[NT] OllyDbg Format String Vulnerability ("INT3 AT")

From: SecuriTeam (support_at_securiteam.com)
Date: 05/15/05

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      OllyDbg Format String Vulnerability ("INT3 AT")
    ------------------------------------------------------------------------

    SUMMARY

    " <http://home.t-online.de/home/Ollydbg/> OllyDbg is a 32-bit assembler
    level analyzing debugger for Microsoft Windows. Emphasis on binary code
    analysis makes it particularly useful in cases where source is
    unavailable."

    The vulnerability in OllyDbg allows attackers to place a formated string
    in the program being debugged that can cause the OllyDbg program to
    execute arbitrary code.

    DETAILS

    Vulnerable Systems:
     * OllyDbg version 1.10

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

    Disassembler Snippets:
    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

    When 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 buffer) and cause Olly to overwrite arbitrary 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.

    ADDITIONAL INFORMATION

    The information has been provided by <mailto:bania.piotr@gmail.com> Piotr
    Bania.
    The original article can be found at:
    <http://pb.specialised.info/all/adv/olly-int3-adv.txt>
    http://pb.specialised.info/all/adv/olly-int3-adv.txt

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