net/textproto: avoid quadratic complexity in Reader.ReadResponse
Reader.ReadResponse constructed a response string from repeated
string concatenation, permitting a malicious sender to cause excessive
memory allocation and CPU consumption by sending a response consisting
of many short lines.
Use a strings.Builder to construct the string instead.
Thanks to Jakub Ciolek for reporting this issue.
Fixes CVE-2025-61724
Fixes #75716
Change-Id: I1a98ce85a21b830cb25799f9ac9333a67400d736
Reviewed-on: https://go-internal-review.googlesource.com/c/go/+/2940 Reviewed-by: Roland Shoemaker <bracewell@google.com> Reviewed-by: Nicholas Husin <husin@google.com>
Reviewed-on: https://go-review.googlesource.com/c/go/+/709859
TryBot-Bypass: Michael Pratt <mpratt@google.com>
Auto-Submit: Michael Pratt <mpratt@google.com> Reviewed-by: Carlos Amedee <carlos@golang.org>