Affected versions of this crate suffered from an integer overflow bug when calculating the size of a buffer to use when encoding base64 using the encode_config_buf and encode_config functions. If the input string was large, this would cause a buffer to be allocated that was too small. Since this function writes to the buffer using unsafe code, it would allow an attacker to write beyond the buffer, causing memory corruption and possibly the execution of arbitrary code.

This flaw was corrected by using checked arithmetic to calculate the size of the buffer.

hyper's HTTP header parser accepted, according to RFC 7230, illegal contents inside Content-Length headers. Due to this, upstream HTTP proxies that ignore the header may still forward them along if it chooses to ignore the error.

To be vulnerable, hyper must be used as an HTTP/1 server and using an HTTP proxy upstream that ignores the header's contents but still forwards it. Due to all the factors that must line up, an attack exploiting this vulnerability is unlikely.

When decoding chunk sizes that are too large, hyper's code would encounter an integer overflow. Depending on the situation, this could lead to data loss from an incorrect total size, or in rarer cases, a request smuggling attack.

To be vulnerable, you must be using hyper for any HTTP/1 purpose, including as a client or server, and consumers must send requests or responses that specify a chunk size greater than 18 exabytes. For a possible request smuggling attack to be possible, any upstream proxies must accept a chunk size greater than 64 bits.