Buffer-overflow in Ultr@VNC 1.0.1 viewer and server

Discussion in 'Forum for discussion of ANTICHAT' started by NeMiNeM, 7 Apr 2006.

  1. NeMiNeM

    NeMiNeM Elder - Старейшина

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    #######################################################################

    Luigi Auriemma

    Application: Ultr@VNC
    http://www.ultravnc.com
    http://ultravnc.sourceforge.net
    Versions: <= 1.0.1 (and current CVS)
    (tabbed_viewer 1.29 is ever the same VNC viewer 1.0.1 and
    so it's vulnerable too)
    Platforms: Windows
    Bugs: A] client Log::ReallyPrint buffer-overflow
    B] server VNCLog::ReallyPrint limited buffer-overflow
    Exploitation: A] remoto, versus client
    B] remoto, versus server
    Date: 04 Apr 2006
    Author: Luigi Auriemma
    e-mail: aluigi (at) autistici (dot) org [email concealed]
    web: http://aluigi.altervista.org

    #######################################################################

    1) Introduction
    2) Bugs
    3) The Code
    4) Fix

    #######################################################################

    ===============
    1) Introduction
    ===============

    Ultr@VNC is a well known open source VNC server and viewer for Windows
    very easy to use and configure.

    #######################################################################

    =======
    2) Bugs
    =======

    ------------------------------------------
    A] client Log::ReallyPrint buffer-overflow
    ------------------------------------------

    During the login process a VNC client can receive three types of
    replies from the server: connection failed, no authentication and
    authentication required.
    The first type of reply (rfbConnFailed) is followed by a text string
    containing the reason of the disconnection.
    Before visualizing this message Ultr@VNC logs everything in the log
    file using the vnclog.Print function which adopts a buffer of 1024
    bytes (LINE_BUFFER_SIZE) for storing the text.
    The result is that a malicious VNC server could be able to execute
    malicious code versus a vulnerable Ultr@VNC client which connects to
    it.

    From vncviewer/Log.cpp:

    void Log::ReallyPrint(LPTSTR format, va_list ap)
    {
    TCHAR line[LINE_BUFFER_SIZE];
    _vstprintf(line, format, ap);
    if (m_todebug) OutputDebugString(line);

    if (m_toconsole) {
    DWORD byteswritten;
    WriteConsole(GetStdHandle(STD_OUTPUT_HANDLE), line, _tcslen(line)*sizeof(TCHAR), &byteswritten, NULL);
    };

    if (m_tofile && (hlogfile != NULL)) {
    DWORD byteswritten;
    WriteFile(hlogfile, line, _tcslen(line)*sizeof(TCHAR), &byteswritten, NULL);

    }
    }

    -----------------------------------------------------
    B] server VNCLog::ReallyPrint limited buffer-overflow
    -----------------------------------------------------

    The logging function used by the Ultr@VNC server is affected by a
    limited buffer-overflow caused by two strcat calls which add a Windows
    error message to the output buffer.
    Anyway there is an important detail about the exploitation of this bug.
    The server is not vulnerable if the admin doesn't touch the "Log debug
    infos to the WinVNC.log file" flag in the configuration, but when the
    admin enables this option his server will be vulnerable forever
    although he will re-disable it.

    From winvnc/winvnc/vnclog.cpp:

    void VNCLog::ReallyPrint(const char* format, va_list ap)
    {
    time_t current = time(0);
    if (current != m_lastLogTime) {
    m_lastLogTime = current;
    ReallyPrintLine(ctime(&m_lastLogTime));
    }

    // - Write the log message, safely, limiting the output buffer size
    TCHAR line[LINE_BUFFER_SIZE];
    TCHAR szErrorMsg[LINE_BUFFER_SIZE];
    DWORD dwErrorCode = GetLastError();
    SetLastError(0);
    FormatMessage(
    FORMAT_MESSAGE_FROM_SYSTEM, NULL, dwErrorCode,
    MAKELANGID(LANG_NEUTRAL, SUBLANG_DEFAULT),(char *)&szErrorMsg,
    LINE_BUFFER_SIZE, NULL);
    _vsnprintf(line, LINE_BUFFER_SIZE, format, ap);
    strcat(line," --");
    strcat(line,szErrorMsg);

    ReallyPrintLine(line);
    }

    #######################################################################

    ===========
    3) The Code
    ===========

    http://aluigi.altervista.org/poc/uvncbof.zip

    #######################################################################

    ======
    4) Fix
    ======

    A patch will be released in the next weeks.

    #######################################################################

    ---
    Luigi Auriemma
    http://aluigi.altervista.org

    --
    http://www.securityfocus.com
     
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