[logs] RE: NT Event Log and Web Server Attacks

From: Rainer Gerhards (rgerhardsat_private)
Date: Fri Jan 17 2003 - 04:52:20 PST

  • Next message: Rainer Gerhards: "[logs] RE: NT Event Log and Web Server Attacks"

    Unfortunately, I need to post a correction on this issue. That
    correction does also address some issues with NT Event logging & IIS in
    general, so maybe Eric would like to jump in?
    The fact to correct is the user that is logged in nt event log.
    Interestingly, when the process (the exe file) is started, that is not
    the expected IUSR_xxx account, but the build-in SYSTEM account. I have
    verified (with cmd.exe) this is not only the case for script processors
    like perl, but with any program (well, at least cmd.exe...).
    The reason seems to be that IIS triggers the 592 event (process
    creation) while NOT being impersonated as the actual user this request
    runs under. Then, in some stage further down the workflow, IIS
    impersonates as the actual user (IUSR_xxx in our case). It is still
    impersonated when the end process is logged via event 593, which
    therefore specifies the IUSR_xxx as the user in question.
    In my personal view, IIS should log the 592 event under the actual user,
    too and not under the system context. This also raises the question
    (which I do not intend to discuss) if IIS is really able to ensure NTFS
    ACLS under all circumstances - or if a failing stage (the one looking up
    ACLs) of IIS could lead to the execution of a file that the actual user
    (e.g. IUSR_xxx) is not intended to. This for sure would require a
    vulnerability in the ACL lookup stage, but there could be some over
    Actually, this finding makes the intrusion detection rule harder. We
    know need to rule out other occasions where the build-in SYSTEM account
    (validly) executes programs. This seems not to be very practical. So we
    can resort to using the 593 (process termination). They still do have
    the name of the file being executed and also have the correct user id in
    them. The only problem here is that we won't detect processes that were
    started by the intruder but NEVER ENDED. Anyhow, I am looking to catch
    (not yet known, maybe not even existing) vulnerabilities and their
    exploits in IIS that will be used to start an intrusion. One of the most
    common things I can think of is calling cmd.exe. That one most probably
    terminates. Most of the attacks I know start with calling cmd.exe or a
    similar program. One, that terminates. Only after a number of requests,
    a non-terminating program like a remote shell or a tftp server is
    started. Of course, if a buffer overrun is used to execute a
    non-terminating program, or a looping batch file is executed, chances
    are good we will NOT see any process termination events. In any case,
    the 593 still provides a good warning sign, altough the 592 would be
    better (if it had the correct user id in it...).
    I will probably play a little more with this issue and post new results
    - if any.
    My apologies for the initial mistake and the time it took to post this
    correction. I hope I miss-lead nobody ;)
    Rainer Gerhards
    > -----Original Message-----
    > From: Rainer Gerhards 
    > Sent: Friday, January 17, 2003 12:26 PM
    > To: Rainer Gerhards
    > Subject: NT Event Log and Web Server Attacks
    > Hi all,
    > I just wanted to let the list know of a way to detect 
    > intrusions via IIS. It may work for other web servers, too.
    > We have just build a new rule to use the nt event log to 
    > detect (so far unknown) attacks... Well effectively, we 
    > detect intrusions after they happend. So the intruder most 
    > probably already got in. This will not save you from the 
    > intrusion, but at least you know you need to react.
    > For it to work, we assume the following:
    > 1. IIS process tracking is activated (and hopefully works 
    > reliable enough ;))
    > 2. The web site is accessed by anonymous users, only
    > 3. The server is a dedicated web server
    > If 2 or 3 are not the case, we can probably still use the 
    > same idea, but the filter needs to be more advanced.
    > It is important to know that anonymous requests are run in 
    > the security context of a specifically designated NT account. 
    > In most cases this is the IUSR_xxx account where xxx is 
    > typically the machine name. This account can be configured in 
    > the IIS admin console. So you need to lookup the details there.
    > What we now do is we look at the event log in short 
    > intervals. When we detect a program start (security event 
    > log, source "Security", event id 592) AND the user is the 
    > IUSR_xxx account AND it is not a program we know IIS needs to 
    > run (e.g. perl.exe is run for PERL scripts; php.exe is for 
    > PHP scripts) then chances are very high that someone has 
    > successfully intruded our machine via IIS. So it would 
    > probably be a good idea to forward the event log record to an 
    > admin in charge (via email or whatever).
    > I hope this is helpful. As always, comments are very welcome. 
    > I would particularly like to know if that works with Apache 
    > under win32 ;)
    > Rainer Gerhards
    > Adiscon
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