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securityvulnsSecurityvulnsSECURITYVULNS:DOC:1612
HistoryMay 15, 2001 - 12:00 a.m.

Security Bulletin MS01-026

2001-05-1500:00:00
vulners.com
20

Title: Superfluous Decoding Operation Could Allow Command
Execution via IIS
Date: May 14, 2001
Software: IIS 4.0 and 5.0
Impact: Three vulnerabilities: Code execution; denial of
service, information disclosure.
Bulletin: MS01-026

Microsoft encourages customers to review the Security Bulletin at:
http://www.microsoft.com/technet/security/bulletin/MS01-026.asp.


Issue:

This patch 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the patches superseded by this patch is provided in the web-hosted
security bulletin, 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.

The patch also eliminates three newly discovered vulnerabilities:

  • A vulnerability that could enable an attacker to run
    operating system commands on an affected server. When
    IIS receives a user request to run a script or other
    server-side program, it performs a decoding pass to
    render the request in a canonical form, then performs
    security checks on the decoded request. A vulnerability
    results because a second, superfluous decoding pass is
    performed after the security checks are completed. If an
    attacker submitted a specially constructed request, it could
    be possible for the request to pass the security checks, but
    then be mapped via the second decoding pass into one that
    should have been blocked – specifically, it could enable
    the request to execute operating system commands or programs
    outside the virtual folder structure. These would be executed
    in the security context of the IUSR_machinename account which,
    by virtue of its membership in the Everyone group, would grant
    the attacker capabilities similar to those of a non-administrative
    user interactively logged on at the console.
  • A vulnerability that could enable denial of service attacks
    against the FTP service. A function that processes wildcard
    sequences in FTP commands doesn't always allocate sufficient
    memory when performing pattern matching. Under unusual
    circumstances, it could be possible for an attacker to levy an
    FTP command containing a wildcard sequence that, when expanded,
    would overrun the allocated memory and cause an access violation.
    This would cause the IIS service (which provides both the web and
    FTP functionality) to fail. As a result, all web or FTP sessions
    in progress at the time would be severed, and no new sessions
    could be established until the IIS service was restarted. In IIS
    5.0, the service would restart automatically. In IIS 4.0, operator
    intervention would be required to restart the service.
  • A vulnerability that could make it easier for an attacker to find
    Guest accounts that had been inadvertently exposed via FTP. By
    design, if a user wishes to log onto an FTP server using a domain
    user account, rather than a local one, he should be required to
    precede it with the name of the domain. However, if an attacker
    preceded an account name with a particular set of characters, the
    FTP service would search the domain, and all trusted domains, for
    the user account. The account would need to be enabled, and the
    attacker would still need to know the correct password in order
    to log into the account. For all practical purposes, this would
    limit the attacker to attacking the Guest account, as it is the
    only account with both a well-known account name and a well-known
    default password.

The patch also corrects errors in three previous patches:

  • The patch originally provided in Microsoft Security Bulletin
    MS00-060 successfully eliminated the vulnerability at issue there,
    but created an opportunity to cause the server to expend an
    inordinate amount of time processing a particular type of invalid
    request.
  • The patches originally provided in Microsoft Security Bulletins
    MS01-014 and MS01-016 (which superseded MS01-014) successfully
    eliminated the vulnerabilities at issue there, but created a
    potential denial of service condition via a memory leak.

Mitigating Factors:

IIS vulnerability:

  • The vulnerability does not provide a way for the attacker to
    learn the folder structure on the server. As a result, if the
    operating system were installed on a separate drive from the
    web root or in non-standard folders, it could prevent an
    attacker from locating programs of interest.
  • The vulnerability does not provide administrative access to
    the server. If the recommendations in the IIS 4.0 and IIS 5.0
    security checklists have been followed, sensitive programs
    will have been moved to folders that can only be accessed by
    the Administrator, and non-administrative access to server
    resources will be have been severely restricted.
    FTP denial of service vulnerability:
  • The attacker would require the ability to start an FTP
    session in order to exploit the vulnerability.
    FTP user account vulnerability:
  • The vulnerability could only be exploited if the FTP server
    was a domain member. However, this is usually not appropriate
    for Internet-connected FTP servers.
  • The vulnerability could only be exploited if the Guest account
    on the local machine was disabled, but the Guest account on a
    trusted domain was enabled. By default, the Guest account is
    disabled in both Windows NT 4.0 and Windows 2000.

Patch Availability:

Acknowledgment:


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