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 golang#75716

Change-Id: I1a98ce85a21b830cb25799f9ac9333a67400d736
Reviewed-on: https://go-internal-review.googlesource.com/c/go/+/2940
Reviewed-by: Roland Shoemaker <[email protected]>
Reviewed-by: Nicholas Husin <[email protected]>
Reviewed-on: https://go-review.googlesource.com/c/go/+/709859
TryBot-Bypass: Michael Pratt <[email protected]>
Auto-Submit: Michael Pratt <[email protected]>
Reviewed-by: Carlos Amedee <[email protected]>

Author: neild

src/net/textproto/reader.go

@@ -285,8 +285,10 @@ func (r *Reader) ReadCodeLine(expectCode int) (code int, message string, err err
 //
 // An expectCode <= 0 disables the check of the status code.
 func (r *Reader) ReadResponse(expectCode int) (code int, message string, err error) {
-	code, continued, message, err := r.readCodeLine(expectCode)
+	code, continued, first, err := r.readCodeLine(expectCode)
 	multi := continued
+	var messageBuilder strings.Builder
+	messageBuilder.WriteString(first)
 	for continued {
 		line, err := r.ReadLine()
 		if err != nil {
@@ -297,12 +299,15 @@ func (r *Reader) ReadResponse(expectCode int) (code int, message string, err err
 		var moreMessage string
 		code2, continued, moreMessage, err = parseCodeLine(line, 0)
 		if err != nil || code2 != code {
-			message += "\n" + strings.TrimRight(line, "\r\n")
+			messageBuilder.WriteByte('\n')
+			messageBuilder.WriteString(strings.TrimRight(line, "\r\n"))
 			continued = true
 			continue
 		}
-		message += "\n" + moreMessage
+		messageBuilder.WriteByte('\n')
+		messageBuilder.WriteString(moreMessage)
 	}
+	message = messageBuilder.String()
 	if err != nil && multi && message != "" {
 		// replace one line error message with all lines (full message)
 		err = &Error{code, message}