Samly lib/samly/helper.ex Samly.Helper.decode_idp_auth_resp authentication replay

Summaryinfo

A vulnerability was found in Samly. It has been rated as critical. Affected is the function Samly.Helper.decode_idp_auth_resp of the file lib/samly/helper.ex. This manipulation causes authentication replay. The identification of this vulnerability is CVE-2026-53424. It is possible to initiate the attack remotely. There is no exploit available.

Detailsinfo

A vulnerability has been found in Samly (version unknown) and classified as critical. Affected by this vulnerability is the function Samly.Helper.decode_idp_auth_resp of the file lib/samly/helper.ex. The manipulation with an unknown input leads to a authentication replay vulnerability. The CWE definition for the vulnerability is CWE-294. A capture-replay flaw exists when the design of the product makes it possible for a malicious user to sniff network traffic and bypass authentication by replaying it to the server in question to the same effect as the original message (or with minor changes). As an impact it is known to affect confidentiality, integrity, and availability. The summary by CVE is:

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 v0.3.0 onward.

It is possible to read the advisory at cna.erlef.org. This vulnerability is known as CVE-2026-53424 since 06/09/2026. The exploitation appears to be easy. The attack can be launched remotely. The exploitation doesn't need any form of authentication. Technical details of the vulnerability are known, but there is no available exploit. The pricing for an exploit might be around USD $0-$5k at the moment (estimation calculated on 08/20/2026). The attack technique deployed by this issue is T1040 according to MITRE ATT&CK.

There is no information about possible countermeasures known. It may be suggested to replace the affected object with an alternative product.

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• Samly

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