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@npmcli/arborist vulnerable to UNIX Symbolic Link (Symlink) Following

High severity GitHub Reviewed Published Aug 31, 2021 in npm/arborist • Updated Jan 27, 2023

Package

npm @npmcli/arborist (npm)

Affected versions

< 2.8.2

Patched versions

2.8.2

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, in part, accomplished by resolving dependency specifiers defined in package.json manifests for dependencies with a specific name, and nesting folders to resolve conflicting dependencies.

When multiple dependencies differ only in the case of their name, Arborist's internal data structure saw them as separate items that could coexist within the same level in the node_modules hierarchy. However, on case-insensitive file systems (such as macOS and Windows), this is not the case. Combined with a symlink dependency such as file:/some/path, this allowed an attacker to create a situation in which arbitrary contents could be written to any location on the filesystem.

For example, a package pwn-a could define a dependency in their package.json file such as "foo": "file:/some/path". Another package, pwn-b could define a dependency such as FOO: "file:foo.tgz". On case-insensitive file systems, if pwn-a was installed, and then pwn-b was installed afterwards, the contents of foo.tgz would be written to /some/path, and any existing contents of /some/path would be removed.

Anyone using npm v7.20.6 or earlier on a case-insensitive filesystem is potentially affected.

Patches

2.8.2 (included in npm v7.20.7 and above)

Fix and Caveats

There are two parts to the fix:

  1. Immediately prior to extraction, if the target folder is not a directory, it is moved aside. (If the installation fails, filesystem entries moved aside in this manner are moved back as part of the rollback process.)
  2. The children map that represents child nodes in the tree is replaced with a case-insensitive map object, such that node.children.get('foo') and node.children.get('FOO') will return the same object, enabling Arborist to detect and handle this class of tree collision.

This second item imposes a caveat on case sensitive filesystems where two packages with names which differ only in case may already exist at the same level in the tree, causing unpredictable behavior in this rare edge case. Note that in such cases, the package-lock.json already creates a situation which is hazardous to use on case-sensitive filesystems, and will likely lead to other problems.

If affected by this caveat, please run npm update to rebuild your tree and generate a new package-lock.json file.

References

@isaacs isaacs published to npm/arborist Aug 31, 2021
Reviewed Aug 31, 2021
Published to the GitHub Advisory Database Aug 31, 2021
Published by the National Vulnerability Database Aug 31, 2021
Last updated Jan 27, 2023

Severity

High

CVSS overall score

This score calculates overall vulnerability severity from 0 to 10 and is based on the Common Vulnerability Scoring System (CVSS).
/ 10

CVSS v3 base metrics

Attack vector
Local
Attack complexity
Low
Privileges required
None
User interaction
Required
Scope
Changed
Confidentiality
High
Integrity
High
Availability
None

CVSS v3 base metrics

Attack vector: More severe the more the remote (logically and physically) an attacker can be in order to exploit the vulnerability.
Attack complexity: More severe for the least complex attacks.
Privileges required: More severe if no privileges are required.
User interaction: More severe when no user interaction is required.
Scope: More severe when a scope change occurs, e.g. one vulnerable component impacts resources in components beyond its security scope.
Confidentiality: More severe when loss of data confidentiality is highest, measuring the level of data access available to an unauthorized user.
Integrity: More severe when loss of data integrity is the highest, measuring the consequence of data modification possible by an unauthorized user.
Availability: More severe when the loss of impacted component availability is highest.
CVSS:3.1/AV:L/AC:L/PR:N/UI:R/S:C/C:H/I:H/A:N

EPSS score

0.065%
(30th percentile)

CVE ID

CVE-2021-39134

GHSA ID

GHSA-2h3h-q99f-3fhc

Source code

Credits

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