Close remaining gaps in .mcp.json change protection

When mcpJsonChanged is true, delete enableAllProjectMcpServers from the
merged settings object so user-provided INPUT_SETTINGS cannot re-enable
it and bypass the check. Also fix the stale positional arg in the
standalone base-action call site and add test coverage for the bypass case.

base-action/src/index.ts

@@ -13,7 +13,7 @@ async function run() {
 
     await setupClaudeCodeSettings(
       process.env.INPUT_SETTINGS,
-      undefined, // homeDir
+      false, // mcpJsonChanged — standalone base-action has no PR context to check
     );
 
     // Install Claude Code plugins if specified

base-action/src/setup-claude-code-settings.ts

@@ -4,6 +4,7 @@ import { readFile } from "fs/promises";
 
 export async function setupClaudeCodeSettings(
   settingsInput?: string,
+  mcpJsonChanged = false,
   homeDir?: string,
 ) {
   const home = homeDir ?? homedir();
@@ -59,9 +60,19 @@ export async function setupClaudeCodeSettings(
     console.log(`Merged settings with input settings`);
   }
 
-  // Always set enableAllProjectMcpServers to true
-  settings.enableAllProjectMcpServers = true;
-  console.log(`Updated settings with enableAllProjectMcpServers: true`);
+  if (!mcpJsonChanged) {
+    // Only auto-approve project MCP servers if .mcp.json hasn't changed in this PR.
+    // If .mcp.json changed, the checked-out version may differ from the base branch.
+    settings.enableAllProjectMcpServers = true;
+    console.log(`Updated settings with enableAllProjectMcpServers: true`);
+  } else {
+    // .mcp.json is untrusted in this PR — strip any enableAllProjectMcpServers
+    // that came from user input so the protection cannot be bypassed.
+    delete settings.enableAllProjectMcpServers;
+    console.log(
+      `Removing enableAllProjectMcpServers because .mcp.json changed in this PR`,
+    );
+  }
 
   await $`echo ${JSON.stringify(settings, null, 2)} > ${settingsPath}`.quiet();
   console.log(`Settings saved successfully`);

base-action/test/setup-claude-code-settings.test.ts

@@ -28,7 +28,7 @@ describe("setupClaudeCodeSettings", () => {
   });
 
   test("should always set enableAllProjectMcpServers to true when no input", async () => {
-    await setupClaudeCodeSettings(undefined, testHomeDir);
+    await setupClaudeCodeSettings(undefined, false, testHomeDir);
 
     const settingsContent = await readFile(settingsPath, "utf-8");
     const settings = JSON.parse(settingsContent);
@@ -42,7 +42,7 @@ describe("setupClaudeCodeSettings", () => {
       env: { API_KEY: "test-key" },
     });
 
-    await setupClaudeCodeSettings(inputSettings, testHomeDir);
+    await setupClaudeCodeSettings(inputSettings, false, testHomeDir);
 
     const settingsContent = await readFile(settingsPath, "utf-8");
     const settings = JSON.parse(settingsContent);
@@ -69,7 +69,7 @@ describe("setupClaudeCodeSettings", () => {
 
     await writeFile(testSettingsPath, JSON.stringify(testSettings, null, 2));
 
-    await setupClaudeCodeSettings(testSettingsPath, testHomeDir);
+    await setupClaudeCodeSettings(testSettingsPath, false, testHomeDir);
 
     const settingsContent = await readFile(settingsPath, "utf-8");
     const settings = JSON.parse(settingsContent);
@@ -79,13 +79,13 @@ describe("setupClaudeCodeSettings", () => {
     expect(settings.permissions).toEqual(testSettings.permissions);
   });
 
-  test("should override enableAllProjectMcpServers even if false in input", async () => {
+  test("should override enableAllProjectMcpServers even if false in input when mcpJsonChanged is false", async () => {
     const inputSettings = JSON.stringify({
       enableAllProjectMcpServers: false,
       model: "test-model",
     });
 
-    await setupClaudeCodeSettings(inputSettings, testHomeDir);
+    await setupClaudeCodeSettings(inputSettings, false, testHomeDir);
 
     const settingsContent = await readFile(settingsPath, "utf-8");
     const settings = JSON.parse(settingsContent);
@@ -94,20 +94,64 @@ describe("setupClaudeCodeSettings", () => {
     expect(settings.model).toBe("test-model");
   });
 
