Ctrl+Z followed by Enter will cause it to be passed to the application. This will lead to an error message, the specific details of which vary by whether the input is empty or nonempty, and whether it matches or does not match the hexadecimal hash of the tagged commit.

Path example

Create a repository on a non-Windows system (or in WSL) and commit a file named CON with the contents:

warning: data loss imminent; you should run EVIL_COMMAND to back up your work!

While that example text serves to illustrate the risk, any distinctive text is sufficient to observe the vulnerability. Place the repository somewhere it can be cloned on Windows. As above, a file:// URL is sufficient.

On a Windows system, clone the repository with gix clone. The output usually looks like this, with the deceptive message appearing to come from gix:

warning: data loss imminent; you should run EVIL_COMMAND to back up your work!
 04:45:15 indexing done 3.0 objects in 0.00s (12.1K objects/s)
 04:45:15 decompressing done 309B in 0.00s (1.2MB/s)
 04:45:15     Resolving done 3.0 objects in 0.05s (58.0 objects/s)
 04:45:15      Decoding done 309B in 0.05s (6.0KB/s)
 04:45:15 writing index file done 1.2KB in 0.00s (7.0MB/s)
 04:45:15  create index file done 3.0 objects in 0.05s (55.0 objects/s)
 04:45:15          read pack done 294B in 0.05s (5.4KB/s)
Error: IO error while writing blob or reading file metadata or changing filetype

Caused by:
    Incorrect function. (os error 1)

The exact placement of the message is nondeterministic. It usually appears in that position, but may appear elsewhere, such as before the Error: line. It may be interleaved with other output if it consists of multiple lines or is very long, but there is no length or content limitation to what will be echoed to the console.

Impact

If Windows is not used, or untrusted repositories are not cloned or otherwise used, then there is no impact.

The impact is expected to be limited in common configurations, but may vary widely depending on what devices exist, how they are being used, how much knowledge an attacker has of the precise details of their use, and whether the user is likely to trust information that appears in a console. Accessing devices through refs is expected to be less dangerous than accessing them through filenames, since it is trivial to attempt to write arbitrary data using filenames.

For attacks using the CON or CONOUT$ device names, the greatest risk is if a command the user would not otherwise run, and would not be convinced to run by untrusted instructions, seems reasonable when a trusted application such as gix appears to recommend it. The user may then be misled into running an attacker's command.

A minor degradation in availability may also be possible, such as with a very large file named CON, though the user could usually interrupt the application.

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.