What is a downgrade attack?
A downgrade attack interferes with the negotiation that secure protocols perform, so two systems settle on an older or weaker option than both actually support. When a browser connects to a server, it offers the protocol versions and cipher suites it understands, and the server picks the strongest one they share. An attacker who can sit between the two, on-path, manipulates that exchange so they agree on something the attacker can read or break.
How it works
The attacker tampers with the handshake, strips the stronger options, or forces one side to retry with a weaker version. Historic examples show the pattern. POODLE in 2014 relied on browsers that retried a failed connection with an older version until they fell back to SSL 3.0, whose padding could then be attacked. FREAK and Logjam in 2015 tricked connections into using deliberately weak export-grade 512-bit keys left over from 1990s regulations. The same idea reaches beyond web traffic: an attacker can push a Wi-Fi client in WPA3 transition mode down to WPA2.
SSL stripping is a downgrade to no encryption
SSL stripping takes a connection from HTTPS down to plain HTTP. When a user types a bare domain, a browser that does not upgrade on its own sends the first request over HTTP, and the server answers with a redirect to HTTPS. An on-path attacker, for example someone running a coffee-shop access point, intercepts that first request, makes the HTTPS connection to the real server themselves, and relays plain HTTP to the victim. The victim sees http:// with no padlock and no certificate warning, because no certificate is ever presented, and types a password that travels in cleartext through the attacker.
How to defend against it
Remove the weak options entirely, because a downgrade can only reach versions both sides still accept. Disable SSL 2.0 and 3.0, TLS 1.0 and 1.1, and weak ciphers such as export, NULL, RC4 and 3DES, and set TLS 1.2 as the minimum with TLS 1.3 preferred. TLS 1.3 also carries built-in downgrade protection: a server that supports it signals that fact in the handshake, so a modern client can detect tampering and abort. Against SSL stripping, HTTP Strict Transport Security (HSTS) tells the browser to use HTTPS for a domain on its own, and the preload list closes the first-visit gap.
Downgrade attack on the CompTIA Security+ exam
Downgrade attacks are listed as indicators of malicious activity under objective 2.5. A scenario that mentions a client suddenly negotiating an old TLS version, a login page loading over plain HTTP when the server already redirects to HTTPS, or a missing padlock on public Wi-Fi points at a downgrade or SSL stripping. The tempting wrong answers are a plain on-path attack with a forged certificate, which does produce a certificate warning, and a simple misconfiguration. The exam-correct fix is to remove the weak versions and ciphers and to enforce HSTS, not to add a CAPTCHA or change passwords.