Details for this vulnerability have not been published by NIST at this point. Descriptions from software vendor advisories for this issue are provided below.
The Go project reports:
net/http (through net/textproto) used to accept and normalize invalid
HTTP/1.1 headers with a space before the colon, in violation of RFC 7230.
If a Go server is used behind a reverse proxy that accepts and forwards
but doesn't normalize such invalid headers, the reverse proxy and the
server can interpret the headers differently. This can lead to filter
bypasses or request smuggling, the latter if requests from separate clients
are multiplexed onto the same connection by the proxy. Such invalid headers
are now rejected by Go servers, and passed without normalization to Go