Browser Use allows bypassing allowed_domains by putting a decoy domain in http auth username portion of a URL
Summary
During a manual source code review, [ARIMLABS.AI](https://arimlabs.ai) researchers identified that the browser_use module includes an embedded whitelist functionality to restrict URLs that can be visited. This restriction is enforced during agent initialization. However, it was discovered that these measures can be bypassed, leading to severe security implications.
Details
File: browser_use/browser/context.py
The BrowserContextConfig class defines an allowed_domains list, which is intended to limit accessible domains. This list is checked in the _is_url_allowed() method before navigation:
```python
@dataclass
class BrowserContextConfig:
"""
[STRIPPED]
"""
cookies_file: str | None = None
minimum_wait_page_load_time: float = 0.5
wait_for_network_idle_page_load_time: float = 1
maximum_wait_page_load_time: float = 5
wait_between_actions: float = 1
disable_security: bool = True
browser_window_size: BrowserContextWindowSize = field(default_factory=lambda: {'width': 1280, 'height': 1100})
no_viewport: Optional[bool] = None
save_recording_path: str | None = None
save_downloads_path: str | None = None
trace_path: str | None = None
locale: str | None = None
user_agent: str = (
'Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/85.0.4183.102 Safari/537.36'
)
highlight_elements: bool = True
viewport_expansion: int = 500
allowed_domains: list[str] | None = None
include_dynamic_attributes: bool = True
_force_keep_context_alive: bool = False
```
The _is_url_allowed() method is responsible for checking whether a given URL is permitted:
```python
def _is_url_allowed(self, url: str) -> bool:
"""Check if a URL is allowed based on the whitelist configuration."""
if not self.config.allowed_domains:
return True
try:
from urllib.parse import urlparse
parsed_url = urlparse(url)
domain = parsed_url.netloc.lower()
# Remove port number if present
if ':' in domain:
domain = domain.split(':')[0]
# Check if domain matches any allowed domain pattern
return any(
domain == allowed_domain.lower() or domain.endswith('.' + allowed_domain.lower())
for allowed_domain in self.config.allowed_domains
)
except Exception as e:
logger.error(f'Error checking URL allowlist: {str(e)}')
return False
```
The core issue stems from the line domain = domain.split(':')[0], which allows an attacker to manipulate basic authentication credentials by providing a username:password pair. By replacing the username with a whitelisted domain, the check can be bypassed, even though the actual domain remains different.
Proof of Concept (PoC)
Set allowed_domains to ['example.com'] and use the following URL:
https://example.com:pass@localhost:8080
This allows bypassing all whitelist controls and accessing restricted internal services.
Impact
- Affected all users relying on this functionality for security.
- Potential for unauthorized enumeration of localhost services and internal networks.
- Ability to bypass domain whitelisting, leading to unauthorized browsing.
