inappropriate_fallback
inappropriate_fallback (86) is sent by a server when a client performed a TLS version downgrade using the TLS_FALLBACK_SCSV signaling cipher suite but the downgraded version is lower than the server's highest supported version. This alert prevents POODLE-style downgrade attacks where a MITM forces the client to negotiate a weaker TLS version.
Alert Code
86
Level
Fatal
Standard
RFC 8446
Description
TLS_FALLBACK_SCSV (RFC 7507) was introduced to prevent downgrade attacks like POODLE. When a TLS client retries a failed connection with a lower version and includes TLS_FALLBACK_SCSV in its cipher suite list, the server checks whether the offered version is lower than its maximum supported version. If it is, the server sends inappropriate_fallback, preventing the downgrade. TLS 1.3 makes this largely obsolete by using the supported_versions extension in the ClientHello, but the mechanism remains important for TLS 1.2 deployments.
Common Causes
- –Client downgraded from TLS 1.3 to TLS 1.2 with TLS_FALLBACK_SCSV – server supports 1.3
- –MITM attack forcing protocol downgrade detected
- –TLS client retry logic producing unnecessary version downgrade