How severe is CVE-2021-21238?

CVE-2021-21238 has a CVSS score of 6.5/10, rated MEDIUM. Review your exposure and patch according to your risk tolerance.

Which packages are affected by CVE-2021-21238?

CVE-2021-21238 affects the following packages: pysaml2 (PyPI). Ecosystems affected: PyPI.

How do I fix CVE-2021-21238?

Update pysaml2 to 6.5.0 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2021-21238 is resolved across your whole dependency graph.

How do I detect CVE-2021-21238 in my PyPI dependencies?

Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for pysaml2, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.

How do I mitigate CVE-2021-21238 if there is no patch (or I can't update yet)?

If you can't upgrade right away: gate or disable the affected feature, validate untrusted input at the boundary, and avoid passing attacker-controlled data into the vulnerable path. O3's runtime protection blocks exploitation in production as an interim safeguard until the upgrade lands.

Is CVE-2021-21238 actively exploited in the wild?

No public exploit code has been indexed for CVE-2021-21238 yet. This does not mean the vulnerability cannot be exploited — absence of public exploits does not imply safety. Apply the recommended fix and use O3 Security to monitor your exposure.

What is the EPSS score for CVE-2021-21238?

CVE-2021-21238 has an EPSS (Exploit Prediction Scoring System) score of 1.1%, placing it in the 66th 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-2021-21238?

CVE-2021-21238 is classified as CWE-347 (CWE-347). This weakness type describe the underlying flaw category, which helps determine the potential impact and the right class of mitigation.

When was CVE-2021-21238 published, and has it been updated?

CVE-2021-21238 was published on January 21, 2021 and was last updated on October 24, 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-2021-21238 — pysaml2

MEDIUMFix: IdentityPython/pysaml2@1d8fd26

CVE-2021-21238 is a medium-severity (CVSS 6.5) CWE-347 vulnerability in pysaml2. A fix is available for pysaml2 — see the affected versions and patch details below.

SAML XML Signature wrapping in PySAML2

Also known asGHSA-f4g9-h89h-jgv9PYSEC-2021-48

Published

Jan 21, 2021

Updated

Oct 24, 2026

Affected

1 pkg

Patched

1 / 1

Exploits

None indexed

Exploitation data as of Nov 2, 2026 · OSV.dev, NVD, FIRST.org (EPSS)

EPSS Exploitation Probability

via FIRST.org ↗

1.1%probability of exploitation in next 30 days

Lower Risk0.00%

Lower risk than most CVEs66th percentile — riskier than 66% of all scored CVEsHighest risk

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

How urgent is this, really

Where this sits among everything scored

Real-World Exposure

1 pkg affected

🐍pysaml2

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

All users of pysaml2 that use the default CryptoBackendXmlSec1 backend and need to verify signed SAML documents are impacted. pysaml2 <= 6.4.1 does not validate the SAML document against an XML schema. This allows invalid XML documents to trick the verification process, by presenting elements with a valid signature inside elements whose content has been malformed. The verification is offloaded to xmlsec1 and xmlsec1 will not validate every signature in the given document, but only the first it finds in the given scope.

Patches

Users should upgrade to pysaml2 v6.5.0.

Workarounds

No workaround provided at this point.

References

No references provided at this point.

Credits

Victor Schönfelder Garcia (isits AG International School of IT Security)
Juraj Somorovsky (Paderborn University)
Vladislav Mladenov (Ruhr University Bochum)

For more information

If you have any questions or comments about this advisory:

Open an issue in pysaml2
Email us at the incident-response address

Affected Packages

1 total 1 fixed

Ecosystem	Package	Vulnerable range	Fix
🐍PyPI	`pysaml2`	all versions	6.5.0`pip install --upgrade 'pysaml2==6.5.0'`

Affected Products

1 product · 1 configurations

Application

pysaml2pysaml2_project

< 6.5.0

range

Detection & mitigation playbook

Open-source dependency

Detect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for pysaml2, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.
Fix
Update pysaml2 to 6.5.0 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2021-21238 is resolved across your whole dependency graph.
Workarounds
If you can't upgrade right away: gate or disable the affected feature, validate untrusted input at the boundary, and avoid passing attacker-controlled data into the vulnerable path. O3's runtime protection blocks exploitation in production as an interim safeguard until the upgrade lands.

Fixing This On Your OS

If you run this on a Linux distribution, patch through your package manager against the distro's own security advisory below — it tracks the exact backported fix for your release, which can ship on a different timeline (and sometimes a different severity) than the upstream project.

Red HatModerate

All versions of Red Hat OpenStack Platform ship but do not use the flawed check-signature functionality of python-pysaml2. The impact for these products is therefore rated as having a security impact of Low and will not be updated at this time.

Workaround published by Red Hat

Mitigation for this issue is either not available or the currently available options do not meet the Red Hat Product Security criteria comprising ease of use and deployment, applicability to widespread installation base or stability.

Source: Red Hat security advisory for CVE-2021-21238 (CC BY 4.0)

Frequently Asked Questions

### Impact All users of pysaml2 that use the default `CryptoBackendXmlSec1` backend and need to verify signed SAML documents are impacted. `pysaml2 <= 6.4.1` does not validate the SAML document against an XML schema. This allows invalid XML documents to trick the verification process, by presenting elements with a valid signature inside elements whose content has been malformed. The verification is offloaded to `xmlsec1` and `xmlsec1` will not validate every signature in the given document, but only the first it finds in the given scope. ### Patches Users should upgrade to pysaml2 `v6.5.0`.