Ruby through 2.4.7 2.5.x through 2.5.6 and 2.6.x through 2.6.4 allows HTTP Response Splitting. If a program using WEBrick inserts untrusted input into the response header an attacker can exploit it to insert a newline character to split a header and inject malicious content to deceive clients. NOTE: this issue exists because of an incomplete fix for CVE-2017-17742 which addressed the CRLF vector but did not address an isolated CR or an isolated LF.