CVE-2021-33026

Flask-Caching 1.10.1 Pickle serialization RCE/privilege escalation if cache compromised

• CVSS 9.8
• CWE-502 Deserialization of Untrusted Data
• Authentication, Authorization, and Session Management
• Remote

** DISPUTED ** The Flask-Caching extension through 1.10.1 for Flask relies on Pickle for serialization, which may lead to remote code execution or local privilege escalation. If an attacker gains access to cache storage (e.g., filesystem, Memcached, Redis, etc.), they can construct a crafted payload, poison the cache, and execute Python code. NOTE: a third party indicates that exploitation is extremely unlikely unless the machine is already compromised; in other cases, the attacker would be unable to write their payload to the cache and generate the required collision.

CVE

CVE-2021-33026

CWE

CWE-502 Deserialization of Untrusted Data

CVSS base score

9.8

Published

2021-05-13

OWASP

A06 Vulnerable and Outdated Components

Orthogonal defect classification

Algorithm

Code defect classification

Incorrect Algorithm

Category

Authentication, Authorization, and Session Management

Subcategory

Privilege Escalation

Accessibility scope

Remote

Impact

Arbitrary Code Execution

Fixed by upgrading

Yes

Solution

Update Flask-Caching to version 1.10.2 or higher.

from flask import Flask
from flask_caching import Cache
import pickle

app = Flask(__name__)

cache = Cache(app, config={'CACHE_TYPE': 'simple', 'CACHE_DEFAULT_TIMEOUT': 300})

@app.route('/set_cache/<key>/<value>')
def set_cache(key, value):
    # VULNERABLE: This code is susceptible to deserialization
    cache.set(key, pickle.dumps(value))

if __name__ == '__main__':
    app.run(debug=True)

from flask import Flask
from flask_caching import Cache
import json

app = Flask(__name__)

cache = Cache(app, config={'CACHE_TYPE': 'simple', 'CACHE_DEFAULT_TIMEOUT': 300})

@app.route('/set_cache/<key>/<value>')
def set_cache(key, value):
    # SECURE: This version prevents deserialization
    cache.set(key, json.dumps(value))

if __name__ == '__main__':
    app.run(debug=True)

@misc{vaitp:cve202133026,
  title        = {{Flask-Caching 1.10.1 Pickle serialization RCE/privilege escalation if cache compromised}},
  author       = {Bogaerts, Fr\'ed\'eric and Ivaki, Naghmeh and Fonseca, Jos\'e},
  year         = {2021},
  note         = {VAITP Python Vulnerability Dataset, entry CVE-2021-33026},
  howpublished = {\url{https://netpack.pt/vaitp/vulnerability/CVE-2021-33026/}}
}