oss-security - Exim 4.96 overflow

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Date: Tue, 9 Aug 2022 15:03:34 +0300
From: Evgeny Legerov <admin@...ndisco.cc>
To: oss-security@...ts.openwall.com
Subject: Exim 4.96 overflow

Hi,


Yet another interesting issue in Exim 4.96, it is OpenBSD specific.

Combination OpenBSD + Exim is very rare, so it probably affects only two 
boxes in the world,one of them is my vm.


OpenBSD dn_expand() source:
int
dn_expand(const u_char *msg, const u_char *eomorig, const u_char *comp_dn,
     char *exp_dn, int length)
{
const u_char *cp;
         char *dn;
         int n, c;
         char *eom;
         int len = -1, checked = 0;

         dn = exp_dn;
         cp = comp_dn;
         if (length > HOST_NAME_MAX)
                 length = HOST_NAME_MAX;
         eom = exp_dn + length;
         while ((n = *cp++)) {
                 switch (n & INDIR_MASK) {
                 case 0:
                         if (dn != exp_dn) {
                                 if (dn >= eom)
                                         return (-1);
                                 *dn++ = '.';
                         }
                         if (dn+n >= eom)
                                 return (-1);
                         checked += n + 1;
                         while (--n >= 0) {
                                 if (((c = *cp++) == '.') || (c == '\\')) {
                                         if (dn + n + 2 >= eom)
                                                 return (-1);
                                         *dn++ = '\\';
                                 }
                                 *dn++ = c;
                                 if (cp >= eomorig)      /* out of range */
                                         return (-1);
                         }
                         break;

                 case INDIR_MASK:
                         if (len < 0)
                                 len = cp - comp_dn + 1;
                         cp = msg + (((n & 0x3f) << 8) | (*cp & 0xff));
                         if (cp < msg || cp >= eomorig)  /* out of range */
                                 return (-1);
                         checked += 2;
                         /*
                          * Check for loops in the compressed name;
                          * if we've looked at the whole message,
                          * there must be a loop.
                          */
                         if (checked >= eomorig - msg)
                                 return (-1);
                         break;

                 default:
                         return (-1);                    /* flag error */
                 }
         }
         *dn = '\0';
         if (len < 0)
                 len = cp - comp_dn;
         return (len);
}

As we can see, dn_expand() does not escape special characters, in 
particular it ignores '\n'.
In case of Exim, after it does a reverse dns lookup, the answer is 
parsed using dn_expand() and

it is stored in 'sender_host_name' global variable.
This variable is written into spool header file.


Many interesting things can happen when we control the contents of spool 
file:

  if (flags & 0x01)      /* one_time data exists */
       {
       int len;
       while (isdigit(*(--p)) || *p == ',' || *p == '-');
       (void)sscanf(CS p+1, "%d,%d", &len, &pno);
       *p = 0;
       if (len > 0)
         {
         p -= len;
[1]        errors_to = string_copy_taint(p, GET_TAINTED);
         }
       }

[2]    *--p = 0;   /* Terminate address */

As long as we control 'len' variable, we have out of bounds read on line 
#1, and out of bounds write on line #2.

It may not be very practical attack,as someone says you need arp 
spoofing for this attack to work.

Your opinions would be very interesting.


regards,

-e
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