Topic:                  unsafe fgets() using in qpopper

Software affected:      qpopper  3.0 fc2, qpopper  2.53  and  probably
                        others

Description:            malicious   user  can  remotely  post  message
                        with  spoofed  or incorrect headers (including
                        "Received:" one)  and  in  some  cases  bypass
                        virus  checking.  This can be used for sending
                        trojans or to attack vulnerabilities in MUA.

Status:                 Vendor  contacted, bug scheduled to be patched
                        in next release, FreeBSD port patched.

Background:

In most unix systems e-mail delivered to user is usually stored in his
mailbox,  which  has  predefined  format  (so-called "unix mailbox" or
"berkley  mailbox").  This  mailbox  holds  messages  in  plain format
separated  by  empty  line ("\n") and specially formed "From " header.
The pattern of the next message in mailbox is "\n\nForm ".

Then  local mail programs (f.e. mail.local) delivers message to user's
mailbox   it  searches  for  this  pattern and if message contains one
"From   "   will  be  commented out by '>' and additional '\n' will be
added  to  message  if  necessary.  This assumes mailbox integrity and
protects from e-mail spoofing.

Problem description:

qpopper  has vulnerability which allows for malicious user to generate
his  own  "From " with followed email headers and text. The problem is
in  the  way  qpopper reads data from mailbox. Qpopper uses fgets() or
fgets()-like routine, mfgets(), which reads data from mailbox into the
fixed  1024  byte  buffer  and  returns  string  in  case  either '\n'
character  received  or  1023  bytes read. Malicious user can put text
like (without leading spaces):

   AAAA...AAA(string of 1023 symbols)\n
   From user Wed Dec  2 05:53 -0700 1992

In this case  fgets() will return 3 strings:
"AAAA...AAA(string of 1023)symbols", without '\n',
"\n",
"From user Wed Dec 2 05:53 -0700 1992"
and  this  will be recognized as a beginning of the new message in the
mailbox.

Text  after "From " string will be recognized as a headers and text of
the   next  message,  allowing  to  generate  any  headers  and  text.
Additionally, this "internal" messages will be treated by any software
as  a  plain  text  inside message, without any MIME attachments. This
allows  to  bypass  virus  checking in case antiviral tools scans only
attached files.

Possible temporary fix for qpopper 3.0 fc2 (not tested):

--- pop_dropcopy.c      Sat Mar 18 02:31:11 2000
+++ pop_dropcopy.c      Wed Apr 12 18:11:11 2000
@@ -205,6 +205,8 @@


 int newline = 1;
+int isbreaked = 0;
+int wasbreaked = 0;

 /*
  *  0 for not a from line
@@ -229,6 +231,14 @@

     /* If the previous line was not a newline then just return */
     /* From message separators are preceeded by a newline */
+    if (isbreaked) {
+       wasbreaked = 1;
+       return ( 0 );
+    }
+    if (wasbreaked) {
+       wasbreaked = 0;
+       return ( 0 );
+    }
     if ( *cp == '\n' ) {
         newline = 1;
         return ( 0 );
@@ -1593,9 +1603,13 @@
     if( size <= 0 ) {
     return NULL;
     }
+    isbreaked = 1;
     while( --size && ((c = getc(stream)) != EOF) ) {
     if( (*p = (char)c) == '\0' ) *p = ' ';
-    if( *p++ == '\n' ) break;
+       if( *p++ == '\n' ) {
+           isbreaked = 0;
+           break;
+       }
     }
     if( p == s ) return NULL;
     *p = '\0';


Additional Info:

mail.local  also  uses  fgets() for reading input message, but default
buffer  size is 2048, so "From " will not be commented and problem can
be  exploited.  If another local mailer is used with same behavior and
buffer  size  1024 or mailer splits strings of 1024 bytes this problem
couldn't be exploited.


http://www.security.nnov.ru
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