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Microsoft Outlook retrieves remote OLE content without prompting

Vulnerability Note VU#974272

Original Release Date: 2018-04-10 | Last Revised: 2018-04-10

Overview

When a Rich Text (RTF) email is previewed in Microsoft Outlook, remotely-hosted OLE content is retrieved without requiring any additional user interaction. This can leak private information including the user's password hash, which may be cracked by an attacker.

Description

Microsoft Outlook will automatically retrieve remote OLE content when an RTF email is previewed. When remote OLE content is hosted on a SMB/CIFS server, the Windows client system will attempt to authenticate with the server using single sign-on (SSO). This may leak the user's IP address, domain name, user name, host name, and password hash. If the user's password is not complex enough, then an attacker may be able to crack the password in a short amount of time.

Impact

By convincing a user to preview an RTF email message with Microsoft Outlook, a remote, unauthenticated attacker may be able to obtain the victim's ip address, domain name, user name, host name, and password hash. This password hash may be cracked offline. This vulnerability may be combined with other vulnerabilities to modify the impact. For example, when combined with VU#867968, an attacker could cause a Windows system to blue-screen crash (BSOD) when a specially-crafted email is previewed with Microsoft Outlook.

Solution

Apply and update

This vulnerability is addressed in the Microsoft update for CVE-2018-0950. This update prevents Outlook from automatically initiating SMB connections when an RTF email is previewed. Note that other techniques requiring additional user interaction will still function after this patch is installed. For example, if an email contains a UNC link, like \\attacker\foo, Outlook will automatically make this link clickable. If a user clicks such a link, the impact will be the same as with this vulnerability. For this reason, please also consider the following workarounds.

Block inbound and outbound SMB connections at your network border

This can be accomplished by blocking ports 445/tcp, 137/tcp, 139/tcp, as well as 137/udp and 139/udp.

Block NTLM Single Sign-on (SSO) authentication

Block NTLM Single Sign-on (SSO) authentication, as specified in Microsoft Security Advisory ADV170014. Starting with Windows 10 and Server 2016, if the EnterpriseAccountSSO registry value is created and set to 0, SSO authentication will be disabled for external and unspecified network resources. With this registry change, accessing SMB resources is still allowed, but external and unspecified SMB resources will require the user to enter credentials as opposed to automatically attempting to use the hash of the currently logged-on user.

Use complex passwords

Assume that at some point your client system will attempt to make an SMB connection to an attacker's server. For this reason, make sure that any Windows login has a sufficiently complex password so that it is resistant to cracking. The following two strategies can help achieve this goal:

    1. Use a password manager to help generate complex random passwords. This strategy can help ensure the use of unique passwords across resources that you use, and it can ensure that the passwords are of a sufficient complexity and randomness.
    2. Use longer passphrases (with mixed-case letters, numbers and symbols) instead of passwords. This strategy can produce memorable credentials that do not require additional software to store and retrieve.

    Vendor Information

    974272
    Expand all

    Microsoft

    Notified:  November 29, 2016 Updated:  April 06, 2018

    Status

      Affected

    Vendor Statement

    No statement is currently available from the vendor regarding this vulnerability.

    Vendor Information

    We are not aware of further vendor information regarding this vulnerability.

    Vendor References

    https://portal.msrc.microsoft.com/en-US/security-guidance/advisory/CVE-2018-0950 https://portal.msrc.microsoft.com/en-US/security-guidance/advisory/ADV170014

    Addendum

    There are no additional comments at this time.

    If you have feedback, comments, or additional information about this vulnerability, please send us email.


    CVSS Metrics

    Group Score Vector
    Base 5.0 AV:N/AC:L/Au:N/C:P/I:N/A:N
    Temporal 4.1 E:F/RL:OF/RC:ND
    Environmental 4.1 CDP:ND/TD:H/CR:ND/IR:ND/AR:ND

    References

    Credit

    This vulnerability was reported by Will Dormann of the CERT/CC.

    This document was written by Will Dormann.

    Other Information

    CVE IDs: CVE-2018-0950
    Date Public: 2018-04-10
    Date First Published: 2018-04-10
    Date Last Updated: 2018-04-10 17:24 UTC
    Document Revision: 24

    Sponsored by the Department of Homeland Security Office of Cybersecurity and Communications.