CVE-2026-53425 Missing InResponseTo validation in Samly allows acceptance of unsolicited SAML responses
Attackers can hijack user sessions by replaying valid SAML responses because the library fails to validate the InResponseTo field against the original request.
- Exploited in the wild
- Not known
- Login needed
- Not needed
- Attackable
- Over the internet
- Victim action needed
- A user has to click or open something
- Fix
- None listed
What is CVE-2026-53425?
CVE-2026-53425 is a high-severity authentication bypass vulnerability in dropbox samly, published on 20 Aug 2026. An attacker can exploit it over the internet without logging in, and only if a user opens a file or follows a link.
Official description
Insufficient Verification of Data Authenticity vulnerability in dropbox samly allows an attacker to establish an authenticated session using a SAML response the service provider never requested.
Samly.SPHandler.validate_authresp/3 in lib/samly/sp_handler.ex validates a SAML response for the SP-initiated flow by comparing only the RelayState value, the IdP identifier, and the presence of a target URL held in the session. It never compares SubjectConfirmationData/@InResponseTo against the ID of the AuthnRequest the service provider issued, and that request ID is never persisted, so no comparison is possible. SAML 2.0 Core section 4.1.4.3 requires a service provider to reject a response whose InResponseTo does not match a request it made. The underlying esaml library checks status, signature, recipient, audience, and staleness, but likewise never inspects InResponseTo, so nothing else closes the gap. Exploitation requires a validly signed assertion from the trusted IdP, which an attacker can obtain for their own account, and a RelayState matching the victim's session; the assertion signature itself remains intact, so this is not a signature-forgery issue.
This issue affects samly: from 0.3.0 onward.
Weakness: CWE-345
Which versions are affected by CVE-2026-53425?
1 open source package ship an affected version, including samly.
| Package | Affected | Fixed in |
|---|---|---|
samly Hex | From 0.3.0 | None |
How do I fix CVE-2026-53425?
No fix has been published for CVE-2026-53425 yet. Limit exposure of the affected software until one is available.
Upgrade, and rotate credentials and sessions that may have been exposed. More about Authentication bypass.
Is CVE-2026-53425 being exploited?
There is no known exploitation of CVE-2026-53425 in the wild and no public exploit code.
Sources
Also known as EEF-CVE-2026-53425, GHSA-4mhx-38g6-r4wq.