Important CSS Directive Causes MS Outlook To Crash (Apr 7, 2017)


Microsoft Outlook is an email client used to send and receive email messages. Recently, SonicWALL received reports of a bug in MS Outlook, wherein a specially crafted email causes it to crash shortly after reading.

The POC shows the email to contain both text and html portions, as shown below:

Retrieving this email via MS Outlook causes a crash as shown:

Debugging Outlook, we see that the crash occurs at wwlib.dll (not necessarily Outlook).

This dll is also used by other Office applications such as Word and Powerpoint. This is used for reading and displaying HTML content.

The problem arises with the “!important” directive in the CSS. Upon testing, removing this directive from the email message sent does not cause a crash.

SonicWALL Threat Research Team has researched this vulnerability and released following signature to protect their customers.

  • IPS 12702 : Microsoft Outlook Denial of Service
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The SonicWall Capture Labs Threat Research Team gathers, analyzes and vets cross-vector threat information from the SonicWall Capture Threat network, consisting of global devices and resources, including more than 1 million security sensors in nearly 200 countries and territories. The research team identifies, analyzes, and mitigates critical vulnerabilities and malware daily through in-depth research, which drives protection for all SonicWall customers. In addition to safeguarding networks globally, the research team supports the larger threat intelligence community by releasing weekly deep technical analyses of the most critical threats to small businesses, providing critical knowledge that defenders need to protect their networks.