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TUCoPS :: Web :: Servers :: nfr.htm

NFR Web Server stack overflow

    NFR Web Server


    NFR Web Server 2.0.2


    Following is based  on Network Associates  Security Advisory.   An
    implementation  fault  in  the  Network  Flight  Recorder  network
    forensics system makes it possible for a remote attacker to obtain
    system management privileges  on boxes running  NFR in a  standard
    configuration.  This problem has been confirmed and is known to be
    exploitable  on  hosts  running  Network  Flight  Recorder's   NFR
    2.0.2-Research release.  Because source code is publicly available
    for this software, it is possible to confirm vulnerability to this
    problem by inspecting the source used to build an installation  of
    NFR on an arbitrary host.

    The Network Flight Recorder custom  web server is used to  present
    an HTTP front-end to the NFR  system.  By default, the web  server
    is called "webd", and is bound  to TCP port 2001.  In  the absence
    of external network access control, arbitrary remote attackers can
    conduct  transactions  with  the  NFR  web  server.   Due  to   an
    implementation  fault  in  "webd",  it  is  possible  for a remote
    attacker to formulate an HTTP transaction that will cause the  web
    server  to  overflow  an  automatic  variable  on  the  stack.  By
    overwriting activation records stored on the stack, it is possible
    to  force  a  transfer  of  control  into  arbitrary  instructions
    provided by the  attacker in the  HTTP transaction, and  thus gain
    total  control  of   the  web  server   process.   In  a   default
    installation, "webd" runs as  the unprivileged "nfr" user.   Thus,
    this attack does not grant an attacker immediate system management
    capabilities.  However, in a standard installation of NFR, program
    binaries that  are run  by the  superuser are  owned by  the "nfr"
    user.  An attacker  that has gained access  to the "nfr" user  via
    the web server  can backdoor these  files to gain  root privileges
    when NFR is restarted.

    Source code for the NFR system is publicly available under license
    from  the  NFR  web  site.  NAI's  advisory makes reference to the
    source code  in the  NFR 2.0.2  Research release  from Wed  Jan 27
    1999.  The  vulnerability discussed in  this advisory occurs  as a
    result of "webd"'s  processing of the  HTTP "POST" command.   POST
    requests are handled in  "nfr/webd/cmdpost.c".  Regardless of  the
    configuration of the NFR web server, the vulnerable POST  handling
    code is exposed to remote attackers.  The HTTP commands handled by
    the  NFR  web  server  are  listed  in the command table, which is
    located in "nfr/webd/ctab.c".  The command table maps HTTP command
    names  to  function  handlers.   The  function  handler for "POST"
    commands which reference programs in the root directory of the web
    server  is  defined  as  "cmd_postbltin()",  which  is  defined in
    "cmdpost.c".  In order to  process the POST command, the  function
    handler attempts to read MIME  headers for the POST data  from the
    HTTP client. This is  handled in "getpostdata()", also  defined in
    "cmdpost.c".   Among  the  headers  recognized  by  the  code   is
    "Content-length", which defines the  amount of data the  client is
    sending the server  in the POST  transaction.  Unfortunately,  the
    MIME header recognition code does not sanity check the value given
    as the Content-length. It is  possible for an attacker to  specify
    an arbitrary Content-length header,  which will be trusted  by the
    server as valid input.  After parsing MIME headers, the web daemon
    attempts to  read as  many bytes  as the  client specified  in the
    Content-length header into an 8k buffer, named "buf", which is  an
    automatic buffer  in cmd_postbltin().   If the  attacker specifies
    more than 8192  bytes of data  in the header,  the additional data
    will overwrite the stack  frame for cmd_postbltin(), allowing  the
    attacker to take over the web daemon process.

    Note that in  some operating systems,  causing a single  read() to
    return more than 8192 bytes is difficult; this problem may not  be
    easily exploited on  these systems.   This problem is  known to be
    exploitable  against  4.4BSD  Unix  operating systems running NFR.
    This is the recommended NFR platform.


    It  is  recommended  that  vulnerable  users  of  NFR  contact NFR
    immediately for a  patch to this  problem.  NFR  has announced the
    release of  a patch  that will  correct this  problem.  It's patch
    number  2.0-p3  and  it  applies  to  product  NFR  Version  2.0.2
    Research.  The  patch is available  as a patch  file which can  be
    applied  to  NFR  Version  2.0.2  Research,  and  as  a   complete
    distribution.  Both versions are available from

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