CVE-2021-43809
ADVISORY - githubSummary
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.
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.
Patches
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.
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
Common Weakness Enumeration (CWE)
Improper Neutralization of Argument Delimiters in a Command ('Argument Injection')
Improper Neutralization of Argument Delimiters in a Command ('Argument Injection')
Improper Neutralization of Argument Delimiters in a Command ('Argument Injection')
GitHub
0.8
CVSS SCORE
6.7medium| Package | Type | OS Name | OS Version | Affected Ranges | Fix Versions |
|---|---|---|---|---|---|
| bundler | gem | - | - | <2.2.33 | 2.2.33 |
CVSS:3 Severity and metrics
The CVSS metrics represent different qualitative aspects of a vulnerability that impact the overall score, as defined by the CVSS Specification.
The vulnerable component is not bound to the network stack and the attacker's path is via read/write/execute capabilities. Either: The attacker exploits the vulnerability by accessing the target system locally (e.g., keyboard, console), or remotely (e.g., SSH); or the attacker relies on User Interaction by another person to perform actions required to exploit the vulnerability (e.g., using social engineering techniques to trick a legitimate user into opening a malicious document).
A successful attack depends on conditions beyond the attacker's control, requiring investing a measurable amount of effort in research, preparation, or execution against the vulnerable component before a successful attack.
The attacker requires privileges that provide basic user capabilities that could normally affect only settings and files owned by a user. Alternatively, an attacker with Low privileges has the ability to access only non-sensitive resources.
Successful exploitation of this vulnerability requires a user to take some action before the vulnerability can be exploited. For example, a successful exploit may only be possible during the installation of an application by a system administrator.
An exploited vulnerability can only affect resources managed by the same security authority. In this case, the vulnerable component and the impacted component are either the same, or both are managed by the same security authority.
There is a total loss of confidentiality, resulting in all resources within the impacted component being divulged to the attacker. Alternatively, access to only some restricted information is obtained, but the disclosed information presents a direct, serious impact. For example, an attacker steals the administrator's password, or private encryption keys of a web server.
There is a total loss of integrity, or a complete loss of protection. For example, the attacker is able to modify any or all files protected by the impacted component. Alternatively, only some files can be modified, but malicious modification would present a direct, serious consequence to the impacted component.
There is a total loss of availability, resulting in the attacker being able to fully deny access to resources in the impacted component; this loss is either sustained (while the attacker continues to deliver the attack) or persistent (the condition persists even after the attack has completed). Alternatively, the attacker has the ability to deny some availability, but the loss of availability presents a direct, serious consequence to the impacted component.
NIST
0.8
CVSS SCORE
6.7mediumAlpine
-
Debian
-
Ubuntu
1.3
CVSS SCORE
7.3mediumAlma
-
CVSS SCORE
N/AmediumRed Hat
1.3
CVSS SCORE
7.3mediumRocky
-
CVSS SCORE
N/AlowOracle
-
CVSS SCORE
N/AmediumPhoton
CVE-2021-43809
-
CVSS SCORE
7.3highintheWild
-
-