CVE-2026-91165: Warpgate: Markup injection in SSO form_post return page via unencoded redirect/error values

Warpgate is an open source SSH, HTTPS and MySQL bastion host for Linux. Prior to 0.27.6, the responsemode=formpost SSO return path in warpgate-protocol-http/src/api/ssoproviderlist.rs uses serdejson::tostring inside ReturnToSsoPostResponse without neutralizing a script-closing sequence. The vulnerable value can enter the script block through the attacker-controlled next redirect parameter stored by warpgate-protocol-http/src/api/ssoproviderdetail.rs or through IdP-derived error messages that makeredirecturl concatenates without URL encoding. The IdP-derived path is reachable when the attacker controls a configured identity provider, or when the attacker controls the email or username claim on an attacker-controlled account and the configured identity provider permits the required unvalidated claim format. A victim must complete the formpost SSO flow for the injected markup to be rendered. The Warpgate Content-Security-Policy blocks injected JavaScript and event handlers, so the demonstrated impact is content spoofing, a false login form, or a meta refresh rather than script execution. This issue is fixed in version 0.27.6.