CVE-2021-39135

Arbitrary file overwrite via symlinked node_modules

Published Aug 31, 2021 · Updated Aug 4, 2024

Symlink following in npm @npmcli/arborist before 2.8.2 allows attackers to write package contents to arbitrary filesystem locations. During dependency extraction, Arborist trusts an existing node_modules path and follows it when the directory has been replaced by a symbolic link. Exploitation requires another means to create that link, such as a preinstall script or a crafted repository that a user installs, and grants writes with the installing process's permissions.

CVSS severity8.2
High
EPSS probability0.55%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

Symlink following in npm @npmcli/arborist before 2.8.2 allows attackers to write package contents to arbitrary filesystem locations. During dependency extraction, Arborist trusts an existing node_modules path and follows it when the directory has been replaced by a symbolic link. Exploitation requires another means to create that link, such as a preinstall script or a crafted repository that a user installs, and grants writes with the installing process's permissions.

The record

CVE
CVE-2021-39135
Published
Aug 31, 2021
Updated
Aug 4, 2024
Vendor
npm
Product
@npmcli/arborist
Classifications
CWE-61, CWE-59, CAPEC-27, T1204.005
Attack vector
local
Privileges
unauthenticated

Timeline

How it unfolded

  1. Aug 31, 2021CVE publishedPublication date reported by the CVE source.
  2. Aug 4, 2024Record updatedLatest update available in the CVE record.

Exploitability

Present is not the same as exploitable

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Is a vulnerable build present?

Compare these published version ranges with your installed build and any vendor patches.

  1. Affected versionversion=< 2.8.2

What conditions does exploitation require?

Attack vectorlocal
Required privilegesunauthenticated

What is affected?

npm · @npmcli/arboristversion=< 2.8.2

Published CVSS scores

4.4NVDAV:L/AC:M/Au:N/C:P/I:P/A:P
8.2GitHubCVSS:3.1/AV:L/AC:L/PR:N/UI:R/S:C/C:H/I:H/A:N
7.8NVDCVSS:3.1/AV:L/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:H

CVSS describes severity. EPSS estimates exploitation probability.

Attacks

What attackers are doing with it

Daily unique IPs observed by Shadowserver honeypots for known exploited vulnerabilities (KEVs). Missing observations do not establish an absence of attacks.

Daily unique IPsNo honeypot observations are available for this CVE in the selected window.

No observations available

Sep 10, 2026Sep 16, 2026
Latest reporting daySep 16, 2026
Latest daily unique IPsUnavailable
Prior 30-day averageUnavailable
SourceShadowserver honeypots (KEV)
Vectorlocal
Privilegesunauthenticated
Known exploitationUnconfirmed
Public exploitUnconfirmed

Weakness, pattern, technique

CWE-61UNIX Symbolic Link (Symlink) Following
CWE-59Improper Link Resolution Before File Access ('Link Following')
CAPEC-27Leveraging Race Conditions via Symbolic Links
T1204.005User Execution: Malicious Library

Public exploit references

No public exploit references are available in this record.

Labels summarize the accepted research assessment. They do not indicate a test against your environment.

Technologies

Your stack

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Your stack

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