fix: harden IPC, screenshot, paging, and tab filter error handling

- tabs.py: validate screenshot data URL prefix and catch binascii.Error
  instead of silently writing a zero-byte file or crashing with a raw traceback
- serve.py: add 30 s recv timeout on client connections to prevent unbounded
  thread accumulation; use hmac.compare_digest for constant-time token check
- native_host.py: bind Unix socket before _registry_add to eliminate the
  window where the registry points to an unbound path; cap paging loop at
  ceil(10000/PAGE_SIZE) iterations to guard against a misbehaving extension;
  remove dead no-hello fast-path queue that was registered but never consumed
- __init__.py: narrow _apply_tab_filter except to (AttributeError, TypeError)
  so broken filter functions raise instead of silently returning wrong results

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

browser_cli/__init__.py

@@ -685,8 +685,8 @@ class BrowserCLI:
 
         try:
             transformed = filter_fn(tabs)
-        except Exception:
-            transformed = None
+        except (AttributeError, TypeError):
+            return [tab for tab in tabs if filter_fn(tab)]
 
         if isinstance(transformed, list):
             return transformed

browser_cli/commands/serve.py

@@ -1,4 +1,4 @@
-import threading, secrets, socket, struct, click, json, sys
+import hmac, threading, secrets, socket, struct, click, json, sys
 from rich.console import Console
 from datetime import datetime
 
@@ -51,7 +51,7 @@ def _proxy_request(client_sock:socket.socket, addr:tuple, profile:str|None, serv
   command = msg.get("command", "?")
 
   if server_token is not None:
-    if msg.get("token") != server_token:
+    if not hmac.compare_digest(msg.get("token") or "", server_token):
       _send_error(msg_id, "unauthorized: invalid or missing token")
       _log(addr, command, None, "DENIED", "bad token")
       return
@@ -110,6 +110,7 @@ def _proxy_request(client_sock:socket.socket, addr:tuple, profile:str|None, serv
     _log(addr, command, resolved_profile, "ERROR", str(e))
 
 def _handle_client(client_sock:socket.socket, addr:tuple, profile:str|None, server_token:str|None) -> None:
+  client_sock.settimeout(30)
   with client_sock:
     _proxy_request(client_sock, addr, profile, server_token)
 

browser_cli/commands/tabs.py

@@ -1,4 +1,5 @@
 import base64
+import binascii
 import click
 from browser_cli.client import BrowserNotConnected, active_browser_targets, remote_browser_targets, send_command
 from rich.console import Console
@@ -288,7 +289,12 @@ def tabs_screenshot(output, tab_id, fmt, quality):
     data_url = result.get("dataUrl", "") if isinstance(result, dict) else ""
     if output:
         header = f"data:image/{fmt};base64,"
-        raw = base64.b64decode(data_url[len(header):])
+        if not data_url.startswith(header):
+            raise click.ClickException("Empty or unexpected screenshot response (incognito/protected tab?)")
+        try:
+            raw = base64.b64decode(data_url[len(header):])
+        except binascii.Error as e:
+            raise click.ClickException(f"Failed to decode screenshot data: {e}")
         with open(output, "wb") as f:
             f.write(raw)
         console.print(f"[green]Screenshot saved:[/green] {output}")

browser_cli/native_host.py

@@ -7,6 +7,7 @@ It relays messages between extension (stdin/stdout Native Messaging protocol)
 and CLI (local IPC endpoint: Unix socket on Unix, named pipe on Windows).
 """
 import json
+import math
 import os
 import queue
 import socket
@@ -121,7 +122,7 @@ def stdin_reader(alias: str):
 
 # --- Thread B: accept CLI socket connections ---
 
-def socket_server(sock_path: str):
+def socket_server(sock_path: str, bound_sock: "socket.socket | None" = None):
     if is_windows():
         while True:
             try:
@@ -132,13 +133,14 @@ def socket_server(sock_path: str):
             threading.Thread(target=handle_cli_connection, args=(conn, listener), daemon=True).start()
         return
 
-    path = Path(sock_path)
-    if path.exists():
-        path.unlink()
-
-    sock = socket.socket(socket.AF_UNIX, socket.SOCK_STREAM)
-    sock.bind(sock_path)
-    sock.listen(16)
+    sock = bound_sock
+    if sock is None:
+        path = Path(sock_path)
+        if path.exists():
+            path.unlink()
+        sock = socket.socket(socket.AF_UNIX, socket.SOCK_STREAM)
+        sock.bind(sock_path)
+        sock.listen(16)
 
     while True:
         try:
@@ -212,8 +214,13 @@ def _collect_paged_browser_command(cmd: dict) -> dict:
     offset = 0
     items = []
     total = None
+    max_pages = math.ceil(10_000 / PAGE_SIZE)
+    pages_fetched = 0
 
     while True:
+        if pages_fetched >= max_pages:
+            return {"id": original_id, "success": False, "error": f"paging loop exceeded {max_pages} pages — extension bug?"}
+        pages_fetched += 1
         page_cmd = dict(cmd)
         page_cmd["id"] = str(uuid.uuid4())
         page_args = dict(cmd.get("args") or {})
@@ -284,21 +291,25 @@ def main():
     if first_msg and first_msg.get("type") == "hello":
         alias = _resolve_profile_alias(first_msg)
     else:
-        # No hello — use a generated alias and re-queue the first command if needed.
+        # No hello — use a generated alias; first_msg is dropped (no response path).
         alias = str(uuid.uuid4())
-        if first_msg:
-            msg_id = first_msg.get("id")
-            if msg_id:
-                q: queue.Queue = queue.Queue()
-                with PENDING_LOCK:
-                    PENDING[msg_id] = q
-                write_native_message(sys.stdout.buffer, first_msg)
 
     runtime_dir().mkdir(mode=0o700, exist_ok=True)
     sock_path = _socket_path_for(alias)
+
+    if not is_windows():
+        path = Path(sock_path)
+        if path.exists():
+            path.unlink()
+        bound_sock = socket.socket(socket.AF_UNIX, socket.SOCK_STREAM)
+        bound_sock.bind(sock_path)
+        bound_sock.listen(16)
+    else:
+        bound_sock = None
+
     _registry_add(alias, sock_path)
 
-    t = threading.Thread(target=socket_server, args=(sock_path,), daemon=True)
+    t = threading.Thread(target=socket_server, args=(sock_path,), kwargs={"bound_sock": bound_sock}, daemon=True)
     t.start()
 
     stdin_reader(alias)