diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index 03129d8..ac7d27a 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -25,6 +25,10 @@ concurrency:
 jobs:
   test:
     runs-on: [self-hosted, Linux, X64]
+    # Do not run untrusted fork PR code on the self-hosted runner.
+    if: >-
+      github.event_name != 'pull_request' ||
+      github.event.pull_request.head.repo.full_name == github.repository
     steps:
       - uses: actions/checkout@v6
       - uses: actions/setup-java@v5