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

From: SecuriTeam (support_at_securiteam.com)
Date: 10/28/03

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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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