What is CVE-2024-21413?
CVE-2024-21413 is a critical security vulnerability in Microsoft Outlook classified as an “Improper Input Validation Vulnerability”. The flaw allows attackers to bypass security protections and execute remote code on a victim’s system simply by sending a malicious email. The vulnerability arises from how Outlook processes URLs, specifically those using the file:// protocol and certain crafted URL structures. The key risk of CVE-2024-21413 is that it can be exploited with minimal user interaction, meaning an attacker can compromise a system just by having the victim preview an email, i.e. no need for clicking a link or downloading an attachment. Microsoft has rated this vulnerability as high severity, emphasizing that exploitation was observed in the wild, making it a zero-day threat at the time of disclosure.
When Was the Vulnerability Discovered?
The vulnerability was initially discovered by Check Point Research. Microsoft was notified, and the flaw was publicly disclosed in February 2024 as part of Microsoft’s Patch Tuesday security updates. Armis CentrixTM for Early Warning, added CVE-2024-21413 to the list of known vulnerabilities being exploited in the wild on February 15, 2024, while CISA added CVE-2024-21413 to their KEV catalog on February 6, 2025, making Armis CentrixTM for Early Warning early by 357 days.
Impact and blast radius: successful exploitation of CVE-2024-21413 can lead to remote code execution, theft of NTLM credentials, and potential full system compromise. The zero-click nature of the attack, which can trigger upon email preview, significantly increases its severity and potential impact. This makes CVE-2024-21413 particularly dangerous for government agencies, enterprises, and organizations using Outlook as their primary email client.
Value of Timely Awareness: immediate awareness and action are crucial, as the vulnerability has proven to be actively exploited in the wild. Delays in applying patches or mitigations can result in unauthorized access, data breaches, and widespread system compromise. Delayed response could allow cybercriminals to gain persistent access to corporate networks, expose sensitive corporate and personal data to attackers, enable ransomware or espionage campaigns using stolen credentials. Microsoft and the wider security researchers community recommend immediate patching and proactive defense measures to mitigate the risk of ongoing exploitation.
Mitigation and Protection:
Proactive defense and workarounds: to defend against CVE-2024-21413, organizations should implement multiple layers of security, including apply security patches immediately (Microsoft has released official patches for affected versions of Outlook and organizations must prioritize deploying these updates across all workstations and servers), as well as – when doable – disable NTLM Authentication (since NTLM credential theft is a major risk, disabling NTLM authentication can reduce exposure to exploitation). If NTLM cannot be disabled, consider enforcing SMB signing and blocking outbound NTLM traffic to untrusted networks. Implementing strict email filtering to detect and quarantine emails containing suspicious URLs is also an effective mitigation measure.
Continuous monitoring and updates: even after upgrading or hardening, it is highly recommended to implement monitoring for unusual outbound connections to detect potential data exfiltration (i.e. flag unexpected SMB traffic leaving the network and watching for high-frequency NTLM authentication requests, which may indicate credential theft attempts), as well as implement application sandboxing (for example restricting Outlook’s ability to launch external applications using Windows Defender Application Control or AppLocker). By implementing these measures, organizations can significantly reduce the risk associated with CVE-2024-21413 and enhance their overall security posture.
Stay vigilant and ensure your systems are up-to-date to defend against evolving cybersecurity threats.
Armis Centrix™ for Early Warning is the proactive cybersecurity solution designed to empower organizations with early warning intelligence to anticipate and mitigate cyber risk effectively. By leveraging AI-driven actionable intelligence, Armis Centrix™ provides insights into the vulnerabilities that threat actors are exploiting in the wild or are about to weaponize, allowing organizations to understand their impact and take preemptive action.
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