CVE-2021-43809 Local Code Execution through Argument Injection via dash leading git url parameter in Gemfile

What is CVE-2021-43809?

CVE-2021-43809 is a medium-severity command injection vulnerability in rubygems, published on 8 Dec 2021. It can only be exploited from the same machine or local network, and only if a user opens a file or follows a link.

Official description

`Bundler` is a package for managing application dependencies in Ruby. In `bundler` versions before 2.2.33, when working with untrusted and apparently harmless `Gemfile`'s, it is not expected that they lead to execution of external code, unless that's explicit in the ruby code inside the `Gemfile` itself. However, if the `Gemfile` includes `gem` entries that use the `git` option with invalid, but seemingly harmless, values with a leading dash, this can be false. To handle dependencies that come from a Git repository instead of a registry, Bundler uses various commands, such as `git clone`. These commands are being constructed using user input (e.g. the repository URL). When building the commands, Bundler versions before 2.2.33 correctly avoid Command Injection vulnerabilities by passing an array of arguments instead of a command string. However, there is the possibility that a user input starts with a dash (`-`) and is therefore treated as an optional argument instead of a positional one. This can lead to Code Execution because some of the commands have options that can be leveraged to run arbitrary executables. Since this value comes from the `Gemfile` file, it can contain any character, including a leading dash.

To exploit this vulnerability, an attacker has to craft a directory containing a `Gemfile` file that declares a dependency that is located in a Git repository. This dependency has to have a Git URL in the form of `-u./payload`. This URL will be used to construct a Git clone command but will be interpreted as the upload-pack argument. Then this directory needs to be shared with the victim, who then needs to run a command that evaluates the Gemfile, such as `bundle lock`, inside.

This vulnerability can lead to Arbitrary Code Execution, which could potentially lead to the takeover of the system. However, the exploitability is very low, because it requires a lot of user interaction. Bundler 2.2.33 has patched this problem by inserting `--` as an argument before any positional arguments to those Git commands that were affected by this issue. Regardless of whether users can upgrade or not, they should review any untrustred `Gemfile`'s before running any `bundler` commands that may read them, since they can contain arbitrary ruby code.

Weakness: CWE-88

Which versions are affected by CVE-2021-43809?

2 open source packages ship an affected version, including bundler. It affects 1 product.

Products include Bundler Bundler.

How do I fix CVE-2021-43809?

Upgrade bundler to 2.2.33 or later.

Upgrade, and pass arguments to programs as a list instead of through a shell. More about Command injection.

Is CVE-2021-43809 being exploited?

There is no known exploitation of CVE-2021-43809 in the wild and no public exploit code.

Other vulnerabilities in rubygems

• CVE-2016-7954 Bundler allows attacker to inject arbitrary code via secondary Gem source Critical 9.8
• CVE-2020-36327 Dependency Confusion in Bundler High 8.8
• CVE-2019-3881 Insecure path handling in Bundler High 7.8
• CVE-2013-0334 Bundler may install gems from a different source than expected Medium 5.0

Sources

• CVE record
• NVD
• OSV
• GitHub advisory
• How this page is made

Also known as ALPINE-CVE-2021-43809, DEBIAN-CVE-2021-43809, GHSA-fj7f-vq84-fh43, MGASA-2022-0102, OESA-2021-1480, OESA-2022-1934 and 2 more.