How severe is CVE-2021-21337?

CVE-2021-21337 has a CVSS score of 6.1/10, rated MEDIUM. Review your exposure and patch according to your risk tolerance.

Which packages are affected by CVE-2021-21337?

CVE-2021-21337 affects the following packages: products-pluggableauthservice (PyPI). Ecosystems affected: PyPI.

How do I fix CVE-2021-21337?

Update products-pluggableauthservice to 2.6.1 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2021-21337 is resolved across your whole dependency graph.

How do I detect CVE-2021-21337 in my PyPI dependencies?

Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for products-pluggableauthservice, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.

How do I mitigate CVE-2021-21337 if there is no patch (or I can't update yet)?

Stop reflecting attacker-controlled destinations: resolve every redirect target against an allowlist of paths or hosts you own, prefer a server-side key over a full URL in the request, and reject absolute URLs entirely where the flow only ever needs a relative one.

Is CVE-2021-21337 actively exploited in the wild?

Yes. There are 2 known exploit references for CVE-2021-21337, including 1 documented proof-of-concept exploit on Exploit-DB and 1 in-the-wild exploitation observed on GitHub and other sources. Treat this as actively exploitable and prioritize patching immediately. All exploit code should only be run in an isolated sandbox environment for research or authorized testing — never against production systems without explicit written authorization.

What is the EPSS score for CVE-2021-21337?

CVE-2021-21337 has an EPSS (Exploit Prediction Scoring System) score of 8.4%, placing it in the 95th percentile of all CVEs. EPSS is maintained by FIRST.org and estimates the probability that a vulnerability will be exploited in the wild within the next 30 days. This score indicates relatively lower exploitation probability, though the CVSS severity should still guide your patching priority.

What type of vulnerability is CVE-2021-21337?

CVE-2021-21337 is classified as Open Redirect (CWE-601). This weakness type describe the underlying flaw category, which helps determine the potential impact and the right class of mitigation.

When was CVE-2021-21337 published, and has it been updated?

CVE-2021-21337 was published on March 8, 2021 and was last updated on July 9, 2026. Advisory data evolves as severity scores, affected ranges, and exploit intelligence are revised — always check the latest version of the advisory before acting.

🐍

🐍 PyPI

Not in CISA KEV

MEDIUM severity

CVE-2021-21337 — products-pluggableauthser…

MEDIUMFix: zopefoundation/Products.PluggableAuthService@7eead06

CVE-2021-21337 is a medium-severity (CVSS 6.1) Open Redirect vulnerability in products-pluggableauthservice. 2 public exploit references exist, so weaponization risk is real. A fix is available for products-pluggableauthservice — see the affected versions and patch details below.

URL Redirection to Untrusted Site ('Open Redirect') in Products.PluggableAuthService

Also known asGHSA-p44j-xrqg-4xrrPYSEC-2021-45

Published

Mar 8, 2021

Updated

Jul 9, 2026

Affected

1 pkg

Patched

1 / 1

Exploits

2 known

Exploitation data as of Oct 10, 2026 · OSV.dev, NVD, FIRST.org (EPSS)

EPSS Exploitation Probability

via FIRST.org ↗

8.4%probability of exploitation in next 30 days

Lower Risk0.00%

Lower risk than most CVEs95th percentile — riskier than 95% of all scored CVEsHighest risk

Probability of exploitation in the next 30 days, from FIRST.org EPSS.

How urgent is this, really

CVE-2021-21337 by exploitation likelihood (EPSS) against impact (CVSS). Outside the shaded patch-first corner.

Where this sits among everything scored

Of 385,987 CVEs with a current EPSS score, this one falls in the < 10% band (highlighted). Counts from FIRST.org, log-scaled.

Real-World Exposure

1 pkg affected

🐍products-pluggableauthservice

Real-time download stats are indexed for npm and PyPI packages. This vulnerability affects PyPI packages — download data is not available via public APIs for these ecosystems.

Description

Impact

What kind of vulnerability is it? Who is impacted?

Open redirect vulnerability - a maliciously crafted link to the login form and login functionality could redirect the browser to a different website.

Patches

Has the problem been patched? What versions should users upgrade to?

The problem has been fixed in version 2.6.1. Depending on how you have installed Products.PluggableAuthService, you should change the buildout version pin to 2.6.1 and re-run the buildout, or if you used pip simply do pip install "Products.PluggableAuthService>=2.6.1"

Workarounds

Is there a way for users to fix or remediate the vulnerability without upgrading?

There is no workaround. Users are encouraged to upgrade.

References

Are there any links users can visit to find out more?

For more information

If you have any questions or comments about this advisory:

Open an issue in the Products.PluggableAuthService issue tracker
Email us at [email protected]

Affected Packages

1 total 1 fixed

Ecosystem	Package	Vulnerable range	Fix
🐍PyPI	`products-pluggableauthservice`	all versions	2.6.1`pip install --upgrade 'products-pluggableauthservice==2.6.1'`

Affected Products

1 product · 1 configurations

Application

products.pluggableauthservicezope

< 2.6.1

range

Exploits & PoCs

Research use only. For defensive security, authorized penetration testing, and academic research only. Never execute exploit code against systems without explicit written authorization.

EDB-49930webappspython

Products.PluggableAuthService 2.6.0 - Open Redirect

by Piyush Patil · Jun 2, 2021

Exploit-DB Source

Detection & mitigation playbook

Open-source dependency

Detect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for products-pluggableauthservice, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.
Fix
Update products-pluggableauthservice to 2.6.1 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2021-21337 is resolved across your whole dependency graph.
Workarounds
Stop reflecting attacker-controlled destinations: resolve every redirect target against an allowlist of paths or hosts you own, prefer a server-side key over a full URL in the request, and reject absolute URLs entirely where the flow only ever needs a relative one.

Frequently Asked Questions

### Impact _What kind of vulnerability is it? Who is impacted?_ Open redirect vulnerability - a maliciously crafted link to the login form and login functionality could redirect the browser to a different website. ### Patches _Has the problem been patched? What versions should users upgrade to?_ The problem has been fixed in version 2.6.1. Depending on how you have installed Products.PluggableAuthService, you should change the buildout version pin to `2.6.1` and re-run the buildout, or if you used `pip` simply do `pip install "Products.PluggableAuthService>=2.6.1"` ### Workarounds _Is the