Deserialization of Untrusted Data in Flask-Caching

Exploitation

No confirmed exploitation recorded. Patch Next marks exploitation only from CISA KEV.

CVSS

4.2 · CVSS:3.1/AV:N/AC:H/PR:H/UI:R/S:U/C:N/I:H/A:N

EPSS

• CVE-2021-33026: 7.35% estimated exploitation activity in the next 30 days; percentile 94.41 (FIRST EPSS, 2026-11-03). This is a model estimate, not proof of exploitation or customer exposure.

As published by the source; long descriptions are shortened.

Flask-Cache adds easy cache support to Flask. 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. However, this is not a high-severity issue, as for an attack like this to work, an attacker must: 1. Be able to write arbitrary values to the cache 2. Be able to generate a cache key that will collide with a value being read by the application 3. Cause the application to read a maliciously-injected value Any situation where all 3 of those is true is a situation where the application has larger problems; for example, if someone's able to inject malicious cached rendered pages into a Flask app's cache, then they can make the website say literally anything they want, regardless of whether it involves the execution of…

Published by the source

18 Jun 2021, 18:33 UTC

Last modified by the source

16 Feb 2024, 08:16 UTC

Source last checked

26 Oct 2026, 13:11 UTC

1. Revision 2 · amended · 26 Oct 2026

   Amended from ghsa record GHSA-656c-6cxf-hvcv

2. Revision 1 · created · 24 Oct 2026

   Published from osv record GHSA-656c-6cxf-hvcv

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# Patch Next advisory brief: CVE-2021-33026

Advisory: Deserialization of Untrusted Data in Flask-Caching
IDs: CVE-2021-33026, GHSA-656c-6cxf-hvcv
Record: https://patchnext.com/advisories/cve-2021-33026 (revision 2, source checked 26 Oct 2026, 13:11 UTC)
Inventory: none attached. This brief comes from a public page and does not say whether any deployment is affected.
Severity: Medium (CVSS 4.2)
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Vendor response not established. A vendor fix does not mean a deployment is fixed.

## Affected components
- PyPI flask-caching: affected <= 1.10.1

## Suggested inspection (read-only)
- `python -m pip show flask-caching`: prints the installed version. Run with the environment that serves production active.

## Supported action
No complete vendor fix is recorded. Follow the primary source's mitigation guidance and check this record again before closing anything.

## Required regression checks
- Run the test suite and a production build.
- Exercise the features that depend on flask-caching.
- Repeat the inspection and confirm that only fixed versions remain installed.

## Rollback considerations
- Keep the previous lockfile and deployment artifact. Rolling back reinstates the vulnerable version, so apply any documented mitigation while you fix forward.

## Unresolved questions
- Which deployments run an affected version?
- Exploitation status can change. Check again before lowering the priority.
- Mark the work resolved only after the fixed version is verified in production.

## Sources
- Verified: https://osv.dev/vulnerability/GHSA-656c-6cxf-hvcv (OSV record GHSA-656c-6cxf-hvcv lists affected ranges: PyPI flask-caching >=0 <=1.10.1)
- Verified: https://github.com/advisories/GHSA-656c-6cxf-hvcv (GitHub-reviewed advisory GHSA-656c-6cxf-hvcv lists affected ranges: PyPI flask-caching >=0 <=1.10.1)

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