Despite HTTP headers having a default limit of 1 MB, the number of cookies that can be parsed did not have a limit.
By sending a lot of very small cookies such as "a=;", an attacker can make an HTTP server allocate a large amount of structs, causing large memory consumption.

net/http now limits the number of cookies accepted to 3000, which can be adjusted using the httpcookiemaxnum GODEBUG option.

Thanks to jub0bs for reporting this issue.

This is CVE-2025-58186 and Go issue https://go.dev/issue/75672.

This is a PRIVATE issue for CVE-2025-58186, tracked in http://b/442560983 and fixed by https://go-internal-review.git.corp.google.com/c/go/+/2720.

/cc @golang/security and @golang/release