It was found that async-http-client did not verify that the server hostname matched the domain name in the subject's Common Name (CN) or subjectAltName field in X.509 certificates. This could allow a man-in-the-middle attacker to spoof an SSL server if they had a certificate that was valid for any domain name.

Async Http Client (aka AHC or async-http-client) before 1.9.0 skips X.509 certificate verification unless both a keyStore location and a trustStore location are explicitly set, which allows man-in-the-middle attackers to spoof HTTPS servers by presenting an arbitrary certificate during use of a typical AHC configuration, as demonstrated by a configuration that does not send client certificates.