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TUCoPS :: Web :: IIS :: iis108~1.htm

IIS - MS update on several patched vulnerabilities



COMMAND

    IIS

SYSTEMS AFFECTED

    IIS 4, 5

PROBLEM

    Following info is based on a Microsoft Security Bulletin MS01-044.
    This  advisory  addresses  the  patch  which is a cumulative patch
    that includes the functionality  of all security patches  released
    to date for IIS  5.0, and all patches  released for IIS 4.0  since
    Windows  NT(r)  4.0  Service  Pack  5.   A complete listing of the
    patches  superseded  by  this  patch  is  provided  below,  in the
    section titled "Additional information about this patch".   Before
    applying the patch, system administrators should take note of  the
    caveats discussed in the same section.

    In addition to including all previously released security patches,
    this patch also includes fixes for five newly discovered  security
    vulnerabilities affecting IIS 4.0 and 5.0:

    - A denial of service vulnerability that could enable an  attacker
      to cause  the IIS  4.0 service  to fail,  if URL redirection has
      been enabled.  The "Code Red" worm generates traffic that can in
      some cases exploit this  vulnerability, with the result  that an
      IIS 4.0  machine that  wasn't susceptible  to infection  via the
      worm could nevertheless have its service disrupted by the worm.

      This  vulnerability  only  affects  IIS  4.0.  IIS  5.0  is  not
      affected.  The vulnerability  only occurs if URL  redirection is
      enabled and does not  provide any capability to  compromise data
      on the server or gain administrative control over it.

    - A denial of service vulnerability that could enable an  attacker
      to temporarily disrupt service on an IIS 5.0 web server.  WebDAV
      doesn't correctly handle particular  type of very long,  invalid
      request.   Such a  request would  cause the  IIS 5.0  service to
      fail;  by  default,   it  would  automatically   restart.    The
      vulnerability only affects IIS 5.0.  IIS 4.0 is not affected.

      The  effect  of  an  attack  via  this  vulnerability  would  be
      temporary.  The server would automatically resume normal service
      as soon  as the  malformed requests  stopped arriving.   The bug
      does not provide  an attacker with  any capability to  carry out
      WebDAV requests.

    - A  denial of  service vulnerability  involving the  way IIS  5.0
      interprets content containing a particular type of invalid  MIME
      header.  If an attacker placed content containing such a  defect
      onto a server and then  requested it, the IIS 5.0  service would
      be  unable  to  serve  any  content  until  a spurious entry was
      removed from the File Type table for the site. The vulnerability
      only affects IIS 5.0. IIS 4.0 is not affected.

      In order to exploit this vulnerability, the attacker would  need
      to have the ability to install content on the server.   However,
      by default, unprivileged users do not have this capability,  and
      best  practices  strongly  recommend  against  granting  it   to
      untrusted users.   Credit goes  to John  Waters of  Deloitte and
      Touche.

    - A buffer overrun vulnerability involving the code that  performs
      server-side include (SSI) directives.   An attacker who had  the
      ability to place content onto a server could include a malformed
      SSI directive that, when the content was processed, would result
      in  code  of  the  attacker's  choice  running  in  Local System
      context.

      In order to exploit this vulnerability, the attacker would  need
      to have the ability to install content on the server.   However,
      by default, unprivileged users do not have this capability,  and
      best  practices  strongly  recommend  against  granting  it   to
      untrusted  users.   Credit  goes  to  The NSFocus Security Team.
      Following is their advisory.

      Microsoft IIS supports SSI (Server Side Include) function.   IIS
      use  ssinc.dll  as  a  SSI  interpreter.   By  default  setting,
      extensions  like  .stm,  .shtm  and  .shtml  would  be mapped to
      interpreter  process  (Ssinc.dll).    SSI  supports   "#include"
      directive, mostly in this form:

        <!--#include file="Filename"-->

      When processing "#include" directive, ssinc.dll would check  for
      the name of the directory  under which the .shtml file  resides,
      append it before  the include file  and form a  new path string.
      For  example,  create  a   file  named  "test.shtml"  with   the
      following content and save it under "wwwroot/abcd/":

        <!--#include file="ABCD"-->

      The new path string would be "/abcd/ABCD".  Ssinc.dll would copy
      it to a buffer of 0x804(2052) bytes.

