CVE-2021-33026: 
Python vulnerability analysis and mitigation

CVE-2021-33026 affects the Flask-Caching extension through version 1.10.1 for Flask. The vulnerability stems from the use of Pickle for serialization, which could potentially lead to remote code execution or local privilege escalation. The issue was discovered in May 2021 and has been marked as DISPUTED (NVD).

The vulnerability arises from Flask-Caching's reliance on Pickle for serialization. If an attacker gains access to cache storage (such as filesystem, Memcached, Redis), they could theoretically construct a crafted payload, poison the cache, and execute Python code. The vulnerability has been assigned a CVSS v3.1 Base Score of 9.8 CRITICAL (Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H) (NVD).

The potential impact of this vulnerability is disputed. For an attack to be successful, three conditions must be met: the attacker must be able to write arbitrary values to the cache, generate a cache key that collides with a value being read by the application, and cause the application to read the maliciously-injected value. In most cases, if these conditions are met, the application would already have more severe security issues (GitHub).

The exploitability of this vulnerability is considered extremely unlikely unless the machine is already compromised. Third-party security experts indicate that in most cases, an attacker would be unable to write their payload to the cache and generate the required collision. There have been no confirmed cases of this exploit being successfully used in the wild (GitHub).

While the vulnerability is disputed, organizations concerned about this issue can consider using alternative serializers such as JSON instead of Pickle. The maintainers are working on transitioning flask-caching to use cachelib, which supports custom/configurable serializers (GitHub).

The security community has largely disputed the severity of this vulnerability. Many experts argue that the CVE should be rejected or have its severity reduced, noting that the vulnerability requires multiple security failures to be exploitable. The maintainers of Flask-Caching consider the CVE invalid, stating that the use of Pickle alone isn't a security issue when not loading arbitrary data (GitHub).