@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 above.