npm v2 lock: aliased packages stay on the registry in the legacy dependencies mirror (hosted silently, vendored with a warning), so npm 6 installs unpatched bytes while VEX attests not_affected

[mikolalysenko]

[agent] Found by the scheduled npm bug-hunt routine (ledger #302).

Summary

Take a lockfileVersion 2 package-lock.json, the format npm 7 and 8 write, which also carries the legacy dependencies mirror that npm 6 reads. When it holds an npm alias ("lp": "npm:left-pad@1.3.0"), hosted and vendored mode rewrite only the packages["node_modules/lp"] entry. The mirror node dependencies.lp ("version": "npm:left-pad@1.3.0") keeps its registry resolved and integrity.

• Hosted (the v5 default) does this silently. Its only warning is redirect_npm_allow_remote.
• Vendored warns vendor_legacy_alias_skipped ("npm 6 clients reading the v2 legacy mirror still install the UNPATCHED registry bytes through it"). See crates/socket-patch-core/src/vendor/npm_lock.rs:950-972.

In both modes, manifest-less socket-patch vex then attests the package not_affected from the lockfile. Meanwhile npm ci with npm 6 on that same committed lock installs the unpatched registry tarball and exits 0.

Non-aliased entries don't have this problem: the mirror is rewritten, and npm 6 installs the patched bytes. That holds for both hosted and vendored, and for nested entries too.

A related symptom in the same area: with an npm 6 lockfileVersion 1 lock and the same alias, hosted scan writes nothing. It warns redirect_npm_entry_not_found: no package-lock.json entry for left-pad@1.3.0 and exits 0 success, even though the lock contains the alias entry "lp": {"version": "npm:left-pad@1.3.0", ...}.

Impact

Docs list npm 6 as able to install a v2 lock (docs/testing/npm-compatibility.md, docs/ecosystems.md:56). A project that commits a v2 lock and has any npm 6 consumer (CI image, developer machine) gets the vulnerable code. Its VEX document still claims the vulnerability is not exploitable, which is the false attestation VEX must never make. Vendored at least warns at write time. Hosted gives no signal at all.

Repro

This uses a local mock of the patch API (batch / by-package / patches/package / patches/view / the tarball route), with --patch-server-url pointing at it. The patched tarball prepends /* SOCKET-PATCHED */ to index.js.

SPA="--api-url http://127.0.0.1:8765 --org o --api-token fake --patch-server-url http://127.0.0.1:8765"
mkdir repro && cd repro
echo '{"name":"t","version":"1.0.0","private":true,"dependencies":{"lp":"npm:left-pad@1.3.0"}}' > package.json
npx -y npm@8.19.4 install            # lockfileVersion 2

socket-patch scan $SPA --json        # hosted: status success, redirected 1, warnings: [redirect_npm_allow_remote]
node -e 'const l=require("./package-lock.json");console.log(l.packages["node_modules/lp"].resolved, l.dependencies.lp.resolved)'
#   http://127.0.0.1:8765/patch/npm/<token>/<uuid>/left-pad-1.3.0.tgz   https://registry.npmjs.org/left-pad/-/left-pad-1.3.0.tgz

# fresh checkouts of the committed files (package.json, package-lock.json, .npmrc)
(cp -r ../repro ../c6  && cd ../c6  && rm -rf node_modules && npx -y npm@6.14.18 ci && head -c 21 node_modules/lp/index.js)   # "/* This program is fr" (UNPATCHED)
(cp -r ../repro ../c12 && cd ../c12 && rm -rf node_modules && npx -y npm@12.1.0 ci && head -c 21 node_modules/lp/index.js)   # "/* SOCKET-PATCHED */"
(cp -r ../repro ../lo  && cd ../lo  && rm -rf node_modules && socket-patch vex $SPA -O v.json)                                 # 1 statement: not_affected

To see the vendored twin, run socket-patch scan --mode vendored $SPA instead of the hosted scan. It prints the vendor_legacy_alias_skipped warning. npm 6 ci again installs unpatched bytes, and lockfile-only vex is again not_affected.

Expected vs actual

• Expected: CLI_CONTRACT.md's lockfile table says for npm lock v2: "v2 legacy dependencies mirror … legacy mirror rewritten". So the alias mirror node should be rewritten like the non-alias ones. Failing that, the run should at least fail loudly in hosted mode (the vendored warning parity). VEX should also withhold the attestation while a lock that some supported npm reads still resolves the package from the registry. That's the same principle as patched_ref_unattributable for the dual-lock shrinkwrap/package-lock case.
• Actual: hosted rewrites only the packages half, silently. Vendored does the same with a warning. VEX attests not_affected in both modes, while npm 6 installs the unpatched bytes.

Matrix (Linux, Node 22.22)

Lock writer	Mode	npm 6.14.18 ci	npm 12.1.0 ci	lockfile-only vex
npm 8.19.4 (v2), alias lp + scoped alias @x/lp	hosted	unpatched	patched	not_affected
npm 8.19.4 (v2), alias	vendored	unpatched (warned at write)	n/a	not_affected
npm 8.19.4 (v2), plain is-number + nested copy (control)	hosted	patched	patched	not_affected (correct)
npm 6.14.18 (v1), alias	hosted	nothing written, redirect_npm_entry_not_found, exit 0	—	no refs

First bad version

This isn't a v5 regression. Release 4.0.0 (scan --mode hosted) leaves the same alias mirror nodes on the registry. Main 2463257 behaves the same way.

Suspect code

• Vendored: crates/socket-patch-core/src/vendor/npm_lock.rs:950 (rewrite_legacy_tree, the vendor_legacy_alias_skipped branch).
• Hosted: the npm package-lock.json redirect under crates/socket-patch-core/src/patch/redirect/ has no counterpart warning and no alias-mirror rewrite.
• VEX: manifest-less npm lock discovery (crates/socket-patch-core/src/vex/discover/) attests from the packages half without checking the legacy mirror.

[mikolalysenko]

[agent] Triaged: priority:p1 (npm). Not a duplicate: #356 is the agent-mode alias miss, this is the v2 lock's legacy dependencies alias mirror in hosted/vendored rewrites plus VEX attesting from the packages half. No open PR covers it.

---

Generated by Claude Code

[mikolalysenko]

[agent] Claiming this issue (shared root cause: npm package-lock writers and VEX skip the v2 legacy dependencies mirror node of an npm alias). Branch: agent/fix-npm-legacy-alias-mirror. Claim-ID: 2026-10-05T00:20:57Z-fb5d72

---

Generated by Claude Code

[mikolalysenko]

[agent] Draft PR: #813

---

Generated by Claude Code