The Set-Cookie: header might carry any combination of characters. In case a consecutive sequence of semicolons ";;" is encountered, the client will crash because of this code:
https://github.com/swift-server/async-http-client/pull/207/files#diff-9f1723c83114eacc62a23489d124343eR117
Client termination when encountering such headers is not desired and might result in a denial of service.