Missing Token Replay Detection in Saml2 Authentication services for ASP.NET

Exploitation

No confirmed exploitation recorded. Patch Next marks exploitation only from CISA KEV.

CVSS

8.2 · CVSS:3.1/AV:N/AC:H/PR:L/UI:N/S:C/C:H/I:H/A:N

EPSS

• CVE-2020-5261: 1.50% estimated exploitation activity in the next 30 days; percentile 74.22 (FIRST EPSS, 2026-11-03). This is a model estimate, not proof of exploitation or customer exposure.

As published by the source; long descriptions are shortened.

Impact Token Replay Detection is an important defence in depth measure for Single Sign On solutions. In all previous 2.X versions, the Token Replay Detection is not properly implemented. Note that version 1.0.1 is not affected. It has a correct Token Replay Implementation and is safe to use. Patches The 2.5.0 version is patched. Workarounds There are no workarounds with existing versions. Fixing the issue requires code updates. References https://en.wikipedia.org/wiki/Replayattack For more information If you have any questions or comments about this advisory: Comment on #711. Email us at security@sustainsys.com if you think that there are further security issues.

Published by the source

25 Mar 2020, 16:52 UTC

Last modified by the source

1 Feb 2023, 05:02 UTC

Source last checked

26 Oct 2026, 16:59 UTC

1. Revision 1 · created · 26 Oct 2026

   Published from ghsa record GHSA-g6j2-ch25-5mmv

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# Patch Next advisory brief: CVE-2020-5261

Advisory: Missing Token Replay Detection in Saml2 Authentication services for ASP.NET
IDs: CVE-2020-5261, GHSA-g6j2-ch25-5mmv
Record: https://patchnext.com/advisories/cve-2020-5261 (revision 1, source checked 26 Oct 2026, 16:59 UTC)
Inventory: none attached. This brief comes from a public page and does not say whether any deployment is affected.
Severity: High (CVSS 8.2)
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Vendor fix available; fixed versions by package: sustainsys.saml2 2.5.0. A vendor fix does not mean a deployment is fixed.

## Affected components
- NuGet sustainsys.saml2: affected >= 2.0.0, < 2.5.0; fixed in 2.5.0

## Suggested inspection (read-only)
- NuGet sustainsys.saml2: check the installed version with your package manager.

## Supported action
Upgrade each affected component to a fixed version on its release line. For a transitive dependency, upgrade the parent package or add an override or constraint, then regenerate the lockfile. Redeploy every environment that ran an affected version.

## Required regression checks
- Run the test suite and a production build.
- Exercise the features that depend on sustainsys.saml2.
- Repeat the inspection and confirm that only fixed versions remain installed.

## Rollback considerations
- Keep the previous lockfile and deployment artifact. Rolling back reinstates the vulnerable version, so apply any documented mitigation while you fix forward.

## Unresolved questions
- Which deployments run an affected version?
- Exploitation status can change. Check again before lowering the priority.
- Mark the work resolved only after the fixed version is verified in production.

## Sources
- Verified: https://github.com/advisories/GHSA-g6j2-ch25-5mmv (GitHub-reviewed advisory GHSA-g6j2-ch25-5mmv lists affected ranges: NuGet sustainsys.saml2 >=2.0.0 <2.5.0)

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