SimpleSAMLphp SP accepts a response from an unexpected IdP when unsigned `Response/InResponseTo` is combined with a signed assertion lacking `SubjectConfirmationData/InResponseTo`
Summary
SimpleSAMLphp's SAML SP ACS path does not enforce the IdP selected for an SP-initiated login. If a saved SP state contains ExpectedIssuer = IdP A, but the ACS receives a valid response from IdP B, the code logs a warning and continues processing instead of rejecting the response.
That behavior becomes security-relevant when combined with the response-processing rule that accepts an unsigned samlp:Response/@InResponseTo outside the signed assertion whenever the signed assertion's SubjectConfirmationData does not carry its own InResponseTo. A response issued by one trusted IdP can therefore be bound to SP state created for another IdP.
Impact
In a multi-IdP deployment, a lower-trust IdP can satisfy SP state created for a different expected IdP. This can bypass an SP flow that intentionally routes the user to a specific IdP, including deployments that set enable_unsolicited to false to prevent IdP-initiated logins.
The impact is highest when the SP trusts multiple IdPs with different assurance levels, tenant boundaries, or attribute namespaces, and application authorization depends on the selected/expected IdP. In those deployments this is an authentication/authorization bypass candidate. Impact strongly depends on whether an attacker can obtain a signed IdP-initiated assertion from a lower-trust trusted IdP and whether the downstream application maps identifiers globally.
Affected software
Click a technology to see all vulnerabilities affecting it.
| Platform | Package | Vulnerable versions | Fixed in |
|---|---|---|---|
| Composer package | simplesamlphp/simplesamlphp | >= 2.5.0, < 2.5.2, < 2.4.7 | 2.5.2, 2.4.7 |
References
- CVSS vector
CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:L/I:H/A:N- Weakness (CWE)
CWE-345- Platform
- Composer
- Package
simplesamlphp/simplesamlphp- Disclosed
- 02.07.2026
- Updated
- 11.10.2026 10:22
Sources and licenses
OSV / GitHub Advisory Database - GHSA-q8r6-xj3f-wrrm - CC BY 4.0
Vulnerability history: simplesamlphp/simplesamlphp
All published vulnerabilities of this package (Composer package), newest first. Shows whether this is a one-off slip or a package that needs watching.
- Total entries
- 23
- First → latest
- 01.2020 07.2026
- Without a fix
- 0
- Exploited in attacks
- 0
Severity breakdown
- Critical 3
- High 4
- Medium 12
- Low 4
Disclosures per year
- 2020: 6
- 2021: 0
- 2022: 12
- 2023: 0
- 2024: 4
- 2025: 0
- 2026: 1
-
SimpleSAMLphp SP accepts a response from an unexpected IdP when unsigned `Response/InResponseTo` is combined with a signed assertion lacking `SubjectConfirmationData/InResponseTo` this entry
Know which vulnerabilities affect your application
With the DockRay integration and software inventory enabled, DockRay checks reported component versions against known vulnerabilities and notifies you about confirmed matches. Coverage depends on the supported technology and available advisory data.