The Sigma ransomware is a type of malicious software designed to encrypt victims' files and demand a ransom payment for their release. This particular ransomware variant has been observed being distributed via a malspam campaign that impersonates Craigslist communications. Attackers send phishing emails that appear to be legitimate Craigslist messages, tricking recipients into opening malicious attachments or clicking on harmful links. These emails leverage social engineering tactics to exploit user trust in Craigslist, increasing the likelihood of user execution of the malware payload. The attack techniques align with MITRE ATT&CK patterns such as spearphishing links (T1192), spearphishing attachments (T1193), user execution (T1204), scripting (T1064), and obfuscated files or information (T1027). Once executed, Sigma ransomware encrypts user data, causing data corruption and loss of availability. The ransomware's obfuscation techniques hinder detection by traditional antivirus solutions. Although the threat level is marked as low and no known exploits in the wild have been reported, the use of social engineering and ransomware payloads poses a tangible risk to targeted users. The campaign's reliance on Craigslist-themed malspam suggests a focus on users who engage with online classifieds, potentially including small businesses and individuals. The absence of affected software versions indicates this is a malware distribution method rather than a vulnerability in a specific product. Overall, Sigma ransomware distributed via fake Craigslist malspam represents a targeted malware threat leveraging phishing to compromise systems and encrypt data.

For European organizations, the Sigma ransomware campaign poses risks primarily through user-targeted phishing attacks that can lead to data encryption and operational disruption. Small and medium enterprises (SMEs) that use online classifieds platforms like Craigslist or similar services for business activities may be particularly vulnerable. Successful infections can result in loss of critical data, downtime, and financial costs associated with ransom payments or recovery efforts. The data corruption caused by the ransomware impacts data integrity and availability, potentially affecting business continuity. Additionally, the social engineering aspect increases the risk of compromise due to user interaction, which remains a common vector for ransomware infections. Although the campaign is assessed as low severity, organizations with limited cybersecurity awareness or insufficient email filtering controls may face higher risks. The threat also underscores the importance of vigilance against phishing campaigns exploiting popular online platforms. European organizations involved in online marketplaces or classified ads should be aware of this threat vector to prevent infection and mitigate potential operational and reputational damage.

To mitigate the risk posed by Sigma ransomware distributed via fake Craigslist malspam, European organizations should implement targeted and practical measures beyond generic advice: 1. Enhance Email Filtering: Deploy advanced email security solutions capable of detecting and quarantining phishing emails, especially those impersonating known platforms like Craigslist. Use threat intelligence feeds to update spam filters with indicators related to Sigma ransomware campaigns. 2. User Awareness Training: Conduct focused phishing awareness training emphasizing the risks of opening attachments or clicking links in unsolicited emails, particularly those mimicking online classifieds or marketplace communications. Use simulated phishing exercises tailored to such scenarios. 3. Application Whitelisting: Implement application control policies that restrict execution of unauthorized scripts or executables, limiting the ability of ransomware to run even if a user executes a malicious attachment. 4. Endpoint Detection and Response (EDR): Utilize EDR tools that can detect obfuscated scripts and suspicious behaviors associated with ransomware execution, enabling rapid identification and containment. 5. Regular Backups and Recovery Testing: Maintain offline, immutable backups of critical data and regularly test restoration procedures to ensure resilience against data corruption caused by ransomware. 6. Network Segmentation: Segment networks to limit lateral movement in case of infection, reducing the scope of impact. 7. Monitor Threat Intelligence: Stay updated with intelligence from sources like CIRCL and other CERTs to detect emerging Sigma ransomware variants or related campaigns. 8. Incident Response Planning: Develop and rehearse incident response plans specific to ransomware scenarios, including communication strategies and legal considerations.