Headline
CVE-2021-37701: Arbitrary File Creation/Overwrite via insufficient symlink protection due to directory cache poisoning using symbolic links
The npm package “tar” (aka node-tar) before versions 4.4.16, 5.0.8, and 6.1.7 has an arbitrary file creation/overwrite and arbitrary code execution vulnerability. node-tar aims to guarantee that any file whose location would be modified by a symbolic link is not extracted. This is, in part, achieved by ensuring that extracted directories are not symlinks. Additionally, in order to prevent unnecessary stat calls to determine whether a given path is a directory, paths are cached when directories are created. This logic was insufficient when extracting tar files that contained both a directory and a symlink with the same name as the directory, where the symlink and directory names in the archive entry used backslashes as a path separator on posix systems. The cache checking logic used both \ and / characters as path separators, however \ is a valid filename character on posix systems. By first creating a directory, and then replacing that directory with a symlink, it was thus possible to bypass node-tar symlink checks on directories, essentially allowing an untrusted tar file to symlink into an arbitrary location and subsequently extracting arbitrary files into that location, thus allowing arbitrary file creation and overwrite. Additionally, a similar confusion could arise on case-insensitive filesystems. If a tar archive contained a directory at FOO, followed by a symbolic link named foo, then on case-insensitive file systems, the creation of the symbolic link would remove the directory from the filesystem, but not from the internal directory cache, as it would not be treated as a cache hit. A subsequent file entry within the FOO directory would then be placed in the target of the symbolic link, thinking that the directory had already been created. These issues were addressed in releases 4.4.16, 5.0.8 and 6.1.7. The v3 branch of node-tar has been deprecated and did not receive patches for these issues. If you are still using a v3 release we recommend you update to a more recent version of node-tar. If this is not possible, a workaround is available in the referenced GHSA-9r2w-394v-53qc.
Impact
Arbitrary File Creation, Arbitrary File Overwrite, Arbitrary Code Execution
node-tar aims to guarantee that any file whose location would be modified by a symbolic link is not extracted. This is, in part, achieved by ensuring that extracted directories are not symlinks. Additionally, in order to prevent unnecessary stat calls to determine whether a given path is a directory, paths are cached when directories are created.
This logic was insufficient when extracting tar files that contained both a directory and a symlink with the same name as the directory, where the symlink and directory names in the archive entry used backslashes as a path separator on posix systems. The cache checking logic used both \ and / characters as path separators, however \ is a valid filename character on posix systems.
By first creating a directory, and then replacing that directory with a symlink, it was thus possible to bypass node-tar symlink checks on directories, essentially allowing an untrusted tar file to symlink into an arbitrary location and subsequently extracting arbitrary files into that location, thus allowing arbitrary file creation and overwrite.
Additionally, a similar confusion could arise on case-insensitive filesystems. If a tar archive contained a directory at FOO, followed by a symbolic link named foo, then on case-insensitive file systems, the creation of the symbolic link would remove the directory from the filesystem, but not from the internal directory cache, as it would not be treated as a cache hit. A subsequent file entry within the FOO directory would then be placed in the target of the symbolic link, thinking that the directory had already been created.
These issues were addressed in releases 4.4.16, 5.0.8 and 6.1.7.
The v3 branch of node-tar has been deprecated and did not receive patches for these issues. If you are still using a v3 release we recommend you update to a more recent version of node-tar. If this is not possible, a workaround is available below.
Patches
4.4.16 || 5.0.8 || 6.1.7
Workarounds
Users may work around this vulnerability without upgrading by creating a custom filter method which prevents the extraction of symbolic links.
const tar = require(‘tar’)
tar.x({ file: 'archive.tgz’, filter: (file, entry) => { if (entry.type === ‘SymbolicLink’) { return false } else { return true } } })
Users are encouraged to upgrade to the latest patched versions, rather than attempt to sanitize tar input themselves.
Fix
The problem is addressed in the following ways:
- All paths are normalized to use / as a path separator, replacing \ with / on Windows systems, and leaving \ intact in the path on posix systems. This is performed in depth, at every level of the program where paths are consumed.
- Directory cache pruning is performed case-insensitively. This may result in undue cache misses on case-sensitive file systems, but the performance impact is negligible.
Caveat
Note that this means that the entry objects exposed in various parts of tar’s API will now always use / as a path separator, even on Windows systems. This is not expected to cause problems, as / is a valid path separator on Windows systems, but may result in issues if entry.path is compared against a path string coming from some other API such as fs.realpath() or path.resolve().
Users are encouraged to always normalize paths using a well-tested method such as path.resolve() before comparing paths to one another.
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