Silent Email Attack CVE-2023-35628 : How to Hack Without an Email Click in Outlook

CVE-2023-35628 is a critical remote code execution (RCE) vulnerability affecting the Microsoft Windows MSHTML platform, with a Common Vulnerability Scoring System (CVSS) score of 8.1, indicating a high level of risk. This flaw is particularly concerning because it can be exploited without any interaction from the user. The vulnerability can be triggered when Microsoft Outlook retrieves and processes a specially crafted email, even before the email is viewed in the Outlook Preview Pane. This makes it a particularly insidious threat, as users may be unaware of the lurking danger​​​​​​.

The nature of CVE-2023-35628 allows a remote, unauthenticated attacker to execute arbitrary code on the victim’s system. The exploit can be initiated by sending a specially crafted email, and it has been noted that ransomware gangs and other malicious entities are likely to find this vulnerability an attractive target. Although the exploit code maturity for CVE-2023-35628 is currently unproven, which means there might not yet be a reliable method for exploiting this vulnerability in the wild, the potential for remote code execution makes it a critical issue for all Windows users​​.

MSHTML platform

The vulnerability in the MSHTML platform, specifically CVE-2023-35628, can be attributed to several factors that are commonly found in software vulnerabilities:

  1. Parsing and Rendering of HTML Content: MSHTML, being a component used for parsing and rendering HTML content in applications like Microsoft Outlook, processes a large amount of untrusted input. This input, which often includes complex HTML and scripting content, can contain flaws or unexpected sequences that are not properly handled by the software.
  2. Memory Management Issues: Vulnerabilities often arise due to memory management issues such as buffer overflows, use-after-free errors, or other similar problems. These issues can occur when the software does not correctly allocate, manage, or free memory when processing HTML content. Attackers can exploit these weaknesses to execute arbitrary code.
  3. Insufficient Input Validation: Software vulnerabilities can also stem from insufficient input validation. If MSHTML does not properly validate or sanitize the HTML content it processes, malicious input could be used to trigger an exploit. This could include specially crafted scripts or malformed HTML structures designed to take advantage of the parser’s weaknesses.
  4. Complexity of Web Standards: The complexity of modern web standards can also contribute to vulnerabilities. As standards evolve and become more complex, it becomes increasingly challenging to ensure that every aspect of the parsing and rendering process is secure against all potential attack vectors.
  5. Integration with Email Clients: The integration of MSHTML with email clients like Outlook adds another layer of complexity. Emails are a common vector for delivering malicious content, and the automatic processing of emails (including the rendering of HTML content) can make it easier for attackers to exploit vulnerabilities without direct interaction from the user.

The No-Click Exploit

An exploit for the CVE-2023-35628 vulnerability in the Windows MSHTML platform would typically involve a few key steps, tailored to leverage the specific nature of this flaw. Here’s a generalized overview of how such an exploit could work:

  1. Crafting a Malicious Email: The attacker starts by creating a specially crafted email. This email would contain malicious code or a payload designed to exploit the vulnerability in the MSHTML platform. The precise nature of this code depends on the specifics of the vulnerability and would be tailored to trigger the flaw in MSHTML.
  2. Email Delivery and Automatic Processing: The crafted email is then sent to the target. In the case of CVE-2023-35628, the critical aspect is that the vulnerability is triggered when Microsoft Outlook retrieves and processes the email. This processing happens automatically, often before the email is even displayed in the Outlook Preview Pane.
  3. Remote Code Execution: Upon processing the malicious email, the exploit code is executed. This code execution occurs within the context of the MSHTML platform, which is a key component used by Outlook for rendering HTML content in emails.
  4. Taking Control or Damaging the System: Once the code is executed, it can perform various malicious activities. This could range from taking control of the user’s system, stealing sensitive information, installing malware, or performing other harmful actions. The extent of the damage or control depends on the nature of the payload and the permissions available to the MSHTML process.

Memory shaping is an advanced exploitation technique often used in sophisticated cyber attacks, particularly those involving complex software systems and secure environments. It’s a method used by attackers to manipulate the layout or state of memory in a target application to facilitate the exploitation of vulnerabilities. Memory shaping can be a part of exploiting vulnerabilities like buffer overflows, use-after-free errors, or other memory corruption issues.

