CVE-2025-27773 is a high-severity (CVSS 8.6) CWE-347 vulnerability in simplesamlphp/saml2. A fix is available for simplesamlphp/saml2 — see the affected versions and patch details below.
SimpleSAMLphp SAML2 library has incorrect signature verification for HTTP-Redirect binding
Exploitation Status
Proof-of-concept exploit code exists
- CISA’s SSVC triage found public proof-of-concept exploit code for this CVE, though no confirmed active exploitation.
- CISA assesses this as automatable — exploitation doesn’t require manual, per-target effort, which raises the odds of mass scanning and opportunistic attacks.
Exploitation and automatability from CISA’s SSVC triage for CVE-2025-27773.
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-2025-27773 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 378,567 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
simplesamlphp/saml2🐘simplesamlphp/saml2🐘simplesamlphp/saml2-legacyReal-time download stats are indexed for npm and PyPI packages. This vulnerability affects Packagist packages — download data is not available via public APIs for these ecosystems.
Description
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 it exists) then SAMLResponse:
Given this, the contents that are processed might not be the same as the data that is actually verified.
Exploiting
Suppose an attacker has a signed HTTP Redirect binding from IdP, say a signed logout response. :
SAMLResponse=idpsigned&RelayState=...&SigAlg=...&Signature
Then an attacker can append SAMLRequest in front:
SAMLRequest=unverifieddata&SAMLResponse=idpsigned&RelayState=...&SigAlg=...&Signature=..
SimpleSAMLPhp will only verify the SAMLResponse, but will actually use the SAMLRequest contents. The impact here is increased because there's no checks that SAMLRequest actually contains a Request, it could instead contain an Response, which allows the attacker to effectively impersonate any user within the SP.
IdPs
Microsoft Azure AD/Entra (and likely ADFS) signs the LogoutResponse via this SimpleSign format in HTTP Redirect binding. If an attacker logs out of Entra, they will be able to extract a valid Signature.
Attached is an HTTP Request when an I initiated a SLO request from the service provider to the IdP (entra). Then IdP POSTed this SAMLResponse with HTTP Redirect binding signature, via the user browser to the SP. It should be possible to carry out the described attack with this.
https://webhook.site/c6038292-6ef5-46ac-973d-d7c25520ec48/logout?SAMLResponse=fVJNa%2bMwEP0rRndZtmw5tnAMy%2fYSaC9N6aGXIsmjVMTRGI9M%2bvObdeihsPQ4w7x5HzM9mcs060c84ZqegWaMBNnhYc%2fejS1UW1TAnVU7XldK8s7JkcvOd60Db3zTsewVFgoY90zmBcsORCscIiUT061VyJqXJS%2fbl7LRUmrZ5mXdvLHsASiFaNKG%2fEhpJi3EFewH4jmnkEC4pqha2UnegFe8bozj3a4a%2bbhzUilZgKtbMW2yb7TxW%2foL7lkM9hTC2XnEOPvZXjDECb2N1lh7mvBsp%2bnsErDs8zJF0lsEe7YuUaOhQDqaC5BOTh%2f%2fPD3qmzE9L5jQ4cSGfrO43KG%2fgwwRLP8ssuHbIiXKryGOeKU8QhLSVN7WteejV8Bru%2bt4WynFbwE3bdVV5ahG0Ys759Dfj3VMJq30s%2fqLI2SvZlrhd020Tevj6hwQMTH04udS8b%2bHGL4A&Signature=Z%2f7gIPv7Gkgvqtwo0bzgXyum9IjHMfP0zTYuNbl%2fBUGlQ%2fU%2bbOZGZJ6Rk9wLUyvNQ5XlZRxZrfESNA%2bn0CVyIedsg9GxQKTi7VqPTJFJqEIP1BZaEpYYP3%2f6sFfLxfTMKecJoQdxnDE5Malte1hMj2UujWnLXOnp0CgO%2f%2fU2K52SoGckIzNDRB%2fJ6%2fysTn%2bDjBrmgdro%2fgdTyby9%2f3vm8dzY8pUkRCgMjlimShrZxr5U33wQvwPLIXlDgActr91RUtWKE0k8sy%2brshrK9DKLPo8AdTLk7NYhjSWdF7OG7uqgEeEo470tacqQuA09E0qDh8CWS%2bycLJijiGYWVyQa4Q%3d%3d&SigAlg=http%3a%2f%2fwww.w3.org%2f2001%2f04%2fxmldsig-more%23rsa-sha256
Affected Packages
| Ecosystem | Package | Vulnerable range | Fix |
|---|---|---|---|
| 🐘Packagist | simplesamlphp/saml2 | all versions | 4.17.0composer require simplesamlphp/saml2:^4.17.0 |
| 🐘Packagist | simplesamlphp/saml2 | ≥ 5.0.0-alpha.1&&< 5.0.0-alpha.20 | 5.0.0-alpha.20composer require simplesamlphp/saml2:^5.0.0-alpha.20 |
| 🐘Packagist | simplesamlphp/saml2-legacy | all versions | 4.17.0composer require simplesamlphp/saml2-legacy:^4.17.0 |
Detection & mitigation playbook
Open-source dependencyDetect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for simplesamlphp/saml2, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.
Fix
Update simplesamlphp/saml2 to 4.17.0 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2025-27773 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-2025-27773 can be triaged on real exposure rather than presence alone.
Tailored to CVE-2025-27773. Runtime protection reduces exposure until a permanent patch is applied and verified — it complements patching, it doesn't replace it.
Fixing This On Your OS
If you run this on a Linux distribution, patch through your package manager against the distro's own security advisory below — it tracks the exact backported fix for your release, which can ship on a different timeline (and sometimes a different severity) than the upstream project.
Frequently Asked Questions
Is CVE-2025-27773 in your dependencies?
O3 Security finds CVE-2025-27773 across Packagist dependencies, including transitive ones, and its impact-aware SCA ranks findings by whether your code actually calls the vulnerable path.