CAPTCHA
acronymsecurityweb dev
Stands for: Completely Automated Public Turing test to tell Computers and Humans Apart
A challenge meant to tell humans and bots apart.
Completely Automated Public Turing test to tell Computers and Humans Apart is a challenge, like distorted text or image selection, that is easy for people and hard for bots. It is a last-resort defense because it adds real user friction.
CAPTCHA tries to distinguish humans from automated clients, and the arms race has been running long enough that the premise is now shaky: machine vision solves the visual puzzles better than people do, and commercial solving services provide humans at low cost regardless.
What replaced the puzzle is behavioural and cryptographic. Risk scoring from browsing signals, proof-of-work challenges that cost an attacker more than a user, and attestation approaches that vouch for a client without identifying it are all attempts to keep the property while dropping the puzzle. The accessibility cost of the old approach was always severe, and any discussion of these systems that omits blind and motor-impaired users is describing a solution that excludes people to keep out machines.
Also known as: captcha