How severe is CVE-2021-39135?

CVE-2021-39135 has a CVSS score of 7.8/10, rated HIGH. Immediate patching is strongly recommended.

Which packages are affected by CVE-2021-39135?

CVE-2021-39135 affects the following packages: @npmcli/arborist (npm). Ecosystems affected: npm.

How do I fix CVE-2021-39135?

Update @npmcli/arborist to 2.8.2 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2021-39135 is resolved across your whole dependency graph.

How do I detect CVE-2021-39135 in my npm dependencies?

Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for @npmcli/arborist, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.

How do I mitigate CVE-2021-39135 if there is no patch (or I can't update yet)?

If you can't upgrade right away: gate or disable the affected feature, validate untrusted input at the boundary, and avoid passing attacker-controlled data into the vulnerable path. O3's runtime protection blocks exploitation in production as an interim safeguard until the upgrade lands.

Is CVE-2021-39135 actively exploited in the wild?

No public exploit code has been indexed for CVE-2021-39135 yet. This does not mean the vulnerability cannot be exploited — absence of public exploits does not imply safety. Apply the recommended fix and use O3 Security to monitor your exposure.

What is the EPSS score for CVE-2021-39135?

CVE-2021-39135 has an EPSS (Exploit Prediction Scoring System) score of 0.6%, placing it in the 45th percentile of all CVEs. EPSS is maintained by FIRST.org and estimates the probability that a vulnerability will be exploited in the wild within the next 30 days. This score indicates relatively lower exploitation probability, though the CVSS severity should still guide your patching priority.

What type of vulnerability is CVE-2021-39135?

CVE-2021-39135 is classified as CWE-61 (CWE-61), CWE-59 (CWE-59). These weakness types describe the underlying flaw category, which helps determine the potential impact and the right class of mitigation.

When was CVE-2021-39135 published, and has it been updated?

CVE-2021-39135 was published on August 31, 2021 and was last updated on July 8, 2026. Advisory data evolves as severity scores, affected ranges, and exploit intelligence are revised — always check the latest version of the advisory before acting.

📦

📦 npm

Not in CISA KEV

HIGH severity

CVE-2021-39135 — @npmcli/arborist

HIGH

CVE-2021-39135 is a high-severity (CVSS 7.8) CWE-61 vulnerability in @npmcli/arborist. A fix is available for @npmcli/arborist — see the affected versions and patch details below.

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

Also known asGHSA-gmw6-94gg-2rc2

Published

Aug 31, 2021

Updated

Jul 8, 2026

Affected

1 pkg

Patched

1 / 1

Exploits

None indexed

Exploitation data as of Oct 10, 2026 · OSV.dev, NVD, FIRST.org (EPSS)

EPSS Exploitation Probability

via FIRST.org ↗

0.6%probability of exploitation in next 30 days

Lower Risk0.00%

Lower risk than most CVEs45th percentile — riskier than 45% of all scored CVEsHighest risk

Probability of exploitation in the next 30 days, from FIRST.org EPSS.

How urgent is this, really

CVE-2021-39135 by exploitation likelihood (EPSS) against impact (CVSS). Outside the shaded patch-first corner.

Where this sits among everything scored

Of 385,386 CVEs with a current EPSS score, this one falls in the < 10% band (highlighted). Counts from FIRST.org, log-scaled.

Real-World Exposure

1 pkg affected

How broadly this vulnerability is actually deployed: weekly install volume shows current usage, and reverse-dependency count shows how many other packages break if it stays unpatched.

214other npm packages depend on this — each one inherits the vulnerability until it's patched upstream

@npmcli/arboristnpm

6.8Mdownloads / week

Description

Impact

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.

A preinstall script could replace node_modules with a symlink. (This is prevented by using --ignore-scripts.)
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.

Patches

2.8.2 (included in npm v7.20.7 and above)

Workarounds

Do not run npm install on untrusted codebases, without first ensuring that the node_modules directory in the project is not a symbolic link.

Fix

Prior to extracting any package contents, the node_modules folder into which it is extracted is verified to be a real directory. If it is not, then it is removed.

Caveat: if you are currently relying on creating a symbolic link to the node_modules folder in order to share dependencies between projects, then that will no longer be possible. Please use the npm link command, explicit file:... dependencies, and/or workspaces to share dependencies in a development environment.

Affected Packages

1 total 1 fixed

Ecosystem	Package	Vulnerable range	Fix
📦npm	`@npmcli/arborist`	all versions	2.8.2`npm install @npmcli/arborist@2.8.2`

Affected Products

3 products · 4 configurations

Application

arboristnpmjs

< 2.8.2

range

Application

graalvmoracle

2 versions

20.3.321.2.0

Application

sinec infrastructure network servicessiemens

< 1.0.1.1

range

Detection & mitigation playbook

Open-source dependency

Detect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for @npmcli/arborist, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.
Fix
Update @npmcli/arborist to 2.8.2 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2021-39135 is resolved across your whole dependency graph.
Workarounds
If you can't upgrade right away: gate or disable the affected feature, validate untrusted input at the boundary, and avoid passing attacker-controlled data into the vulnerable path. O3's runtime protection blocks exploitation in production as an interim safeguard until the upgrade lands.

Frequently Asked Questions

### Impact 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 lin