A user can tell curl >= 7.20.0 and <= 7.78.0 to require a successful upgrade to TLS when speaking to an IMAP, POP3 or FTP server ("--ssl-reqd" on the command line or"CURLOPT_USE_SSL" set to "CURLUSESSL_CONTROL" or "CURLUSESSL_ALL" withlibcurl). This requirement could be bypassed if the server would return a properly crafted but perfectly legitimate response.This flaw would then make curl silently continue its operations withoutTLS contrary to the instructions and expectations, exposing possibly sensitive data in clear text over the network.