CVE-2026-53424 PUBLISHED

Missing one-time-use enforcement in Samly allows replay of SAML bearer assertions

Assigner: EEF
Reserved: 09.06.2026 Published: 20.08.2026 Updated: 20.08.2026

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.

Metrics

CVSS Vector: CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:H/VI:H/VA:N/SC:N/SI:N/SA:N
CVSS Score: 9.1

Product Status

Vendor dropbox
Product samly
Versions Default: unaffected
  • affected from 0.3.0 to * (excl.)
Vendor dropbox
Product samly
Versions Default: unaffected
  • affected from 8a5bb1b4a4753d05470da2036323477f63cfdf4c to * (excl.)
Vendor handnot2
Product samly
Versions Default: unaffected
  • affected from 8a5bb1b4a4753d05470da2036323477f63cfdf4c to * (excl.)

Credits

  • Mark Madsen reporter
  • Jonatan Männchen / EEF coordinator

References

Problem Types

  • CWE-294 Authentication Bypass by Capture-replay CWE

Impacts

  • CAPEC-60 Reusing Session IDs (aka Session Replay)