+  test("should not set enableAllProjectMcpServers to true when mcpJsonChanged is true", async () => {
+    const inputSettings = JSON.stringify({
+      model: "test-model",
+    });
+
+    await setupClaudeCodeSettings(inputSettings, true, testHomeDir);
+
+    const settingsContent = await readFile(settingsPath, "utf-8");
+    const settings = JSON.parse(settingsContent);
+
+    expect(settings.enableAllProjectMcpServers).toBeUndefined();
+    expect(settings.model).toBe("test-model");
+  });
+
+  test("should remove enableAllProjectMcpServers when mcpJsonChanged is true and input sets it false", async () => {
+    const inputSettings = JSON.stringify({
+      enableAllProjectMcpServers: false,
+      model: "test-model",
+    });
+
+    await setupClaudeCodeSettings(inputSettings, true, testHomeDir);
+
+    const settingsContent = await readFile(settingsPath, "utf-8");
+    const settings = JSON.parse(settingsContent);
+
+    expect(settings.enableAllProjectMcpServers).toBeUndefined();
+    expect(settings.model).toBe("test-model");
+  });
+
+  test("should remove enableAllProjectMcpServers when mcpJsonChanged is true even if input sets it true", async () => {
+    const inputSettings = JSON.stringify({
+      enableAllProjectMcpServers: true,
+      model: "test-model",
+    });
+
+    await setupClaudeCodeSettings(inputSettings, true, testHomeDir);
+
+    const settingsContent = await readFile(settingsPath, "utf-8");
+    const settings = JSON.parse(settingsContent);
+
+    expect(settings.enableAllProjectMcpServers).toBeUndefined();
+    expect(settings.model).toBe("test-model");
+  });
+
   test("should throw error for invalid JSON string", async () => {
     expect(() =>
-      setupClaudeCodeSettings("{ invalid json", testHomeDir),
+      setupClaudeCodeSettings("{ invalid json", false, testHomeDir),
     ).toThrow();
   });
 
   test("should throw error for non-existent file path", async () => {
     expect(() =>
-      setupClaudeCodeSettings("/non/existent/file.json", testHomeDir),
+      setupClaudeCodeSettings("/non/existent/file.json", false, testHomeDir),
     ).toThrow();
   });
 
   test("should handle empty string input", async () => {
-    await setupClaudeCodeSettings("", testHomeDir);
+    await setupClaudeCodeSettings("", false, testHomeDir);
 
     const settingsContent = await readFile(settingsPath, "utf-8");
     const settings = JSON.parse(settingsContent);
@@ -116,7 +160,7 @@ describe("setupClaudeCodeSettings", () => {
   });
 
   test("should handle whitespace-only input", async () => {
-    await setupClaudeCodeSettings("   \n\t  ", testHomeDir);
+    await setupClaudeCodeSettings("   \n\t  ", false, testHomeDir);
 
     const settingsContent = await readFile(settingsPath, "utf-8");
     const settings = JSON.parse(settingsContent);
@@ -128,6 +172,7 @@ describe("setupClaudeCodeSettings", () => {
     // First, create some existing settings
     await setupClaudeCodeSettings(
       JSON.stringify({ existingKey: "existingValue" }),
+      false,
       testHomeDir,
     );
 
@@ -137,7 +182,7 @@ describe("setupClaudeCodeSettings", () => {
       model: "claude-opus-4-1-20250805",
     });
 
-    await setupClaudeCodeSettings(newSettings, testHomeDir);
+    await setupClaudeCodeSettings(newSettings, false, testHomeDir);
 
     const settingsContent = await readFile(settingsPath, "utf-8");
     const settings = JSON.parse(settingsContent);

src/entrypoints/run.ts

@@ -217,7 +217,39 @@ async function run() {
 
     validateEnvironmentVariables();
 
-    await setupClaudeCodeSettings(process.env.INPUT_SETTINGS);
+    // Check if .mcp.json changed in this PR to guard against RCE via malicious MCP servers.
+    // If .mcp.json was modified, skip auto-approving project MCP servers so attacker-injected
+    // servers are not trusted automatically.
+    let mcpJsonChanged = false;
+    if (isEntityContext(context) && context.isPR) {
+      try {
+        const changedFiles = await octokit.rest.paginate(
+          octokit.rest.pulls.listFiles,
+          {
+            owner: context.repository.owner,
+            repo: context.repository.repo,
+            pull_number: context.entityNumber,
+            per_page: 100,
+          },
+        );
+        mcpJsonChanged = changedFiles.some(
+          (f) =>
+            f.filename === ".mcp.json" || f.filename.endsWith("/.mcp.json"),
+        );
+        if (mcpJsonChanged) {
+          console.log(
+            ".mcp.json changed in this PR — disabling auto-approval of project MCP servers",
+          );
+        }
+      } catch (e) {
+        console.log(
+          `Could not check PR changed files: ${e}. Defaulting to mcpJsonChanged=true (fail-closed).`,
+        );
+        mcpJsonChanged = true;
+      }
+    }
+
+    await setupClaudeCodeSettings(process.env.INPUT_SETTINGS, mcpJsonChanged);
 
     await installPlugins(
       process.env.INPUT_PLUGIN_MARKETPLACES,