Harden CI: validate PR artifact inputs, migrate off deprecated Semgrep action (#5232)

* Harden CI workflows: validate PR artifact inputs and migrate off deprecated Semgrep action

Address two CI/supply-chain hardening items from the 2026-07-03 security
audit:

- comment_check_amalgamation.yml (todo 117): the privileged `workflow_run`
  job consumes an untrusted PR artifact. Validate `author` against a strict
  GitHub-username pattern and `number` as a positive integer before use, and
  extract the artifact into a dedicated directory (`unzip -o pr.zip -d
  ./pr_artifact`), reading only the two expected files by fixed path. This
  prevents Markdown/mention injection via the attacker-controlled `author`
  text and avoids a malicious archive touching the workspace.

- semgrep.yml (todo 118): `returntocorp/semgrep-action` is deprecated (the
  org was renamed to `semgrep/*`). Replace it with an explicit `semgrep ci`
  invocation via the maintained CLI; the deployment is inferred from
  SEMGREP_APP_TOKEN.

Todo 116 (CIFuzz `@master` refs) already carries a comment documenting the
OSS-Fuzz-recommended exception, so no change is needed there.

Signed-off-by: Niels Lohmann <mail@nlohmann.me>
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

* Fix Semgrep step: use `semgrep scan` instead of token-gated `semgrep ci`

The CI `Scan` job failed with "Path does not exist: semgrep.sarif" because
`semgrep ci` requires a login token (SEMGREP_APP_TOKEN), which this repo does
not have configured, so it bailed without producing a SARIF file. The former
returntocorp/semgrep-action, given no token, fell back to plain
`semgrep scan --sarif`; match that with `semgrep scan --config auto`, which
needs no token and always produces the SARIF for upload.

Signed-off-by: Niels Lohmann <mail@nlohmann.me>
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

---------

Signed-off-by: Niels Lohmann <mail@nlohmann.me>
Co-authored-by: Claude Opus 4.8 <noreply@anthropic.com>

.github/workflows/comment_check_amalgamation.yml

@@ -47,7 +47,10 @@ jobs:
 
             var hasPatch = artifacts.data.artifacts.some((artifact) => artifact.name == "amalgamation-patch");
             core.setOutput('has_patch', String(hasPatch));
-      - run: unzip pr.zip
+      # Extract the untrusted PR artifact into a dedicated empty directory and
+      # read only the two expected files by fixed path afterwards. This avoids a
+      # malicious archive overwriting workspace files or escaping via ../ paths.
+      - run: unzip -o pr.zip -d ./pr_artifact
 
       - name: 'Comment on PR'
         uses: actions/github-script@3a2844b7e9c422d3c10d287c895573f7108da1b3 # v9.0.0
@@ -55,8 +58,19 @@ jobs:
           github-token: ${{ secrets.GITHUB_TOKEN }}
           script: |
             var fs = require('fs');
-            const author = fs.readFileSync('./author')
-            const issue_number = Number(fs.readFileSync('./number'));
+            // Both values come from a fork-triggered workflow and are therefore
+            // attacker-controlled. Validate them strictly before use to prevent
+            // Markdown/mention injection and bogus REST API filters.
+            const author = fs.readFileSync('./pr_artifact/author', 'utf8').trim();
+            if (!/^[A-Za-z0-9-]{1,39}$/.test(author)) {
+              core.setFailed(`Refusing to proceed: untrusted author value '${author}' is not a valid GitHub username.`);
+              return;
+            }
+            const issue_number = Number(fs.readFileSync('./pr_artifact/number', 'utf8').trim());
+            if (!Number.isInteger(issue_number) || issue_number <= 0) {
+              core.setFailed('Refusing to proceed: untrusted PR number is not a positive integer.');
+              return;
+            }
             const opts = github.rest.issues.listForRepo.endpoint.merge({
               owner: context.repo.owner,
               repo: context.repo.repo,

.github/workflows/semgrep.yml

@@ -41,12 +41,23 @@ jobs:
         with:
           persist-credentials: false
 
-      # Scan code using project's configuration on https://semgrep.dev/manage
-      - uses: returntocorp/semgrep-action@713efdd345f3035192eaa63f56867b88e63e4e5d
-        with:
-          publishToken: ${{ secrets.SEMGREP_APP_TOKEN }}
-          publishDeployment: ${{ secrets.SEMGREP_DEPLOYMENT_ID }}
-          generateSarif: "1"
+      # The former returntocorp/semgrep-action is deprecated (the org was renamed
+      # to semgrep/*); the maintained approach is to install the CLI and invoke
+      # it directly. We use `semgrep scan` (not `semgrep ci`, which requires a
+      # login token): with no SEMGREP_APP_TOKEN configured this is exactly what
+      # the old action fell back to, running community rules with no token.
+      # SEMGREP_APP_TOKEN is still passed through so registry auth works if a
+      # token is ever added.
+      - name: Install Semgrep
+        run: python3 -m pip install --user semgrep
+
+      # `semgrep scan --sarif` always exits 0 even with findings; continue-on-error
+      # is a safety net so the SARIF upload still runs if the scan itself errors.
+      - name: Run Semgrep
+        run: semgrep scan --config auto --sarif --output=semgrep.sarif
+        continue-on-error: true
+        env:
+          SEMGREP_APP_TOKEN: ${{ secrets.SEMGREP_APP_TOKEN }}
 
       # Upload SARIF file generated in previous step
       - name: Upload SARIF file