Kong Gateway EUVD-2026-80387

Exploitation requires the Kong SAML plugin to be in use AND the validate_assertion_signature option to be EXPLICITLY set to false. This is the critical gate: the option is enabled (true) by default, so default configurations are NOT exploitable - only deployments where an operator deliberately disabled assertion-signature validation are affected. When that condition holds, no further prerequisites apply: a remote unauthenticated attacker (AV:N/PR:N/UI:N/AC:L) can submit a crafted SAML response with an unsigned assertion and impersonate arbitrary users including administrators. The limiting factor is entirely the non-default configuration; there is no required user interaction or authentication.

This is a genuine authentication-bypass flaw (CWE-288) but its real-world risk is strongly gated by a non-default configuration. The vendor CVSS 4.0 vector (AV:N/AC:L/AT:N/PR:N/UI:N, subsequent-system SC:H) confirms a remote, unauthenticated, low-complexity network vector against the SAML plugin - signals that in isolation read as critical. The decisive limiter, however, is stated plainly in the description: the flaw only manifests when validate_assertion_signature is EXPLICITLY set to false, and that option is enabled (true) by default. So default deployments are NOT vulnerable, which materially lowers the population at risk versus the raw pre-auth score. No EPSS score, no CISA KEV listing, and no public POC were provided, so there is no public exploit identified at time of analysis and no evidence of active or automatable exploitation. Net: treat as a real, patch-worthy authentication bypass for the subset of tenants that disabled signature validation, but it is not a blanket-critical, mass-exploitable issue - the configuration gate makes it targeted rather than widespread. Note also a modeling tension worth verifying with the vendor: the vendor 4.0 vector records only subsequent confidentiality (SC:H) with SI:N/SA:N, yet the description says an attacker can impersonate administrators, which would normally imply an integrity impact on protected downstream resources as well.

An operator has disabled validate_assertion_signature on the Kong SAML plugin. A remote unauthenticated attacker crafts a SAML response containing an unsigned assertion that names an administrator as the subject and submits it to the gateway's SAML assertion consumer endpoint; the plugin extracts the identity without verifying any cryptographic signature and authenticates the attacker as that admin. No public POC was provided, but exploitation is low-complexity given the missing signature check.