Curl_urldecode: no peeking beyond end of input buffer

Security problem: CVE-2013-2174

If a program would give a string like "%FF" to curl_easy_unescape() but
ask for it to decode only the first byte, it would still parse and
decode the full hex sequence. The function then not only read beyond the
allowed buffer but it would also deduct the *unsigned* counter variable
for how many more bytes there's left to read in the buffer by two,
making the counter wrap. Continuing this, the function would go on
reading beyond the buffer and soon writing beyond the allocated target
buffer...

Bug: http://curl.haxx.se/docs/adv_20130622.html
Reported-by: Timo Sirainen

diff --git a/lib/escape.c b/lib/escape.c
index 6a26cf8ef9ff0515c07cce92a63e16da66383c83..aa7db2c5b99d94287b4bac94c0b9be0ede22a2db 100644 (file)
--- a/lib/escape.c
+++ b/lib/escape.c
@@ -5,7 +5,7 @@
  *                            | (__| |_| |  _ <| |___
  *                             \___|\___/|_| \_\_____|
  *
- * Copyright (C) 1998 - 2011, Daniel Stenberg, <daniel@haxx.se>, et al.
+ * Copyright (C) 1998 - 2013, Daniel Stenberg, <daniel@haxx.se>, et al.
  *
  * This software is licensed as described in the file COPYING, which
  * you should have received as part of this distribution. The terms
@@ -159,7 +159,8 @@ CURLcode Curl_urldecode(struct SessionHandle *data,
 
   while(--alloc > 0) {
     in = *string;
-    if(('%' == in) && ISXDIGIT(string[1]) && ISXDIGIT(string[2])) {
+    if(('%' == in) && (alloc > 2) &&
+       ISXDIGIT(string[1]) && ISXDIGIT(string[2])) {
       /* this is two hexadecimal digits following a '%' */
       char hexstr[3];
       char *ptr;