diff --git a/src/cleanup.ts b/src/cleanup.ts
index 5a1c450..8d8e531 100644
--- a/src/cleanup.ts
+++ b/src/cleanup.ts
@@ -26,6 +26,11 @@ import { isPlatformSupported, isAgentInstalled } from "./utils";
     return;
   }
 
+  if (isGithubHosted() && process.platform === "linux" && !process.env.USER) {
+    console.log(common.UBUNTU_SLIM_MESSAGE);
+    return;
+  }
+
   if (isARCRunner()) {
     console.log(`[!] ${common.ARC_RUNNER_MESSAGE}`);
     return;
diff --git a/src/common.ts b/src/common.ts
index 81f36bf..0409778 100644
--- a/src/common.ts
+++ b/src/common.ts
@@ -148,3 +148,6 @@ export const ARM64_RUNNER_MESSAGE =
 
 export const ARM64_WINDOWS_RUNNER_MESSAGE =
   "Windows ARM runners are not yet supported by Harden-Runner.";
+
+export const UBUNTU_SLIM_MESSAGE =
+  "This job is running on an ubuntu-slim runner (GitHub-hosted Linux container without a standard USER environment variable). Harden Runner is not supported on ubuntu-slim runners. This job will not be monitored.";
diff --git a/src/setup.ts b/src/setup.ts
index bd45f33..b22fc75 100644
--- a/src/setup.ts
+++ b/src/setup.ts
@@ -64,6 +64,11 @@ interface MonitorResponse {
       return;
     }
 
+    if (isGithubHosted() && process.platform === "linux" && !process.env.USER) {
+      console.log(common.UBUNTU_SLIM_MESSAGE);
+      return;
+    }
+
     var correlation_id = uuidv4();
     var api_url = STEPSECURITY_API_URL;
     var web_url = STEPSECURITY_WEB_URL;