fix(core): secure argsPattern and revert WEB_FETCH_TOOL_NAME escalation (#22104)

Co-authored-by: Taylor Mullen <ntaylormullen@google.com>

packages/core/src/policy/utils.ts

@@ -89,6 +89,25 @@ export function buildArgsPatterns(
   return [argsPattern];
 }
 
+/**
+ * Builds a regex pattern to match a specific parameter and value in tool arguments.
+ * This is used to narrow tool approvals to specific parameters.
+ *
+ * @param paramName The name of the parameter.
+ * @param value The value to match.
+ * @returns A regex string that matches "<paramName>":<value> in a JSON string.
+ */
+export function buildParamArgsPattern(
+  paramName: string,
+  value: unknown,
+): string {
+  const encodedValue = JSON.stringify(value);
+  // We wrap the JSON string in escapeRegex and prepend/append \\0 to explicitly
+  // match top-level JSON properties generated by stableStringify, preventing
+  // argument injection bypass attacks.
+  return `\\\\0${escapeRegex(`"${paramName}":${encodedValue}`)}\\\\0`;
+}
+
 /**
  * Builds a regex pattern to match a specific file path in tool arguments.
  * This is used to narrow tool approvals for edit tools to specific files.
@@ -97,11 +116,18 @@ export function buildArgsPatterns(
  * @returns A regex string that matches "file_path":"<path>" in a JSON string.
  */
 export function buildFilePathArgsPattern(filePath: string): string {
-  const encodedPath = JSON.stringify(filePath);
-  // We must wrap the JSON string in escapeRegex to ensure regex control characters
-  // (like '.' in file extensions) are treated as literals, preventing overly broad
-  // matches (e.g. 'foo.ts' matching 'fooXts').
-  return escapeRegex(`"file_path":${encodedPath}`);
+  return buildParamArgsPattern('file_path', filePath);
+}
+
+/**
+ * Builds a regex pattern to match a specific directory path in tool arguments.
+ * This is used to narrow tool approvals for list_directory tool.
+ *
+ * @param dirPath The path to the directory.
+ * @returns A regex string that matches "dir_path":"<path>" in a JSON string.
+ */
+export function buildDirPathArgsPattern(dirPath: string): string {
+  return buildParamArgsPattern('dir_path', dirPath);
 }
 
 /**
@@ -112,7 +138,5 @@ export function buildFilePathArgsPattern(filePath: string): string {
  * @returns A regex string that matches "pattern":"<pattern>" in a JSON string.
  */
 export function buildPatternArgsPattern(pattern: string): string {
-  const encodedPattern = JSON.stringify(pattern);
-  // We use escapeRegex to ensure regex control characters are treated as literals.
-  return escapeRegex(`"pattern":${encodedPattern}`);
+  return buildParamArgsPattern('pattern', pattern);
 }