Vulnerability · Published Aug 20, 2026 · Assigned by EEF

CVE-2026-53424Missing one-time-use enforcement in Samly allows replay of SAML bearer assertions

Critical 9.1

In plain English

A critical-severity security flaw in samly from dropbox. There are no confirmed reports of exploitation so far. The chance it is exploited in the next 30 days is estimated at 0.6% (higher than 49% of all known vulnerabilities).

Severity (CVSS 4.0)9.1Critical

Exploit odds (30 days)0.6%top 51.0% of all CVEs

Exploited in the wildNo reportsCISA SSVC: none

Attack toolingNoneno public tooling known

Official description

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.

Affected products (as reported)

Vendor	Product	Versions
dropbox	samly	0.3.0 – before *
dropbox	samly	8a5bb1b4a4753d05470da2036323477f63cfdf4c – before *
handnot2	samly	8a5bb1b4a4753d05470da2036323477f63cfdf4c – before *

Weakness: CWE-294 · 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

References

https://cna.erlef.org/cves/CVE-2026-53424.html related, third-party-advisory
https://osv.dev/vulnerability/EEF-CVE-2026-53424 related