diff --git a/.github/workflows/ci.yml b/.github/workflows/ci.yml
index 0049301..ce8da9c 100644
--- a/.github/workflows/ci.yml
+++ b/.github/workflows/ci.yml
@@ -9,6 +9,10 @@ on:
 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
       - name: Wait for dpkg lock
@@ -32,6 +36,10 @@ jobs:
 
   docker-build:
     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
       - name: Build Docker image