From: Damien Neil <dneil@google.com>
Date: Tue, 30 Sep 2025 22:11:16 +0000 (-0700)
Subject: [release-branch.go1.25] net/textproto: avoid quadratic complexity in Reader.ReadResponse
X-Git-Tag: go1.25.2~7
X-Git-Url: http://www.git.cypherpunks.su/?a=commitdiff_plain;h=5d7a787aa2b486f77537eeaed9c38c940a7182b8;p=gostls13.git

[release-branch.go1.25] 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
For #75716
Fixes #75718

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-internal-review.googlesource.com/c/go/+/2960
Reviewed-by: Damien Neil <dneil@google.com>
Reviewed-on: https://go-review.googlesource.com/c/go/+/709846
Reviewed-by: Carlos Amedee <carlos@golang.org>
TryBot-Bypass: Michael Pratt <mpratt@google.com>
Auto-Submit: Michael Pratt <mpratt@google.com>
---

diff --git a/src/net/textproto/reader.go b/src/net/textproto/reader.go
index d375340121..94e3e0b53d 100644
--- a/src/net/textproto/reader.go
+++ b/src/net/textproto/reader.go
@@ -284,8 +284,10 @@ 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 {
@@ -296,12 +298,15 @@ 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}