Home / Threat Intelligence bulletins / Microsoft Outlook remote code execution vulnerability

Target Industry

Indiscriminate, opportunistic targeting.


Microsoft has disclosed a high-severity Outlook remote code execution (RCE) vulnerability, tracked as CVE-2023-33131. Although the attack complexity is low, a threat actor could take advantage of the flaw in an email attack by emailing the victim a specially designed file and convincing them to open it.

In a web-based attack scenario, an attacker may run a website that contains a specially created file intended to exploit the vulnerability. Or they might make use of a website that has been compromised that accepts or hosts user-provided content.

There is no mechanism for an attacker to convince users to visit the website. Instead, a victim would need to be persuaded to follow a link – through a hyperlink in a phishing email or instant message – and then be persuaded to open the specially constructed file.

The vulnerability must be opened by the user to be exploited.


  • Data Breach: Phishing attempts using malicious zip archive tools can result in data breaches, revealing sensitive customer data, proprietary information, and corporate secrets.
  • Financial Loss: Phishing attacks may result in financial losses for businesses. Financial fraud, unauthorised transactions, lost company prospects, and possible legal costs related to data breach clean-up could also occur.
  • Operational Disruption: Phishing attacks that are successful can stop business operations by compromising infrastructure, networks, and systems. Customer satisfaction may suffer because of downtime, decreased productivity, and disruption of essential services.
  • Reputation damage: A phishing attack can damage a company’s reputation. Customers might stop trusting the company, stakeholders might question its security procedures, and rivals might take advantage of the circumstance to gain a competitive edge.
  • Regulation and Legal Repercussions: Phishing attacks frequently involve the theft of financial and personal information, which can have regulatory and legal repercussions.

Vulnerability Detection

A security patch for this vulnerability has been released by Microsoft. Previous product versions therefore remain vulnerable to potential exploitation.

Affected Products

Microsoft Outlook.

Containment, Mitigations & Remediations

  • End user training: Employees should be trained to detect markers of phishing emails and to abstain from opening files from unverified sources. Run phishing simulations frequently throughout your company. By evaluating their capacity to recognise phishing emails and react correctly, these simulated phishing emails assist employees in identifying and avoiding genuine phishing attempts.
  • Endpoint Protection: Endpoint security tools, such antivirus and anti-malware software, should be used to scan and prevent the execution of malicious files, including those included in zip packages, before they can be executed on endpoints.
  • All devices should implement the most recent vendor updates available as these will contain updates to their security features to help prevent exploitation from known threats.
  • Put in place email filtering technologies that are effective at detecting and preventing phishing emails before they reach employees’ inboxes to identify and remove harmful attachments or links, use anti-malware software.

Indicators of Compromise

No specific Indicators of Compromise (IoCs) are available currently.

Threat Landscape

Phishing attacks now apply more complex social engineering tactics and more convincingly designed emails, websites, and login pages to lure their targets. In addition, threat actors are increasingly focusing their efforts on particular users or organisations, a practise known as spear phishing or whaling. By personalising the messages to appear more trustworthy, this increases the potential success of the attack effort.

Due to their efficacy and prevalence, RCE type attacks have been widely used for a long time. Vulnerable programmes frequently contain insecure classes and functions that provide direct code execution. The variety of forms that RCE assaults can take and their ability to directly influence a system through an insecure programme – often without compromising the victim’s network – are what give them their efficacy.

Mitre Methodologies

Tactic: Execution:

TA0002 – Execution

Threat Group

No attribution to specific threat actors or groups has been identified at the time of writing.

Further Information

Microsoft Advisory