What is CVE-2021-21239?

CVE-2021-21239 is a medium-severity vulnerability in Identitypython Pysaml2, classified under Improper Verification of Cryptographic Signature. CVSS score: 6.5/10. Published 2021-01-21.

How severe is CVE-2021-21239?

Medium severity. CVSS v3 base score is 6.5 out of 10.

Is CVE-2021-21239 known to be exploited?

5 public proof-of-concept repositories are indexed. Not currently listed in the CISA KEV catalog.

Vulnerability in Identitypython Pysaml2

CVE-2021-21239

PySAML2 is a pure python implementation of SAML Version 2 Standard. PySAML2 before 6.5.0 has an improper verification of cryptographic signature vulnerability. Users of pysaml2 that use the default CryptoBackendXmlSec1 backend and need to verify signed SAML documents are impacted. PySAML2 does not ensure that a signed SAML document is correctly signed. The default CryptoBackendXmlSec1 backend is using the xmlsec1 binary to verify the signature of signed SAML documents, but by default xmlsec1 accepts any type of key found within the given document. xmlsec1 needs to be configured explicitly to only use only _x509 certificates_ for the verification process of the SAML document signature. This is fixed in PySAML2 6.5.0.

Published 2021-01-21 · last modified 2026-06-17.

CVSS v3 metric

CVSS v3 base score 6.5 (Medium). Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:N/I:H/A:N.

EPSS exploit prediction

EPSS: 0.013 (68.7th percentile), scored 2026-09-30.

Low probability of exploitation in the next 30 days, but still tracked. 69th percentile — 68.7% of CVEs in the catalogue have a lower EPSS than this one. How to read EPSS.

EPSS over last 30 days · oldest: 0.012 · newest: 0.013 · change: +0.001

Affected products

Identitypython Pysaml2 — versions < 6.5.0
Pysaml2 Project
Debian Linux — versions 9.0

Weakness classification (CWE)

CWE-347 — Improper Verification of Cryptographic Signature

Public proof-of-concept exploits

References

github.com/IdentityPython/pysaml2/security/advisories/GHSA-5p3x-r448-pc62 (Third Party Advisory)
pypi.org/project/pysaml2 (Product, Third Party Advisory)
www.aleksey.com/pipermail/xmlsec/2013/009717.html (Exploit, Mailing List, Third Party Advisory)
github.com/IdentityPython/pysaml2/releases/tag/v6.5.0 (Third Party Advisory)
github.com/IdentityPython/pysaml2/commit/46578df0695269a16f1c94171f1429873f90ed… (Patch, Third Party Advisory)
lists.debian.org/debian-lts-announce/2021/02/msg00038.html (mailing-list, Mailing List, Third Party Advisory)

Frequently asked questions

What is CVE-2021-21239?: CVE-2021-21239 is a medium-severity vulnerability in Identitypython Pysaml2, classified under Improper Verification of Cryptographic Signature. CVSS score: 6.5/10. Published 2021-01-21.
How severe is CVE-2021-21239?: Medium severity. CVSS v3 base score is 6.5 out of 10.
Is CVE-2021-21239 known to be exploited?: 5 public proof-of-concept repositories are indexed. Not currently listed in the CISA KEV catalog.