CVE-2021-21273

Synapse Matrix server IP address restriction issue

• CVSS 6.1
• CWE-601 URL Redirection to Untrusted Site ('Open Redirect')
• Configuration Issues
• Remote

Synapse is a Matrix reference homeserver written in python (pypi package matrix-synapse). Matrix is an ecosystem for open federated Instant Messaging and VoIP. In Synapse before version 1.25.0, requests to user provided domains were not restricted to external IP addresses when calculating the key validity for third-party invite events and sending push notifications. This could cause Synapse to make requests to internal infrastructure. The type of request was not controlled by the user, although limited modification of request bodies was possible. For the most thorough protection server administrators should remove the deprecated `federation_ip_range_blacklist` from their settings after upgrading to Synapse v1.25.0 which will result in Synapse using the improved default IP address restrictions. See the new `ip_range_blacklist` and `ip_range_whitelist` settings if more specific control is necessary.

CVE

CVE-2021-21273

CWE

CWE-601 URL Redirection to Untrusted Site ('Open Redirect')

CVSS base score

6.1

Published

2021-02-26

OWASP

A10 Server Side Request Forgery (SSRF)

Orthogonal defect classification

Function

Code defect classification

Incorrect Functionality

Category

Configuration Issues

Subcategory

Server-Side Request Forgery (SSRF)

Accessibility scope

Remote

Impact

Unauthorized Access

Fixed by upgrading

Yes

Solution

Update Synapse to version 1.25.0 or higher.

def handle_third_party_invite(event, user_ip):
    request_url = event['invite_url']
    response = make_request_to_url(request_url, event['request_body'])
    
    print("Invite processed with response:", response)

def make_request_to_url(url, body):
    return "Response from {}".format(url)

import ipaddress

def is_ip_allowed(ip, allowed_ranges):
    ip_obj = ipaddress.ip_address(ip)
    for ip_range in allowed_ranges:
        if ip_obj in ipaddress.ip_network(ip_range):
            return True
    return False

def handle_third_party_invite(event, user_ip):
    allowed_ip_ranges = [
        '192.0.2.0/24',
        '203.0.113.0/24'
    ]

    if not is_ip_allowed(user_ip, allowed_ip_ranges):
        raise ValueError("IP address not allowed for this request.")

    print("Invite processed successfully.")

@misc{vaitp:cve202121273,
  title        = {{Synapse Matrix server IP address restriction issue}},
  author       = {Bogaerts, Fr\'ed\'eric and Ivaki, Naghmeh and Fonseca, Jos\'e},
  year         = {2021},
  note         = {VAITP Python Vulnerability Dataset, entry CVE-2021-21273},
  howpublished = {\url{https://netpack.pt/vaitp/vulnerability/CVE-2021-21273/}}
}