CVE-2021-41168

Snudown Markdown parser Hash collision DoS

• CVSS 6.5
• CWE-407
• Resource Management
• Remote

Snudown is a reddit-specific fork of the Sundown Markdown parser used by GitHub, with Python integration added. In affected versions snudown was found to be vulnerable to denial of service attacks to its reference table implementation. References written in markdown ` [reference_name]: https://www.example.com` are inserted into a hash table which was found to have a weak hash function, meaning that an attacker can reliably generate a large number of collisions for it. This makes the hash table vulnerable to a hash-collision DoS attack, a type of algorithmic complexity attack. Further the hash table allowed for duplicate entries resulting in long retrieval times. Proofs of concept and further discussion of the hash collision issue are discussed on the snudown GHSA(https://github.com/reddit/snudown/security/advisories/GHSA-6gvv-9q92-w5f6). Users are advised to update to version 1.7.0.

CVE

CVE-2021-41168

CWE

CWE-407

CVSS base score

6.5

Published

2021-10-21

OWASP

A08 Software and Data Integrity Failures

Orthogonal defect classification

Algorithm

Code defect classification

Incorrect Algorithm

Category

Resource Management

Subcategory

Resource Exhaustion

Accessibility scope

Remote

Impact

Denial of Service (DoS)

Fixed by upgrading

Yes

Solution

Update Snudown to version 1.7.0 or higher.

class ReferenceTable:
    def __init__(self):
        self.references = {}

    def add_reference(self, name, url):
        if name in self.references:
            self.references[name].append(url)
        else:
            self.references[name] = [url]

    def get_reference(self, name):
        return self.references.get(name, None)

table = ReferenceTable()
for i in range(1000):
    table.add_reference("collision_key", f"https://www.example.com/{i}")
print(table.get_reference("collision_key"))

import re

class ReferenceTable:
    def __init__(self, max_refs_per_key=100):
        self.references = {}
        self.max_refs_per_key = max_refs_per_key

    def is_valid_url(self, url):
        return re.match(r'^https://[a-zA-Z0-9./_-]+$', url) is not None

    def add_reference(self, name, url):
        if not self.is_valid_url(url):
            return

        refs = self.references.get(name, [])
        if len(refs) < self.max_refs_per_key:
            refs.append(url)
            self.references[name] = refs

    def get_reference(self, name):
        return self.references.get(name, [])

table = ReferenceTable()
for i in range(1000):
    table.add_reference("collision_key", f"https://www.example.com/{i}")
print(table.get_reference("collision_key"))

@misc{vaitp:cve202141168,
  title        = {{Snudown Markdown parser Hash collision DoS}},
  author       = {Bogaerts, Fr\'ed\'eric and Ivaki, Naghmeh and Fonseca, Jos\'e},
  year         = {2021},
  note         = {VAITP Python Vulnerability Dataset, entry CVE-2021-41168},
  howpublished = {\url{https://netpack.pt/vaitp/vulnerability/CVE-2021-41168/}}
}