CVE-2026-118656: Echo has Open Redirect via Control-Character Prefix Defeating sanitizeURI

Echo’s dedicated open-redirect guard, sanitizeURI, decides whether a redirect target is a local path by inspecting exactly two byte positions: uri[0] and uri[1]. If the value begins with a control character such as a tab or a newline, followed by a separator pair, the guard does not fire and the value is written to the Location header unchanged.

Browsers remove tab and newline characters from URLs before parsing them, and treat a backslash as a forward slash for HTTP and HTTPS URLs. A Location value of /\t/attacker.example is therefore resolved by the browser as an absolute URL on attacker.example. Confirmed end to end in a headless browser.