CVE-2020-5268 is a high-severity (CVSS 7.3) CWE-303 vulnerability in Sustainsys.Saml2. A fix is available for Sustainsys.Saml2 — see the affected versions and patch details below.
Subject Confirmation Method not validated in Saml2 Authentication Services for ASP.NET
EPSS Exploitation Probability
EPSS (Exploit Prediction Scoring System) is a daily probability model maintained by FIRST.org. It estimates the likelihood a CVE will be exploited in production environments within the next 30 days, derived from real-world threat intelligence signals.
How urgent is this, really
CVE-2020-5268 plotted by exploitation likelihood (EPSS) against impact (CVSS). The shaded corner — EPSS 50%+ and CVSS 7.0+ — is where this CVE doesn't sit, though severity or exploitability alone can still warrant action.
Where this sits among everything scored
Of 378,156 CVEs with a current EPSS score, this one falls in the < 10% band (highlighted). Real counts from FIRST.org, not a sample — log-scaled since the landscape is heavily right-skewed.
Real-World Exposure
Sustainsys.Saml2.NETSustainsys.Saml2Real-time download stats are indexed for npm and PyPI packages. This vulnerability affects NuGet packages — download data is not available via public APIs for these ecosystems.
Description
Impact
Saml2 tokens are usually used as bearer tokens - a caller that presents a token is assumed to be the subject of the token. There is also support in the Saml2 protocol for issuing tokens that is tied to a subject through other means, e.g. holder-of-key where possession of a private key must be proved. The Sustainsys.Saml2 library incorrectly treats all incoming tokens as bearer tokens, even though they have another subject confirmation method specified. This could be used by an attacker that could get access to Saml2 tokens with another subject confirmation method than bearer. The attacker could then use such a tocken to create a log in session.
Patches
Version 1.0.2 and 2.7.0 are patched.
Workarounds
Ensure that any IdentityProvider trusted by the Sustainsys.Saml2 SP only issues bearer tokens if the audience matches the Sustainsys.Saml2 SP.
For more information
If you have any questions or comments about this advisory:
- Comment on #103
- Email us at [email protected] if you think that there are further security issues.
Affected Packages
| Ecosystem | Package | Vulnerable range | Fix |
|---|---|---|---|
| .NETNuGet | Sustainsys.Saml2 | all versions | 1.0.2dotnet add package Sustainsys.Saml2 --version 1.0.2 |
| .NETNuGet | Sustainsys.Saml2 | ≥ 2.0.0&&< 2.7.0 | 2.7.0dotnet add package Sustainsys.Saml2 --version 2.7.0 |
Detection & mitigation playbook
Open-source dependencyDetect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for Sustainsys.Saml2, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.
Fix
Update Sustainsys.Saml2 to 1.0.2 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2020-5268 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.
How O3 protects you
O3 Security's impact-aware SCA analyses which vulnerable code paths your application actually calls, so a match like CVE-2020-5268 can be triaged on real exposure rather than presence alone.
Tailored to CVE-2020-5268. Runtime protection reduces exposure until a permanent patch is applied and verified — it complements patching, it doesn't replace it.
Frequently Asked Questions
Is CVE-2020-5268 in your dependencies?
O3 Security finds CVE-2020-5268 across NuGet dependencies, including transitive ones, and its impact-aware SCA ranks findings by whether your code actually calls the vulnerable path.