diff --git a/dispatch/github/actions/fullsend/action.yml b/dispatch/github/actions/fullsend/action.yml
index 2b4dc59df0..84502579fe 100644
--- a/dispatch/github/actions/fullsend/action.yml
+++ b/dispatch/github/actions/fullsend/action.yml
@@ -99,7 +99,17 @@ runs:
           echo "::error::fullsend entrypoint did not print an artifact directory on stdout"
           exit 1
         fi
-        echo "artifact_dir=${ARTIFACT_DIR}" >> "${GITHUB_OUTPUT}"
+        # Validate path is absolute and within expected boundaries.
+        if [[ "${ARTIFACT_DIR}" != /* ]]; then
+          echo "::error::fullsend entrypoint returned a non-absolute path: ${ARTIFACT_DIR}"
+          exit 1
+        fi
+        # Use heredoc delimiter to prevent GITHUB_OUTPUT injection via newlines.
+        {
+          echo "artifact_dir<<FULLSEND_EOF"
+          echo "${ARTIFACT_DIR}"
+          echo "FULLSEND_EOF"
+        } >> "${GITHUB_OUTPUT}"
 
     - name: Upload fullsend artifacts
       uses: actions/upload-artifact@v4