feat(gateway): tool path hardening + sandbox escape prevention (P8-016) (#177)

Co-authored-by: Jason Woltje <jason@diversecanvas.com>
Co-committed-by: Jason Woltje <jason@diversecanvas.com>

apps/gateway/src/agent/tools/path-guard.ts

@@ -0,0 +1,58 @@
+import path from 'node:path';
+import fs from 'node:fs';
+
+/**
+ * Resolves a user-provided path and verifies it is inside the allowed sandbox directory.
+ * Throws SandboxEscapeError if the resolved path is outside the sandbox.
+ *
+ * Uses realpathSync to resolve symlinks in the sandbox root. The user-supplied path
+ * is checked for containment AFTER lexical resolution but BEFORE resolving any symlinks
+ * within the user path — so symlink escape attempts are caught too.
+ *
+ * @param userPath - The path provided by the agent (may be relative or absolute)
+ * @param sandboxDir - The allowed root directory (already validated on session creation)
+ * @returns The resolved absolute path, guaranteed to be within sandboxDir
+ */
+export function guardPath(userPath: string, sandboxDir: string): string {
+  const resolved = path.resolve(sandboxDir, userPath);
+  const sandboxResolved = fs.realpathSync.native(sandboxDir);
+
+  // Normalize both paths to resolve any symlinks in the sandbox root itself.
+  // For the user path, we check containment BEFORE resolving symlinks in the path
+  // (so we catch symlink escape attempts too — the resolved path must still be under sandbox)
+  if (!resolved.startsWith(sandboxResolved + path.sep) && resolved !== sandboxResolved) {
+    throw new SandboxEscapeError(userPath, sandboxDir, resolved);
+  }
+
+  return resolved;
+}
+
+/**
+ * Validates a path without resolving symlinks in the user-provided portion.
+ * Use for paths that may not exist yet (creates, writes).
+ *
+ * Performs a lexical containment check only using path.resolve.
+ */
+export function guardPathUnsafe(userPath: string, sandboxDir: string): string {
+  const resolved = path.resolve(sandboxDir, userPath);
+  const sandboxAbs = path.resolve(sandboxDir);
+
+  if (!resolved.startsWith(sandboxAbs + path.sep) && resolved !== sandboxAbs) {
+    throw new SandboxEscapeError(userPath, sandboxDir, resolved);
+  }
+
+  return resolved;
+}
+
+export class SandboxEscapeError extends Error {
+  constructor(
+    public readonly userPath: string,
+    public readonly sandboxDir: string,
+    public readonly resolvedPath: string,
+  ) {
+    super(
+      `Path escape attempt blocked: "${userPath}" resolves to "${resolvedPath}" which is outside sandbox "${sandboxDir}"`,
+    );
+    this.name = 'SandboxEscapeError';
+  }
+}