The SimpleSAMLphp SAML2 library incorrectly verifies signatures for HTTP-Redirect binding

Exploitation

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

CVSS

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

EPSS

• CVE-2025-27773: 0.41% estimated exploitation activity in the next 30 days; percentile 35.84 (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.

Summary There's a signature confusion attack in the HTTPRedirect binding. An attacker with any signed SAMLResponse via the HTTP-Redirect binding can cause the application to accept an unsigned message. I believe that it exists for v4 only. I have not yet developed a PoC. V5 is well designed and instead builds the signed query from the same message that will be consumed. Details What is verified The data['SignedQuery'] is the string that will be verified by the public key. It is defined here: https://github.com/simplesamlphp/saml2/blob/9545abd0d9d48388f2fa00469c5c1e0294f0303e/src/SAML2/HTTPRedirect.php#L178-L217 THe code will iterate through each parameter name. Notably, sigQuery is overridden each time when processing, making the last of SAMLRequest/SAMLResponse used for sigQuery. For example, given: SAMLRequest=a&SAMLResponse=idpsigned SAMLResponse=idpsigned will be set as sigQuery, then later verified What is actually processed Processing uses SAMLRequest parameter value first, (if…

Published by the source

11 Mar 2025, 19:23 UTC

Last modified by the source

9 May 2025, 21:31 UTC

Source last checked

26 Oct 2026, 14:33 UTC

1. Revision 1 · created · 26 Oct 2026

   Published from ghsa record GHSA-46r4-f8gj-xg56

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# Patch Next advisory brief: CVE-2025-27773

Advisory: The SimpleSAMLphp SAML2 library incorrectly verifies signatures for HTTP-Redirect binding
IDs: CVE-2025-27773, GHSA-46r4-f8gj-xg56
Record: https://patchnext.com/advisories/cve-2025-27773 (revision 1, source checked 26 Oct 2026, 14:33 UTC)
Inventory: none attached. This brief comes from a public page and does not say whether any deployment is affected.
Severity: High (CVSS 8.6)
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Vendor fix available; fixed versions by package: simplesamlphp/saml2-legacy 4.17.0; simplesamlphp/saml2 5.0.0-alpha.20, 4.17.0. A vendor fix does not mean a deployment is fixed.

## Affected components
- Packagist simplesamlphp/saml2-legacy: affected <= 4.16.15; fixed in 4.17.0
- Packagist simplesamlphp/saml2: affected >= 5.0.0-alpha.1, <= 5.0.0-alpha.19; <= 4.16.15; fixed in 4.17.0, 5.0.0-alpha.20

## Suggested inspection (read-only)
- Packagist simplesamlphp/saml2-legacy: check the installed version with your package manager.
- Packagist simplesamlphp/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 simplesamlphp/saml2-legacy, simplesamlphp/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-46r4-f8gj-xg56 (GitHub-reviewed advisory GHSA-46r4-f8gj-xg56 lists affected ranges: Packagist simplesamlphp/saml2 >=5.0.0-alpha.1 =0 =0 <=4.16.15)

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