diff --git a/.github/actions/sign-image/action.yml b/.github/actions/sign-image/action.yml
index 9aef124..cb3900a 100644
--- a/.github/actions/sign-image/action.yml
+++ b/.github/actions/sign-image/action.yml
@@ -31,7 +31,11 @@ runs:
         registry-type: public
 
     - name: Install cosign
-      uses: sigstore/cosign-installer@v4.1.2
+      uses: sigstore/cosign-installer@6f9f17788090df1f26f669e9d70d6ae9567deba6 # v4.1.2
+      with:
+        # Explicit and pinned: the installer's default drifts with its own
+        # releases; v3.1.3 is the latest stable with no known advisories.
+        cosign-release: 'v3.1.3'
 
     - name: Sign image
       shell: bash