diff --git a/src/main.ts b/src/main.ts
index aab606c..41ec229 100644
--- a/src/main.ts
+++ b/src/main.ts
@@ -40,7 +40,13 @@ import {
   RetryPolicy,
   VpcConnectorEgressSettings,
 } from './client';
-import { formatEntry, parseEventTriggerFilters, parseSecrets, stringToInt } from './util';
+import {
+  formatEntry,
+  parseEventTriggerFilters,
+  parseSecrets,
+  stringToInt,
+  validateUniverse,
+} from './util';
 
 async function run() {
   try {
@@ -49,6 +55,11 @@ async function run() {
     const region = presence(getInput('region')) || 'us-central1';
     const universe = getInput('universe') || 'googleapis.com';
 
+    // Validate universe before it is interpolated into the Cloud Functions
+    // endpoint URL, otherwise a value carrying URL syntax can route the
+    // credentialed request to an attacker-controlled host (SSRF).
+    validateUniverse(universe);
+
     // top-level inputs
     const name = getInput('name', { required: true });
     const description = presence(getInput('description'));
diff --git a/src/util.ts b/src/util.ts
index 9bc2e60..e4aab40 100644
--- a/src/util.ts
+++ b/src/util.ts
@@ -225,3 +225,33 @@ export function parseSecrets(
 
   return [secretEnvVars, secretVolumes];
 }
+
+/**
+ * universePattern matches a well-formed DNS hostname (RFC 1123 labels, total
+ * length <= 253). It deliberately excludes scheme, path, port, userinfo, query,
+ * and fragment characters so the value can only ever be a host.
+ */
+const universePattern =
+  /^(?=.{1,253}$)[a-z0-9]([a-z0-9-]{0,61}[a-z0-9])?(\.[a-z0-9]([a-z0-9-]{0,61}[a-z0-9])?)+$/;
+
+/**
+ * validateUniverse ensures the universe value is a bare DNS hostname before it
+ * is interpolated into the Cloud Functions API endpoint
+ * (`https://cloudfunctions.${universe}/v2`). A value carrying URL syntax can
+ * redirect the credentialed request — including the GCP access token in the
+ * Authorization header — to an attacker-controlled host (e.g. "attacker.com#"
+ * truncates the real host via the fragment delimiter). Validating hostname form
+ * (rather than allowlisting googleapis.com) still accepts every legitimate
+ * universe — googleapis.com, Trusted Partner Cloud, and Google Distributed
+ * Cloud domains — without breaking sovereign deployments.
+ *
+ * @param universe Universe value to validate.
+ */
+export function validateUniverse(universe: string): void {
+  if (!universePattern.test(universe)) {
+    throw new Error(
+      `Invalid universe "${universe}": must be a bare DNS hostname ` +
+        `(e.g. "googleapis.com"), with no scheme, path, port, or other URL characters.`,
+    );
+  }
+}
diff --git a/tests/util.test.ts b/tests/util.test.ts
index 442c0b6..edf7a58 100644
--- a/tests/util.test.ts
+++ b/tests/util.test.ts
@@ -20,7 +20,7 @@ import assert from 'node:assert';
 import StreamZip from 'node-stream-zip';
 import { assertMembers, randomFilepath } from '@google-github-actions/actions-utils';
 
-import { parseEventTriggerFilters, stringToInt, zipDir } from '../src/util';
+import { parseEventTriggerFilters, stringToInt, validateUniverse, zipDir } from '../src/util';
 
 test('#zipDir', { concurrency: true }, async (suite) => {
   const cases = [
@@ -182,3 +182,30 @@ async function getFilesInZip(zipFilePath: string): Promise<string[]> {
   }
   return filesInsideZip;
 }
+
+test('#validateUniverse', { concurrency: true }, async (suite) => {
+  await suite.test('accepts legitimate universes (public, TPC, GDC)', () => {
+    for (const universe of ['googleapis.com', 'us-central1.rep.googleapis.com', 'apis-tpc.goog']) {
+      assert.doesNotThrow(() => validateUniverse(universe), `expected "${universe}" to be valid`);
+    }
+  });
+
+  await suite.test('rejects values carrying URL syntax (SSRF guard)', () => {
+    for (const universe of [
+      'attacker.com#.googleapis.com', // fragment truncates the real host
+      'attacker.com#',
+      'attacker.com/path',
+      'attacker.com:8080',
+      'user@attacker.com',
+      'https://attacker.com',
+      'attacker .com',
+      '',
+    ]) {
+      assert.throws(
+        () => validateUniverse(universe),
+        /Invalid universe/,
+        `expected "${universe}" to be rejected`,
+      );
+    }
+  });
+});