Summary
Local Code Execution through Argument Injection via dash leading git url parameter in Gemfile.
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.
Exploitation
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.
Workarounds
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.
References
Impact
This vulnerability can lead to Arbitrary Code Execution, which could potentially lead to the takeover of the system. However, as explained above, the exploitability is very low, because it requires a lot of user interaction. It still could put developers at risk when dealing with untrusted files in a way they think is safe, because the exploit still works when the victim tries to make sure nothing can happen, e.g. by manually reviewing the Gemfile (although they would need the weird URL with a leading dash to not raise any flags).
This kind of attack vector has been used in the past to target security researchers by sending them projects to collaborate on.
CVE-2021-43809 has a CVSS score of 6.7 (Medium). The vector is requires local access, low privileges required, and user interaction required. A CVSS score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether this affects your application depends on whether the vulnerable code is present and reachable in your environment. A fixed version is available (2.2.33); upgrading removes the vulnerable code path.
Affected versions
Security releases
Kodem intelligence
Severity tells you how bad this could be in the worst case. It does not tell you whether you are exposed. Exploitability and impact are functions of runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A vulnerable package can sit in your dependency tree and never run.
Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter. Kodem's runtime-powered SCA identifies whether this CVE is reachable in your applications.
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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.
Frequently Asked Questions
- What is CVE-2021-43809? CVE-2021-43809 is a medium-severity security vulnerability in bundler (rubygems), affecting versions < 2.2.33. It is fixed in 2.2.33.
- How severe is CVE-2021-43809? CVE-2021-43809 has a CVSS score of 6.7 (Medium). This score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether it represents real risk in your environment depends on whether the vulnerable code is present and reachable.
- Which versions of bundler are affected by CVE-2021-43809? bundler (rubygems) versions < 2.2.33 is affected.
- Is there a fix for CVE-2021-43809? Yes. CVE-2021-43809 is fixed in 2.2.33. Upgrade to this version or later.
- Is CVE-2021-43809 exploitable, and should I be worried? Whether CVE-2021-43809 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
- What actually determines whether CVE-2021-43809 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
- How do I fix CVE-2021-43809? Upgrade
bundlerto 2.2.33 or later.