Open redirect in pgAdmin 4's multi-factor authentication flow. The MFA validate and register endpoints honoured the user-supplied 'next' query/form parameter without confirming the target pointed back inside pgAdmin, so an authenticated victim who clicked /mfa/validate?next=<external> -- a link typically delivered by phishing -- would be sent to an attacker-controlled host directly out of the trusted auth flow. The defect is a trusted-domain redirect, not a privilege bypass: the attacker gains no read/write access to pgAdmin or the victim's database, but the redirect launders the attacker's destination through pgAdmin's URL, which raises the success rate of credential-phishing follow-on against the victim. Fix introduces a same-origin _is_safe_redirect_url helper and gates every MFA redirect that consumes user-supplied 'next' values through it. The helper allows only relative paths and absolute URLs whose scheme is http(s) and whose host matches the current request host.

it rejects external hosts in absolute and protocol-relative form, non-http schemes (javascript:, data:, mailto:), userinfo tricks (http://localhost@attacker/), and backslash variants that some browsers normalize to forward slashes. Unsafe targets fall back to the internal browser index. A dedicated regression test exercises each accept/reject category and the original reporter PoC. This issue affects pgAdmin 4: from 6.0 before 9.16.