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.

main/java/com/ning/http/client/AsyncHttpClientConfig.java in Async Http Client (aka AHC or async-http-client) before 1.9.0 does not require a hostname match during verification of X.509 certificates, which allows man-in-the-middle attackers to spoof HTTPS servers via an arbitrary valid certificate.