The threat titled "Excel(ent) Obfuscation: Regex Gone Rogue" describes a novel attack technique exploiting Microsoft Excel's recently introduced regex functions, specifically REGEXEXTRACT, to perform advanced code obfuscation within Excel files. Attackers embed malicious PowerShell commands inside large blocks of text in Excel spreadsheets and use REGEXEXTRACT to dynamically extract these commands at runtime. This obfuscation method significantly reduces the visibility of malicious code to traditional antivirus solutions, as demonstrated by a proof-of-concept that lowered VirusTotal detection rates from 22 to just 2. Furthermore, this technique evades heuristic analysis tools such as OLEVBA, which are commonly used to analyze VBA macros, by hiding payloads within regex-extracted strings rather than explicit macro code. Currently, exploitation is limited by Microsoft's default macro security settings, which typically block or warn users about macros, and by the limited availability of the regex functions in Excel versions. However, as these functions become more widespread, attackers could combine this obfuscation method with social engineering or other exploits to bypass macro protections. The provided file hashes correspond to samples of malicious Excel files using this technique, useful for detection and analysis. Overall, this represents an evolution in Excel-based malware obfuscation, leveraging new Excel functionality to evade detection and complicate forensic analysis.

For European organizations, this threat poses a medium-level risk with the potential for significant operational disruption if exploited successfully. Excel is widely used across European enterprises, including critical sectors such as finance, manufacturing, government, and healthcare, making it a common vector for initial compromise. The advanced obfuscation technique reduces detection rates, increasing the likelihood that malicious payloads could execute undetected, potentially leading to data breaches, ransomware deployment, or lateral movement within networks. Since PowerShell is frequently used for post-exploitation activities, attackers could leverage this method to establish persistent access or exfiltrate sensitive data. The evasion of heuristic tools complicates incident response and forensic investigations, potentially delaying detection and remediation efforts. Although Microsoft's default macro security settings currently mitigate widespread exploitation, organizations with lax macro policies or users who enable macros without verification are at higher risk. In regulated European sectors, such as finance and healthcare, successful exploitation could lead to violations of GDPR and other data protection regulations, resulting in financial penalties and reputational damage.

To effectively mitigate this threat, European organizations should implement targeted measures beyond generic macro restrictions: 1) Update endpoint detection and response (EDR) tools and antivirus solutions to recognize obfuscation patterns involving REGEXEXTRACT and related Excel functions. Collaborate with security vendors to incorporate detection signatures based on the provided malicious file hashes and behavior analytics. 2) Enforce strict Group Policy settings to disable macros by default and restrict macro execution to digitally signed and trusted sources only. 3) Conduct user awareness training focused on the risks of enabling macros, especially in unsolicited or unexpected Excel files, emphasizing the new obfuscation techniques involving regex functions. 4) Implement network-level controls to monitor and restrict PowerShell execution originating from Office applications, using application whitelisting and script block logging to detect anomalous activity. 5) Regularly audit and update Excel installations to the latest versions, ensuring macro security features and function availability align with organizational policies. 6) Employ advanced threat hunting practices that include searching for large text blocks with embedded regex extraction patterns in Excel files and correlating these with PowerShell execution events. 7) Integrate sandboxing solutions capable of dynamically analyzing Excel files with macros and regex functions to detect hidden payloads before delivery to end users. These focused actions will reduce the attack surface and improve early detection of this sophisticated obfuscation technique.