CVE-2023-35311 - An In-Depth Guide to the Microsoft Outlook Security Feature Bypass Vulnerability

In June 2023, Microsoft patched a critical vulnerability in Outlook — tagged CVE-2023-35311. This security flaw allowed attackers to bypass built-in protection features, exposing users to phishing attacks and more. In this post, we’ll break down the technical details, show examples, summarize Microsoft’s advisory, and explain how this vulnerability could be exploited (with code snippets). Finally, we’ll share remediation steps and references to keep your organizations safe.

What is CVE-2023-35311?

CVE-2023-35311 is a “Security Feature Bypass” vulnerability discovered in Microsoft Outlook. The problem arises from how Outlook handles click-on URLs in email messages — specifically, how it interacts with Microsoft’s Protected View and Office’s security features meant to stop users from accidentally opening dangerous files or links.

In Simple Terms

Attackers could craft malicious email content that would bypass the warning dialog you normally see when clicking sketchy links. This could let them trick users into visiting dangerous websites and opening files, even if Outlook’s security measures are in place.

How Does the Vulnerability Work?

Normally, when users receive a link in Outlook that points to an Internet or Untrusted Zone, Microsoft apps like Word or Excel enable "Protected View" — a safe environment with limited permissions.

With CVE-2023-35311, it’s possible to disguise dangerous links so Outlook (and the Office apps) doesn’t recognize them as potentially harmful, which lets users open them without warning.

Example Malicious URL in Email

<a href="http://evil.com/officefile.docx">Click here to view your document</a>

But the trick is, the attacker crafts the email so Outlook doesn’t treat this URL as “untrusted." This could be done using certain encoding tricks or URL formatting, which fools the security system.

Attackers might use hexadecimal, unicode or IP-encoded URLs

<!-- Hexadecimal IP-encoded link -->
<a href="http://x7f000001/officefile.docx">Check your document</a>
<!-- Unicode disguised link (lookalike domain) -->
<a href="http://xn--evl-na.com/maliciousfile.docx">Important Info</a>

Clicking these links may not trigger the normal Outlook/Office warnings, especially if they exploit the vulnerability.

All without any security warning.

Microsoft rated this flaw as "Important" (CVE score: 8.8 - HIGH).

According to the official Microsoft Security Response Center (MSRC)

> “An attacker who successfully exploited this vulnerability could bypass security features and cause a user to open a malicious file or link.”

Microsoft released fixes as part of their June 2023 Patch Tuesday update. If your Outlook or Office is up to date, you’re likely protected.

- Microsoft Security Response Center (MSRC) CVE-2023-35311 Advisory
- Microsoft Docs: Security Update Guide

Exploit Details & Proof of Concept

While public, weaponized exploits are not widely available, the core of exploiting CVE-2023-35311 relies on *malicious link crafting* and *user interaction*. Here’s a sanitized demonstration:

PoC: HTML Email with Encoded Link

<!-- Crafted email body snippet -->
<p>Dear User,</p>
<p>Your document is ready. <a href="file://\\evil.com\payload.docx">Click here</a> to view it.</p>

Or using a clever encoding

<!-- Trick URL to appear to be trusted -->
<a href="http://me%2Fevil.com/document.docx">View document</a>

In affected Outlook versions, these links don't trigger Protected View for some file types, causing the file to open outside the safety layer.

References

- CVE-2023-35311 Microsoft Advisory
- Microsoft June 2023 Patch Tuesday
- Security Feature Bypass Vulnerabilities Explained

Summary

CVE-2023-35311 is a serious Outlook vulnerability that lets attackers disguise dangerous links, bypassing Microsoft’s security warnings. Patch your systems now, educate your users, and stay vigilant to keep your organization safe. If you suspect you’ve been affected, check your email logs and update your software ASAP!

Timeline

Published on: 07/11/2023 18:15:00 UTC
Last modified on: 07/14/2023 14:26:00 UTC