EEF-CVE-2026-53424
Missing one-time-use enforcement in Samly allows replay of SAML bearer assertions
Details
## Summary
Authentication Bypass by Capture-replay vulnerability in dropbox samly allows an attacker to authenticate as the subject of a captured SAML assertion by resubmitting it.
Samly.Helper.decode\_idp\_auth\_resp/3 in lib/samly/helper.ex calls esaml\_sp:validate\_assertion/2, whose default duplicate detector is a no-op. The /3 arity accepting a DuplicateFun exists in esaml and implements the check, but Samly never calls it and offers no configuration to supply one, so the SAML 2.0 Web Browser SSO Profile requirement that a bearer assertion be used once is unenforced. An attacker holding a valid SAMLResponse obtained from the network, from browser history, or from logs can submit the identical bytes repeatedly until the assertion's NotOnOrAfter passes, each time establishing a session as the assertion's subject.
This issue affects samly: from 0.3.0 onward.
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Affected packages
0.3.0 No fixed version published yet for samly (hex). Pin to a known-safe version or switch to an alternative.