CVE-2021-41250

Bypassing token filter with non-blacklisted URL

• CVSS 4.3
• CWE-20 Improper Input Validation
• Input Validation and Sanitization
• Remote

Python discord bot is the community bot for the Python Discord community. In affected versions when a non-blacklisted URL and an otherwise triggering filter token is included in the same message the token filter does not trigger. This means that by including any non-blacklisted URL moderation filters can be bypassed. This issue has been resolved in commit 67390298852513d13e0213870e50fb3cff1424e0

CVE

CVE-2021-41250

CWE

CWE-20 Improper Input Validation

CVSS base score

4.3

Published

2021-11-05

OWASP

A05 Security Misconfiguration

Orthogonal defect classification

Function

Code defect classification

Incorrect Functionality

Category

Input Validation and Sanitization

Subcategory

Insecure Parsing or Deserialization

Accessibility scope

Remote

Impact

Unauthorized Access

Fixed by upgrading

Yes

Solution

Update Python Discord bot to version with commit 67390298852513d13e0213870e50fb3cff1424e0 or higher.

import re

BLACKLISTED_URLS = ["http://example.com", "http://example2.com"]

def is_blacklisted_url(url):
    return url in BLACKLISTED_URLS

def filter_message(message):
    filter_tokens = ["badword", "trigger"]
    
    if any(token in message for token in filter_tokens):
        return "Message contains a triggering filter token."

    if any(is_blacklisted_url(url) for url in re.findall(r'http[s]?://\S+', message)):
        return "Message contains a blacklisted URL."

    return "Message is clean."

message_with_url_and_token = "Check this link http://example.com and this badword."
print(filter_message(message_with_url_and_token))

import re
import os

BLACKLISTED_URLS = os.getenv('BLACKLISTED_URLS', '').split(',')
FILTER_TOKENS = os.getenv('FILTER_TOKENS', '').split(',')

def is_blacklisted_url(url):
    url = url.strip().lower()
    return url in BLACKLISTED_URLS

def filter_message(message):
    if not isinstance(message, str):
        return "Message is invalid."

    if any(token.lower() in message.lower() for token in FILTER_TOKENS):
        return "Message contains a triggering filter token."

    urls = re.findall(r'https?://[a-zA-Z0-9-_.]+(?:/[a-zA-Z0-9-_.]*)*', message)
    
    if any(is_blacklisted_url(url) for url in urls):
        return "Message contains a blacklisted URL."

    return "Message is clean."

message_with_url_and_token = "Check this link http://example.com and this badword."
print(filter_message(message_with_url_and_token))

@misc{vaitp:cve202141250,
  title        = {{Bypassing token filter with non-blacklisted URL}},
  author       = {Bogaerts, Fr\'ed\'eric and Ivaki, Naghmeh and Fonseca, Jos\'e},
  year         = {2021},
  note         = {VAITP Python Vulnerability Dataset, entry CVE-2021-41250},
  howpublished = {\url{https://netpack.pt/vaitp/vulnerability/CVE-2021-41250/}}
}