Negotiating Down
A downgrade attack manipulates negotiation so two parties settle on a weaker protocol or cipher than both support. An on path attacker strips or tampers with capability advertisements so the connection falls back to something exploitable.
Common Forms
- Stripping HTTPS to plain HTTP when a user types a bare domain.
- Forcing an older TLS version or a weak cipher suite during the handshake.
- Removing modern features so a legacy, vulnerable path is taken.
Defenses
- Send HSTS so browsers refuse plain HTTP for known sites.
- Disable obsolete protocol versions and weak ciphers entirely.
- Use handshake integrity checks that detect tampering with negotiated parameters.
Key idea
Downgrade attacks force a weaker negotiated protocol, so disable obsolete versions, enforce HSTS, and validate handshake integrity.