diff --git a/actions/setup/js/sanitize_content.test.cjs b/actions/setup/js/sanitize_content.test.cjs
index 2a215245e3..b8c83f8b75 100644
--- a/actions/setup/js/sanitize_content.test.cjs
+++ b/actions/setup/js/sanitize_content.test.cjs
@@ -131,6 +131,61 @@ describe("sanitize_content.cjs", () => {
     });
   });
 
+  describe("@mention bypass prevention (underscore-prefixed)", () => {
+    // Security tests for CVE-like vulnerability where underscore before @ could bypass sanitization
+    // These test cases are from the security report documenting the bypass patterns
+
+    it("should neutralize @mentions preceded by underscore in function names", () => {
+      const result = sanitizeContent("test_@user");
+      expect(result).toBe("test_`@user`");
+    });
+
+    it("should neutralize @mentions preceded by underscore in variable names", () => {
+      const result = sanitizeContent("production_@maintainer");
+      expect(result).toBe("production_`@maintainer`");
+    });
+
+    it("should neutralize @mentions preceded by underscore with hyphens", () => {
+      const result = sanitizeContent("validate_@security-team");
+      expect(result).toBe("validate_`@security-team`");
+    });
+
+    it("should neutralize @mentions preceded by underscore in commands", () => {
+      const result = sanitizeContent("run_@admin");
+      expect(result).toBe("run_`@admin`");
+    });
+
+    it("should neutralize @mentions preceded by multiple underscores", () => {
+      const result = sanitizeContent("My_Project_@owner");
+      expect(result).toBe("My_Project_`@owner`");
+    });
+
+    it("should neutralize @mentions with just underscore prefix", () => {
+      const result = sanitizeContent("_@user");
+      expect(result).toBe("_`@user`");
+    });
+
+    it("should neutralize @mentions preceded by underscore with possessive", () => {
+      const result = sanitizeContent("is_@user's project");
+      expect(result).toBe("is_`@user`'s project");
+    });
+
+    it("should neutralize multiple underscore-prefixed @mentions", () => {
+      const result = sanitizeContent("config_@admin and deploy_@maintainer");
+      expect(result).toBe("config_`@admin` and deploy_`@maintainer`");
+    });
+
+    it("should neutralize underscore-prefixed org/team mentions", () => {
+      const result = sanitizeContent("api_@org/team");
+      expect(result).toBe("api_`@org/team`");
+    });
+
+    it("should handle mixed normal and underscore-prefixed mentions", () => {
+      const result = sanitizeContent("Hello @user and test_@admin");
+      expect(result).toBe("Hello `@user` and test_`@admin`");
+    });
+  });
+
   describe("@mention allowedAliases", () => {
     it("should not neutralize mentions in allowedAliases list", () => {
       const result = sanitizeContent("Hello @author", { allowedAliases: ["author"] });
diff --git a/actions/setup/js/sanitize_content_core.cjs b/actions/setup/js/sanitize_content_core.cjs
index 77d39936fb..8e4773a5fc 100644
--- a/actions/setup/js/sanitize_content_core.cjs
+++ b/actions/setup/js/sanitize_content_core.cjs
@@ -297,7 +297,9 @@ function neutralizeCommands(s) {
 function neutralizeAllMentions(s) {
   // Replace @name or @org/team outside code with `@name`
   // No filtering - all mentions are neutralized
-  return s.replace(/(^|[^\w`])@([A-Za-z0-9](?:[A-Za-z0-9_-]{0,37}[A-Za-z0-9])?(?:\/[A-Za-z0-9._-]+)?)/g, (m, p1, p2) => {
+  // Changed [^\w`] to [^A-Za-z0-9`] to include underscore as a valid preceding character
+  // This prevents bypass patterns like "test_@user" from escaping sanitization
+  return s.replace(/(^|[^A-Za-z0-9`])@([A-Za-z0-9](?:[A-Za-z0-9_-]{0,37}[A-Za-z0-9])?(?:\/[A-Za-z0-9._-]+)?)/g, (m, p1, p2) => {
     // Log when a mention is escaped to help debug issues
     if (typeof core !== "undefined" && core.info) {
       core.info(`Escaped mention: @${p2} (not in allowed list)`);