bugtraq - OllyDbg "INT3 AT" Format String Vulnerability

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Date: Fri, 13 May 2005 15:04:16 +0200
From: Piotr Bania <bania.piotr@...il.com>
To: SBUGTRAQ <bugtraq@...urityfocus.com>,
	FULLDISC <full-disclosure@...ts.grok.org.uk>,
	vulnwatch@...nwatch.org, news@...uriteam.com
Subject: OllyDbg "INT3 AT" Format String Vulnerability





      OllyDbg "INT3 AT" Format String Vulnerability
      by Piotr Bania <bania.piotr@...il.com>
      http://pb.specialised.info


      Original location:
      http://pb.specialised.info/all/adv/olly-int3-adv.txt
	
      Severity: 			High / Medium - code execution.
      Version affected:  	Probably all versions, tested on
				v1.10.

	

      I. BACKGROUND

      "OllyDbg is a 32-bit assembler level analysing debugger for
       Microsoft Windows. Emphasis on binary code analysis makes it
       particularly useful in cases where source is unavailable."


      II. DESCRIPTION		

      Vulnerability takes place when module (with special crafted file
      name) executes int 3 instruction (trap to debugger).

      Here is the vulnerable code:

      .text:0042FBE0                 lea     eax, [ebp+buffer]
      .text:0042FBE6                 push    eax        ; format string
      .text:0042FBE7                 mov     edx, [ebp+var_28]
      .text:0042FBEA                 push    edx
      .text:0042FBEB                 call    sub_42E100 ; _vsprintf->
                                                        ;___vprinter


      Where format is an ascii string like: "INT3 command at
      <module_name>.addr".

      Attacker can place a format string chars inside "<module_name>"
      (part of format buffor) and cause Olly to overwrite arbitary data.

      NOTE: Even with "IGNORE INT3 BREAKS" option checked, OllyDbg is
            still vulnerable. Attacker can also load some special crafted
            module (with special crafted name) while debugging, to make
            the attack more stealthy.


      III. IMPACT

      This vulnerability after successful exploitation can allow the
      attacker to run arbitrary code in context of current user.
      Of course if the exploitation was not successful OllyDbg will fault
      and loose all debugged data.



best regards,
Piotr Bania

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