[UNIX] Remote Overflow in tHTTPd (< > replacing)

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To: list@securiteam.com
Date: 28 Oct 2003 10:41:35 +0200

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  Remote Overflow in tHTTPd (< > replacing)
------------------------------------------------------------------------

SUMMARY

A vulneability in tHTTPd allows remote attackers to overflow an internal
buffer and partially overwrite EBP register and execute arbitrary code.

DETAILS

Vulnerable systems:
 * tHTTPd version 2.21 up to version 2.23b1

Immune systems:
 * tHTTPd version 2.24

The problem is found in libhttpd.c in the function defang()
static void defang( char* str, char* dfstr, int dfsize )
    {
    char* cp1;
    char* cp2;

    for ( cp1 = str, cp2 = dfstr;
   *cp1 != '\0' && cp2 - dfstr < dfsize - 1;
   ++cp1, ++cp2 )
 {
 switch ( *cp1 )
     {
     case '<':
     *cp2++ = '&';
     *cp2++ = 'l';
     *cp2++ = 't';
     *cp2 = ';';
     break;
     case '>':
     *cp2++ = '&';
     *cp2++ = 'g';
     *cp2++ = 't';
     *cp2 = ';';
     break;
     default:
     *cp2 = *cp1;
     break;
     }
 }
    *cp2 = '\0';
    }

So when '<' or '>' are found in the input we "pay for 1 and get 3 for
free", this allows us overwrite bits of EBP and indirectly control EIP
(assuming its been compiled with gcc < 3.0).

Workaround:
Upgrade to version 2.24

Disclosure Timeline:
09/08/2003: Vendor notified by e-mail
09/12/2003: Vendor replies with working fix
10/27/2003: Public release

ADDITIONAL INFORMATION

The information has been provided by <mailto:advisories@texonet.com>
texonet.com.

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