diff --git a/src/config.ts b/src/config.ts
index 18d3d85..efde5b0 100644
--- a/src/config.ts
+++ b/src/config.ts
@@ -49,9 +49,11 @@ const config = new Config(
   browserstackLocalOptions,
   process.env.USE_OWN_LOCAL_BINARY_PROCESS === "true",
   process.env.REMOTE_MCP === "true",
+  // Fail-closed: uploads are contained to the working directory unless the
+  // user explicitly widens the boundary via MCP_UPLOAD_BASE_DIR (PMAA-107).
   process.env.MCP_UPLOAD_BASE_DIR && process.env.MCP_UPLOAD_BASE_DIR.length > 0
     ? process.env.MCP_UPLOAD_BASE_DIR
-    : undefined,
+    : process.cwd(),
 );
 
 export default config;
diff --git a/src/lib/upload-validator.ts b/src/lib/upload-validator.ts
index d516d12..dc21a87 100644
--- a/src/lib/upload-validator.ts
+++ b/src/lib/upload-validator.ts
@@ -17,7 +17,8 @@ export interface UploadValidationOptions {
  *  - File extension is in `allowedExtensions` (case-insensitive)
  *  - No path segment is a hidden dir/file (starts with `.`); blocks ~/.ssh,
  *    ~/.aws, .env, etc. even after symlink resolution
- *  - If `allowedBaseDir` is set, the canonical path must live inside it
+ *  - The canonical path must live inside `allowedBaseDir` (defaults to the
+ *    process working directory when not provided — containment is fail-closed)
  */
 export function validateUploadPath(
   filePath: string,
@@ -71,23 +72,22 @@ export function validateUploadPath(
     );
   }
 
-  if (options.allowedBaseDir) {
-    let baseCanonical: string;
-    try {
-      baseCanonical = fs.realpathSync(path.resolve(options.allowedBaseDir));
-    } catch {
-      throw new Error(
-        `Upload rejected: configured MCP_UPLOAD_BASE_DIR does not exist (${options.allowedBaseDir}).`,
-      );
-    }
-    const baseWithSep = baseCanonical.endsWith(path.sep)
-      ? baseCanonical
-      : baseCanonical + path.sep;
-    if (canonical !== baseCanonical && !canonical.startsWith(baseWithSep)) {
-      throw new Error(
-        `Upload rejected: file must be located inside ${baseCanonical}.`,
-      );
-    }
+  const allowedBaseDir = options.allowedBaseDir ?? process.cwd();
+  let baseCanonical: string;
+  try {
+    baseCanonical = fs.realpathSync(path.resolve(allowedBaseDir));
+  } catch {
+    throw new Error(
+      `Upload rejected: configured MCP_UPLOAD_BASE_DIR does not exist (${allowedBaseDir}).`,
+    );
+  }
+  const baseWithSep = baseCanonical.endsWith(path.sep)
+    ? baseCanonical
+    : baseCanonical + path.sep;
+  if (canonical !== baseCanonical && !canonical.startsWith(baseWithSep)) {
+    throw new Error(
+      `Upload rejected: file must be located inside ${baseCanonical}. Set MCP_UPLOAD_BASE_DIR to allow uploads from a different directory.`,
+    );
   }
 
   return canonical;
diff --git a/tests/tools/upload-validator.test.ts b/tests/tools/upload-validator.test.ts
index 51eed47..444aa4c 100644
--- a/tests/tools/upload-validator.test.ts
+++ b/tests/tools/upload-validator.test.ts
@@ -1,7 +1,7 @@
 import fs from "fs";
 import os from "os";
 import path from "path";
-import { afterEach, beforeEach, describe, expect, it } from "vitest";
+import { afterEach, beforeEach, describe, expect, it, vi } from "vitest";
 import {
   validateUploadPath,
   APP_BINARY_EXTENSIONS,
@@ -32,10 +32,35 @@ describe("validateUploadPath", () => {
     const resolved = validateUploadPath(file, {
       allowedExtensions: APP_BINARY_EXTENSIONS,
       maxSizeBytes: MAX_APP_UPLOAD_BYTES,
+      allowedBaseDir: workDir,
     });
     expect(resolved).toBe(fs.realpathSync(file));
   });
 
+  it("defaults containment to the process working directory (fail-closed)", () => {
+    const outside = write("app.apk");
+    expect(() =>
+      validateUploadPath(outside, {
+        allowedExtensions: APP_BINARY_EXTENSIONS,
+        maxSizeBytes: MAX_APP_UPLOAD_BYTES,
+      }),
+    ).toThrow(/must be located inside/);
+  });
+
+  it("allows files inside the working directory when no base dir is set", () => {
+    const cwdSpy = vi.spyOn(process, "cwd").mockReturnValue(workDir);
+    try {
+      const file = write("app.apk");
+      const resolved = validateUploadPath(file, {
+        allowedExtensions: APP_BINARY_EXTENSIONS,
+        maxSizeBytes: MAX_APP_UPLOAD_BYTES,
+      });
+      expect(resolved).toBe(fs.realpathSync(file));
+    } finally {
+      cwdSpy.mockRestore();
+    }
+  });
+
   it("rejects an empty path", () => {
     expect(() =>
       validateUploadPath("   ", {
@@ -185,6 +210,7 @@ describe("validateUploadPath", () => {
     const resolved = validateUploadPath(file, {
       allowedExtensions: APP_BINARY_EXTENSIONS,
       maxSizeBytes: MAX_APP_UPLOAD_BYTES,
+      allowedBaseDir: workDir,
     });
     expect(resolved).toBe(fs.realpathSync(file));
   });