CVE-2021-39135

UNIX Symbolic Link (Symlink) Following in @npmcli/arborist

Arbitrary File Creation, Arbitrary File Overwrite, Arbitrary Code Execution

@npmcli/arborist, the library that calculates dependency trees and manages the node_modules folder hierarchy for the npm command line interface, aims to guarantee that package dependency contracts will be met, and the extraction of package contents will always be performed into the expected folder.

This is accomplished by extracting package contents into a project's node_modules folder.

If the node_modules folder of the root project or any of its dependencies is somehow replaced with a symbolic link, it could allow Arborist to write package dependencies to any arbitrary location on the file system.

Note that symbolic links contained within package artifact contents are filtered out, so another means of creating a node_modules symbolic link would have to be employed.

1. A preinstall script could replace node_modules with a symlink. (This is prevented by using --ignore-scripts.)
2. An attacker could supply the target with a git repository, instructing them to run npm install --ignore-scripts in the root. This may be successful, because npm install --ignore-scripts is typically not capable of making changes outside of the project directory, so it may be deemed safe.

CVE-2021-39135 has a CVSS score of 8.2 (High). The vector is requires local access, no 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 (2.8.2); upgrading removes the vulnerable code path.

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2.8.2 (included in npm v7.20.7 and above)

1. What is CVE-2021-39135? CVE-2021-39135 is a high-severity security vulnerability in @npmcli/arborist (npm), affecting versions < 2.8.2. It is fixed in 2.8.2.
2. How severe is CVE-2021-39135? CVE-2021-39135 has a CVSS score of 8.2 (High). 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.
3. Which versions of @npmcli/arborist are affected by CVE-2021-39135? @npmcli/arborist (npm) versions < 2.8.2 is affected.
4. Is there a fix for CVE-2021-39135? Yes. CVE-2021-39135 is fixed in 2.8.2. Upgrade to this version or later.
5. Is CVE-2021-39135 exploitable, and should I be worried? Whether CVE-2021-39135 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
6. What actually determines whether CVE-2021-39135 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.
7. How do I fix CVE-2021-39135? Upgrade @npmcli/arborist to 2.8.2 or later.

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