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Fencer Vulns
critical 9.8

CVE-2021-33026 json

Deserialization of Untrusted Data in Flask-Caching

published 2021-05-13 5 y ago modified 2026-06-17 4 mo ago also GHSA-656c-6cxf-hvcv , EUVD-2021-0084

cvss
9.8
version 3.1, CNA
cisa kev
Not listed
not in CISA's catalog
epss
7.3%
EPSS 7.3% chance of exploitation in 30 days, percentile 94%
public exploits
0
none known

Summary

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

Patch / fix

Vendor advisories

What the vendors themselves say about their products: affected, fixed or not affected, and what to do.

red hat product security vex CVE-2021-33026 updated cvss 7.5

python-flask-caching: crafted payload may lead to remote code execution or local privilege escalation

  • Red Hat All currently supported Red Hat products
    not affected: vulnerable code not present

Red Hat CSAF VEX © Red Hat, Inc., from https://security.access.redhat.com/data/csaf/v2/vex/; changed: reduced to product statuses and remediations. CC BY 4.0 · csaf

CISA SSVC

Exploitation
none
Automatable
yes
Technical impact
total
Assessed
2 y ago

Exploit prediction

7.35% chance of exploitation in the next 30 days, higher than 94% of all scored CVEs (FIRST EPSS, 2026-11-08).

CVSS scores

assigned by version score vector
CISA ADP 3.1 9.8 CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H
NVD 3.1 9.8 CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H
CNA 3.1 9.8 CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H
NVD 2.0 7.5 AV:N/AC:L/Au:N/C:P/I:P/A:P
GitHub 3.1 4.2 CVSS:3.1/AV:N/AC:H/PR:H/UI:R/S:U/C:N/I:H/A:N

Affected packages

ecosystem package affected fixed in
pypi flask-caching
<=1.10.1
 no fix listed

Affected products

  • flask-caching_project flask-caching

Classification

Weakness
  • CWE-502 Deserialization of Untrusted Data
  • CWE-269 Improper Privilege Management

References

Sources

Fencer Vulns keeps one entry per vulnerability: the CVE id wins, then GHSA, and records that alias each other are merged, except that two CVEs always keep separate entries. The same data is available as JSON.