[NT] OllyDbg Format String Bug

From: SecuriTeam (support_at_securiteam.com)
Date: 07/20/04

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Date: 20 Jul 2004 10:12:59 +0200

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      OllyDbg Format String Bug
    ------------------------------------------------------------------------

    SUMMARY

    OllyDbg is a "32-bit assembler level analyzing debugger for Microsoft
    Windows". There exists a format string bug in the code that handles
    Debugger Messages in OllyDbg. This means any traced application can crash
    OllyDbg and execute machine code.

    DETAILS

    Vulnerable Systems:
     * OllyDbg version 1.10

    Typically OllyDbg attaches to a process and allows the user how to
    customize the session; whether they trace, or they breakpoint some stuff
    or whatever. The Windows API is actually very debugger friendly and has
    many functions to interact with debuggers (most likely built for their own
    (safe) debugger WinDbg). One of these functions, OutputDebugString sends a
    string directly to the debugger for interpretation, which OllyDbg displays
    to the user via a status line along the bottom, lacks a format specifiers,
    which means the user supplied string is used as the format specifiers.

    To reproduce this excellent bug, these steps can be taken:
    1. Download <http://python.org> Python and
    <http://starship.python.net/crew/mhammond/win32/Downloads.html> win32com.
    These two are _essential_ to any hacker's Windows box.

    2. Run 'python' so you get an interactive shell.

    3. Attach to the 'python' process with OllyDbg, press 'F9' to continue
    execution.

    4. Type 'import win32api' and press enter in the python screen.

    5. Type 'win32api.OutputDebugString("%s" * 50)' to crash OllyDbg.
    Typically, if you have OllyDbg set as the JIT Debugger, another OllyDbg
    screen will pop up OR

    6. Type 'win32api.OutputDebugString("%8.8x" * 15)' to view what is
    currently in the stack

    7. The python process will now have died since OllyDbg died, so do the
    process again

    ADDITIONAL INFORMATION

    The information has been provided by <mailto:nd@felinemenace.org> ned.

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