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fix(ci): flag refs/pull checkouts under pull_request_target

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The `pull_request_target` rule's `expressionPattern` matches only
the canonical `github.event.pull_request.head.{ref,sha,repo.full_name}`
interpolations. It does not match the second canonical form of
the same exploit — fetching `refs/pull/<N>/{head,merge}` directly:

  - uses: actions/checkout@v4
    with:
      ref: refs/pull/${{ github.event.pull_request.number }}/merge

The merge-ref variant is what GitHub's own security guidance calls
out as the highest-severity privilege-escalation pattern under
`pull_request_target`: it materialises the PR's merge commit
(attacker code spliced with base), executes inside a workflow that
has full repo-scoped tokens, and gives the attacker the chance to
exfiltrate secrets or push to default branches. `refs/pull/N/head`
is functionally equivalent — same source, same trust boundary.

Reproduced on `main` before this commit:

  $ cat /tmp/bad.yml
  name: bad
  on: { pull_request_target: { types: [opened] } }
  permissions: { contents: read }
  jobs:
    do:
      runs-on: ubuntu-latest
      steps:
        - uses: actions/checkout@v4
          with:
            ref: refs/pull/${{ github.event.pull_request.number }}/merge
            persist-credentials: false
        - run: npm ci --ignore-scripts

  $ ECC_WORKFLOWS_DIR=/tmp node scripts/ci/validate-workflow-security.js
  Validated workflow security for 1 workflow files
  $ echo $?
  0

Expected: violation flagging the refs/pull checkout under pull_request_target.
Actual: passes silently.

Fix: add a `refPattern` to the `pull_request_target` rule:

    /^\s*ref:\s*['"]?[^'"\n]*refs\/(?:remotes\/)?pull\/[^'"\n\s]+/m

and apply it per checkout step inside the existing
event-gated loop. The pattern matches the ref VALUE so it catches
all interpolation shapes — `refs/pull/123/head`,
`refs/pull/${{ github.event.pull_request.number }}/merge`,
`${{ env.FOO }}/refs/pull/N/head` — without enumerating the
possible interpolations themselves.

Scoping: the rule is already gated on the workflow containing
`pull_request_target:`, so non-privileged `pull_request` workflows
that legitimately check out a PR ref are not affected.

After this commit the reproduction above exits 1 with:

  ERROR: bad.yml:10 - pull_request_target must not checkout an untrusted pull_request head ref/repository

Three new regression tests in `tests/ci/validate-workflow-security.test.js`:
  - rejects pull_request_target + refs/pull/<N>/merge
  - rejects pull_request_target + hardcoded refs/pull/<N>/head
  - allows pull_request_target with no `with.ref:` (base-ref checkout —
    the safe pattern from GitHub's own guidance)

Test count: 17 → 20 in this file; full `yarn test` still green.

Together with the previous commit, this closes the two
independent `validate-workflow-security.js` bypasses I found.
This commit is contained in:
Jamkris
2026-05-15 15:00:42 +09:00
committed by Affaan Mustafa
parent 7f971b7e6f
commit cdbc925d89
2 changed files with 49 additions and 0 deletions
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28
tests/ci/validate-workflow-security.test.js
View File

@@ -99,6 +99,34 @@ function run() {
assert.match(result.stderr, /pull_request\.head\.sha/);
})) passed++; else failed++;
// `refs/pull/<N>/{head,merge}` under `pull_request_target` is the canonical
// privilege-escalation pattern that the standard `github.event.pull_request.head.*`
// expression check did not cover. Either form pulls attacker-controlled code
// into a privileged workflow.
if (test('rejects pull_request_target checkout fetching refs/pull/N/merge', () => {
const result = runValidator({
'unsafe-pr-target-merge-ref.yml': `name: Unsafe\non:\n pull_request_target:\n types: [opened]\njobs:\n inspect:\n runs-on: ubuntu-latest\n steps:\n - uses: actions/checkout@v4\n with:\n ref: refs/pull/\${{ github.event.pull_request.number }}/merge\n persist-credentials: false\n`,
});
assert.notStrictEqual(result.status, 0, 'Expected validator to fail on refs/pull/N/merge under pull_request_target');
assert.match(result.stderr, /pull_request_target must not checkout an untrusted pull_request head ref/);
})) passed++; else failed++;
if (test('rejects pull_request_target checkout fetching hardcoded refs/pull/N/head', () => {
const result = runValidator({
'unsafe-pr-target-head-ref.yml': `name: Unsafe\non:\n pull_request_target:\njobs:\n inspect:\n runs-on: ubuntu-latest\n steps:\n - uses: actions/checkout@v4\n with:\n ref: refs/pull/123/head\n persist-credentials: false\n`,
});
assert.notStrictEqual(result.status, 0, 'Expected validator to fail on hardcoded refs/pull/N/head');
assert.match(result.stderr, /pull_request_target must not checkout an untrusted pull_request head ref/);
})) passed++; else failed++;
if (test('allows pull_request_target checkout of the base ref (no with.ref)', () => {
const result = runValidator({
'safe-pr-target-base.yml': `name: Safe\non:\n pull_request_target:\njobs:\n inspect:\n runs-on: ubuntu-latest\n steps:\n - uses: actions/checkout@v4\n with:\n persist-credentials: false\n - run: echo inspecting base\n`,
});
assert.strictEqual(result.status, 0, result.stderr || result.stdout);
})) passed++; else failed++;
if (test('rejects shared cache use in pull_request_target workflows', () => {
const result = runValidator({
'unsafe-pr-target-cache.yml': `name: Unsafe\non:\n pull_request_target:\n branches: [main]\njobs:\n inspect:\n runs-on: ubuntu-latest\n steps:\n - uses: actions/cache@v5\n with:\n path: ~/.npm\n key: cache\n - run: echo inspect\n`,