Here’s a simplified example to illustrate how memory shaping and its exploitation might work:

  1. Identifying a Vulnerability: First, the attacker finds a vulnerability in the target application that can be exploited to corrupt memory. For instance, this could be a buffer overflow, where the application fails to check the length of input, allowing an attacker to write more data to a buffer than it can hold.
  2. Analyzing Memory Layout: The attacker then studies the application’s memory layout to understand how data is stored and managed. This involves identifying where in memory different types of data are located and how they are accessed by the application.
  3. Memory Shaping: Once the attacker has a good understanding of the memory layout, they begin the process of memory shaping. This involves crafting inputs or actions that modify the application’s memory in a controlled way. For example, they might allocate and free memory in a specific pattern to arrange chunks of memory in a desired layout.
  4. Exploiting the Vulnerability: With the memory shaped to their advantage, the attacker then exploits the identified vulnerability. Using the buffer overflow example, they might overflow a buffer with data that includes malicious code (the payload) and carefully calculated addresses or commands that redirect the application’s execution flow to the payload.
  5. Executing Arbitrary Code: If successful, the exploit allows the attacker’s code to be executed with the privileges of the target application. This could lead to various malicious outcomes, such as data theft, installation of malware, or gaining control over the system.

It’s important to note that memory shaping is a complex and technical process that requires in-depth knowledge of both the target application and general exploitation techniques. It’s typically used in scenarios where standard exploitation methods are not effective, often due to security measures like Address Space Layout Randomization (ASLR) or other protections.

Due to the complexity and potential for misuse, specific exploit code or detailed methodologies for memory shaping are not shared publicly. The goal of cybersecurity research in this area is to understand and mitigate such advanced threats, ensuring software and systems are secure against potential attacks.

It’s important to note that the complexity of the exploit for CVE-2023-35628 is considered high. It requires specific knowledge and techniques, particularly related to memory shaping, to successfully exploit the vulnerability. This complexity might limit the exploitation to more skilled attackers.

The attack complexity is considered high due to the reliance on complex memory-shaping techniques to successfully exploit the vulnerability. Despite this complexity, the high impact of the vulnerability necessitates prompt attention and action. Microsoft has addressed this flaw in their December 2023 Patch Tuesday updates, recommending users to update their systems as a preventative measure​​.

It’s important to note that CVE-2023-35628 is just one of several vulnerabilities addressed in the December 2023 Patch Tuesday updates. Other notable vulnerabilities include CVE-2023-35630 and CVE-2023-35641, which are remote code execution vulnerabilities affecting Microsoft Internet Connection Sharing (ICS) with a CVSS score of 8.8, and a critical spoofing vulnerability in Microsoft Power Platform Connector (CVE-2023-36019) with a CVSS score of 9.6​​.

Mitigation & Scope

The CVE-2023-35628 vulnerability, which is a critical remote code execution flaw in the Windows MSHTML platform, affects a range of Microsoft products, including Office 365 and on-premises versions. This vulnerability is significant due to its potential to allow exploitation as soon as Outlook retrieves and processes a specially crafted malicious email, even before the user interacts with the email. This means that exploitation could occur without any action from the user, not even requiring the Preview Pane in Outlook.

In terms of impact on Office 365 and on-premises environments, it’s important to note that the MSHTML proprietary browser engine, which is the component affected by this vulnerability, is used by Outlook among other applications to render HTML content. The fact that this engine remains installed within Windows, regardless of the status of Internet Explorer 11, means that systems where Internet Explorer 11 has been fully disabled are still vulnerable until patched.

For addressing this vulnerability, Microsoft released patches as part of their December 2023 Patch Tuesday. These patches are essential for mitigating the risk posed by this vulnerability and are available for various versions of Windows and related software components. Given the critical nature of this vulnerability and its potential impact on confidentiality, integrity, and availability, it’s strongly recommended for users and administrators of both Office 365 and on-premises environments to apply these updates promptly.

The December 2023 Patch Tuesday from Microsoft addressed a total of 34 vulnerabilities, including this critical RCE vulnerability in MSHTML. It’s noteworthy that there were no security patches for Exchange, SharePoint, Visual Studio/.NET, or SQL Server in this particular update cycle.

The details about the patches and the specific versions they apply to can be found in Microsoft’s security bulletins and support documentation. For users and administrators, it is crucial to review these resources and ensure that all applicable security updates are applied to protect against potential exploits of this vulnerability​

Given the severity and the ease with which this vulnerability can be exploited, it is crucial for Windows users, particularly those using Microsoft Outlook, to ensure their systems are updated with the latest security patches provided by Microsoft. Regular review of patching strategies and overall cybersecurity methods is advisable to maintain a robust security posture.