Summary
SAML XML Signature wrapping in PySAML2
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
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.
CVE-2021-21238 has a CVSS score of 6.5 (Medium). The vector is network-reachable, no privileges required, and user interaction required. A CVSS score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether this affects your application depends on whether the vulnerable code is present and reachable in your environment. A fixed version is available (6.5.0); upgrading removes the vulnerable code path.
Affected versions
Security releases
Kodem intelligence
Severity tells you how bad this could be in the worst case. It does not tell you whether you are exposed. Exploitability and impact are functions of runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A vulnerable package can sit in your dependency tree and never run.
Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter. Kodem's runtime-powered SCA identifies whether this CVE is reachable in your applications.
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Users should upgrade to pysaml2 v6.5.0.
Frequently Asked Questions
- What is CVE-2021-21238? CVE-2021-21238 is a medium-severity security vulnerability in pysaml2 (pip), affecting versions < 6.5.0. It is fixed in 6.5.0.
- How severe is CVE-2021-21238? CVE-2021-21238 has a CVSS score of 6.5 (Medium). This score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether it represents real risk in your environment depends on whether the vulnerable code is present and reachable.
- Which versions of pysaml2 are affected by CVE-2021-21238? pysaml2 (pip) versions < 6.5.0 is affected.
- Is there a fix for CVE-2021-21238? Yes. CVE-2021-21238 is fixed in 6.5.0. Upgrade to this version or later.
- Is CVE-2021-21238 exploitable, and should I be worried? Whether CVE-2021-21238 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
- What actually determines whether CVE-2021-21238 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
- How do I fix CVE-2021-21238? Upgrade
pysaml2to 6.5.0 or later.