CVE-2026-61629: nginx ignition has ParseAcceptLanguage `_` separator bypass that enables ~75x CPU amplification via Accept-Language header in i18nMiddleware

The gin i18n middleware in nginx-ignition’s API server runs in front of every HTTP request and calls golang.org/x/text/language.ParseAcceptLanguage on the raw Accept-Language header without imposing any size or shape filter. The underlying parser has quadratic-time behaviour on long lists of malformed language tags. The CVE-2022-32149 guard that golang.org/x/text added in v0.3.8 caps the number of - characters in the input at 1000, but it does not cap _ characters even though the parser’s internal scanner aliases _ to - before parsing. A single unauthenticated GET request with an Accept-Language header built out of _ separators burns about 2.4 seconds of server CPU on the host running nginx-ignition; ten concurrent attackers saturate a ten-core box for the duration of the attack while consuming ~10 MiB/s of upstream bandwidth.