GHSA-8qq5-rm4j-mr97: node-tar Arbitrary File Overwrite via Hardlink Escape

## Summary
`node-tar` ≤ 7.5.2 does not sanitize the `linkpath` field of `Link` entries when extracting with `preservePaths=false`. A malicious TAR can hardlink a file inside the extraction root (e.g., `payload`) to an absolute path outside the root (e.g., `/tmp/secret.txt`). When the extractor later writes to the in-root path, it actually overwrites the arbitrary external file, enabling path-escape and arbitrary file overwrite.

## Impact
- **Affected component:** `node-tar` hardlink extraction logic (`unpack.ts`)
- **Affected versions:** 7.5.2 and earlier (fixed in 7.5.3)
- **Risk:** High. Attackers controlling TAR input can overwrite any writable file reachable by the extracting process, enabling configuration tampering or code execution in build/CI pipelines.

## Root Cause
`node-tar` resolves hardlink targets with `path.resolve(this.cwd, entry.linkpath)` but never checks that the resulting path still resides under `cwd`. Because `path.resolve` returns the second operand unchanged when it is absolute, an attacker can supply an absolute `linkpath`. The library then creates a hardlink from the in-root `entry.path` to the attacker-chosen absolute file. Subsequent writes to `entry.path` therefore modify the external target. Fix commit (upstream): https://github.com/isaacs/node-tar/commit/340eb285b6d986e91969a1170d7fe9b0face405e adds validation to reject absolute link targets.

## Reproduction Steps
1. Run `repro/reproduction_steps.sh`.
2. Script installs Node dependencies (including `node-tar@7.5.2`), crafts a malicious TAR with a hardlink pointing to `secret.txt` outside the extraction root, extracts it with `tar.x({preservePaths:false})`, writes `EXPLOIT` into `out/payload`, and finally reads `secret.txt`.
3. Reproduction succeeds when the script reports `secret.txt now contains: EXPLOIT` and exits 0.

## Evidence
- Logs: `logs/repro_20260117075559.log`, `logs/repro_20260117075603.log`
- Key excerpt: `secret.txt now contains: EXPLOIT` followed by `Reproduction successful: secret.txt overwritten via hardlink escape`
- Environment: Node v22.21.1, npm 10.9.4 as captured in the log prologue.

## Recommendations / Next Steps
- Upgrade to `node-tar` ≥ 7.5.3 where absolute link targets are rejected.
- When possible, reject TAR archives containing `Link`/`SymbolicLink` entries or perform manual validation that resolved paths stay inside the extraction root.
- Add regression tests that extract crafted archives with absolute `linkpath`s to ensure future sanitization.

## Additional Notes
- Script is idempotent: executed twice consecutively; both runs succeeded and produced separate timestamped logs.
- Limitation: PoC demonstrates hardlink overwrite only; symlink poisoning is related but not exercised here.

The agent's step-by-step process — every tool call, every handoff, the moment the exploit fired. Phases: support triages the advisory · repro reproduces it · vuln_variant confirms the fix blocks it · judge verifies.

Scripts, logs, diffs, and output captured during the reproduction.