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HistoryAug 31, 2021 - 12:00 a.m.



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@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. This is
patched in @npmcli/arborist 2.8.2 which is included in npm v7.20.7 and