Affected versions of this crate called mem::uninitialized() in the HTTP1 parser to create values of type httparse::Header (from the httparse crate). This is unsound, since Header contains references and thus must be non-null. The flaw was corrected by avoiding the use of mem::uninitialized(), using MaybeUninit instead.
When used on Windows platforms, all versions of Hyper prior to 0.9.4 did not perform hostname verification when making HTTPS requests. This allows an attacker to perform MitM attacks by preventing any valid CA-issued certificate, even if there's a hostname mismatch. The problem was addressed by leveraging rust-openssl's built-in support for hostname verification.
hyper's HTTP server code had a flaw that incorrectly understands some requests with multiple transfer-encoding headers to have a chunked payload, when it should have been rejected as illegal. This combined with an upstream HTTP proxy that understands the request payload boundary differently can result in "request smuggling" or "desync attacks".
Vulnerable versions of hyper allow GET requests to have bodies, even if there is no Transfer-Encoding or Content-Length header. As per the HTTP 1.1 specification, such requests do not have bodies, so the body will be interpreted as a separate HTTP request. This allows an attacker who can control the body and method of an HTTP request made by hyper to inject a request with headers that would not otherwise …
Serializing of headers to the socket did not filter the values for newline bytes (\r or \n), which allowed for header values to split a request or response. People would not likely include newlines in the headers in their own applications, so the way for most people to exploit this is if an application constructs headers based on unsanitized user input. This issue was fixed by replacing all newline characters …
hyper's HTTP/1 server code had a flaw that incorrectly parses and accepts requests with a Content-Length header with a prefixed plus sign, when it should have been rejected as illegal. This combined with an upstream HTTP proxy that doesn't parse such Content-Length headers, but forwards them, can result in "request smuggling" or "desync attacks".
hyper's HTTP server and client code had a flaw that could trigger an integer overflow when decoding chunk sizes that are too big. This allows possible data loss, or if combined with an upstream HTTP proxy that allows chunk sizes larger than hyper does, can result in "request smuggling" or "desync attacks".