      When  obtaining  Server-side  include  filename from test.shtml,
      ssinc.dll would perform  length check for  it.  In  case that it
      is longer  than 0x801  bytes, ssinc.dll  would cut  it to  0x801
      bytes and  append '\0'  at the  end. Thus,  the include filename
      (including the trailing '\0')  won't be longer than  0x802(2550)
      bytes.

      But  it  does  not  check  the  length  of  the  new path string
      appending current directory name.  Thus, if we set the contained
      filename  to  be  a  string  longer  than  0x801  bytes and save
      "test.shtml" under a directory (name  of which is longer than  9
      bytes), a buffer overflow would occur and overwrite the EBP  and
      EIP saved in stack completely (The trailing '\0' would overwrite
      the first argument).

      As ssinc.dll is  running in LOCAL  SYSTEM context, in  case that
      an attacker carefully  form the overflow  data, he might  change
      the procedure flow and run arbitrary code with SYSTEM privilege.

      To launch an attack, the  attacker would need the following  two
      conditions:
      1. Privilege to create file or directory under Web directory.
      2. Ability to access created file through Web service.

      Exploit:
      1. Create a file "test.shtml" with following file content:

        <!--#include file="AAAA[...]AA"-->

         Number of 'A' should be over 2049.
      2. Create   a  directory   "a"  under   Web  directory.     Copy
         "test.shtml" to "a" directory.
      3. Request "test.shtml" through web browser:

        http://webhost/a/test.shtml

      4. IIS  would  return  a  blank  page  which  indicates that  an
         overflow  has  occurred.   Meanwhile  the  trailing  '\0' has
         overwritten the last byte of saved EBP.

      On the contrary, in case  that the contained file has  a shorter
      name  like  'AA',  IIS  would  return  a  SSI file '/a/AA' error
      message when receiving the request.

    - A privilege  elevation vulnerability that  results because of  a
      flaw in a table that  IIS 5.0 consults when determining  whether
      a  process  should  in-process   or  out-of-process.   IIS   5.0
      contains a table that lists the system files that should  always
      run  in-process.   However,  the  list  provides the files using
      relative as well  as absolute addressing,  with the result  that
      any file whose  name matched that  of a file  on the list  would
      run in-process.  The vulnerability only affects IIS 5.0. IIS 4.0
      is not affected.

      In order to exploit this vulnerability, the attacker would  need
      to have the ability to install content on the server.   However,
      by default, unprivileged users do not have this capability,  and
      best  practices  strongly  recommend  against  granting  it   to
      untrusted users.  Credit goes  to Oded Horovitz.  Here  is their
      advisory.

      The exploit allows a GUEST  user (who has the rights  to execute
      code on the system) to elevate his privileges.  Once the exploit
      is executed, it allows an attacker to run arbitrary code on  the
      machine  with  SYSTEM  privileges.   Usually,  by  using certain
      well-known attacks, the user can  upload the exploit to the  IIS
      virtual directory, and then remotely execute it.  Alternatively,
      anyone  with  a  valid  username  and  password can log into the
      system, upload the exploit file  into the IIS virtual tree,  and
      then execute it.

      IIS supports three different modes of process isolation.   These
      modes control  how well  the IIS  process is  isolated from  the
      processes  that  are  being  invoked  as  part  of  the  request
      processing.   Due to  a weakness  in IIS,  several dll files are
      always executed by the  least secure isolation level  regardless
      of  the  actual  process  isolation  settings.   By  adding   or
      replacing  one  of  these  dlls  with  a  malicious  version, an
      attacker can run arbitrary code with SYSTEM privileges.

SOLUTION

    In addition, this patch eliminates  a side effect of the  previous
    IIS  cumulative  patch  (discussed  in  the  Caveats  section   of
    Microsoft  Security   Bulletin  MS01-026)   by  restoring   proper
    functioning of UPN-style logons via FTP and W3SVC.

    A patch is  available to fix  these vulnerabilities.   Please read
    the Security Bulletin

        http://www.microsoft.com/technet/security/bulletin/ms01-044.asp

    for information on obtaining this patch.


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