Active Exploitation of Microsoft Outlook RCE Vulnerability (CVE-2024-21413)
A critical remote code execution (RCE) vulnerability, CVE-2024-21413, affecting Microsoft Outlook has been actively exploited.
CISA has directed U.S. federal agencies to secure their systems against ongoing cyberattacks targeting this vulnerability, tracked as CVE-2024–21413. The flaw was originally discovered by Check Point vulnerability researcher Haifei Li and is a result of improper input validation when processing emails containing malicious links.
| OEM | Microsoft |
| Severity | Critical |
| CVSS | 9.8 |
| CVEs | CVE-2024-21413 |
| Exploited in Wild | Yes |
| Patch/Remediation Available | Yes |
| Advisory Version | 1.0 |
Overview
The flaw allows attackers to bypass security protections, leading to NTLM credential theft and arbitrary code execution. The vulnerability is critical, and Microsoft has released patches to mitigate the risk.
| Vulnerability Name | CVE ID | Product Affected | Severity |
| Remote Code Execution Vulnerability | CVE-2024-21413 | Microsoft | Critical |
Technical Summary
The CVE-2024-21413 vulnerability arises due to improper input validation in Microsoft Outlook when handling emails containing malicious links. Exploitation of this flaw enables attackers to bypass Protected View, a security feature designed to prevent execution of harmful content embedded in Office files.
By manipulating URLs with the file:// protocol and inserting an exclamation mark followed by arbitrary text, attackers can evade Outlook’s built-in security measures, tricking users into opening malicious Office files in editing mode instead of read-only mode. The Preview Pane also serves as an attack vector, enabling zero-click exploitation. Here is the POC also available for this vulnerabilty.
| CVE ID | System Affected | Vulnerability Details | Impact |
| CVE-2024-21413 | Microsoft Office LTSC 2021, Microsoft 365 Apps, Microsoft Outlook 2016, Microsoft Office 2019 | Exploits improper input validation to bypass Outlook security protections using manipulated hyperlinks. | NTLM credential theft, remote code execution, potential full system compromise |
Remediation:
- Apply Security Patches: Ensure that all the Microsoft Office products are updated with the latest security patches.
- Disable NTLM Authentication: Where feasible, reduce reliance on NTLM authentication to prevent credential theft.
General Remediation:
- Monitor Network Activity: Watch unusual outbound connections to attacker-controlled servers.
- User Awareness Training: Educate employees on recognizing phishing attempts and avoiding click on suspicious links or attachments.
- Enable Advanced Threat Protection: Use security tools like Microsoft Defender to enhance security monitoring and detection.
- Regularly Update Software: Maintain a routine patching schedule to ensure all systems are protected against known vulnerabilities.
- Restrict Macros and External Content: Configure Microsoft Office to block macros and disable automatic external content execution.
Conclusion:
The exploitation of CVE-2024-21413 underscores the ongoing threat posed by improperly validated inputs in widely used enterprise software. With this vulnerability being actively exploited and the POC publicly available, organizations must prioritize patching, strengthen monitoring, and follow best security practices to minimize risks. CISA has included CVE-2024-21413 in its Known Exploited Vulnerabilities (KEV) catalog, emphasizing the need for immediate action.
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