Yurei Ransomware Encryption Analysis Go Builder Insights

AlienVault released a detailed threat report on November 14 2025 that examines the encryption architecture of the Yurei ransomware family. The analysis focuses on the Go based builder that is responsible for generating the cryptographic keys, packaging the payload, and orchestrating the attack against corporate networks across Sri Lanka and Nigeria. The report is highly reliable with a confidence level of 100% and is considered a primary source for security analysts looking to understand the technical and operational aspects of Yurei.

Yurei first appeared in September 2025 and has quickly adopted a standard ransomware model. Target industries include transportation, information technology, marketing, and food production. The group leverages a sophisticated encryption pipeline that uses ChaCha20-Poly1305 for file payload encryption and secp256k1-ECIES for protecting the session keys. Each file is encrypted with a unique 256‑bit key and a 96‑bit nonce, ensuring that even if a single key is compromised, the rest of the encrypted data remains secure. The builder also implements a directory exclusion list that skips system directories, temporary files, and critical infrastructure files to maintain system operability after the attack.

The encryption process is executed in a multi‑stage manner. First, the Go builder generates a random symmetric key and nonce. It then encrypts the target files using ChaCha20-Poly1305, a modern authenticated encryption algorithm that provides both confidentiality and integrity. After all files are encrypted, the builder uses the secp256k1 elliptic curve to perform ECIES encryption on the symmetric key, binding it to a public key that only the threat actor possesses. The encrypted key is stored in a small metadata file that is distributed across the infected host. This design makes key recovery impossible without the attacker’s private key.

Yurei’s ransom note is a standard threat document that appears in the victim’s desktop after the encryption completes. The note demands a payment in cryptocurrency within five days or the data will be publicly leaked and regulatory notifications will be issued. The note also contains a link to a phishing site that hosts the decryption tool, which is a secondary vector for credential theft and further compromise.

From a detection perspective, the report highlights several indicators of compromise. The presence of a Go binary with a specific hash, the creation of a large number of encrypted files with .enc extensions, and the existence of a hidden metadata file that contains ECIES encrypted keys are strong signals. Network traffic to known command and control servers, especially those that use HTTPS on non‑standard ports, should also be monitored.

Security analysts can use the following recommendations to mitigate the risk of a Yurei attack:

• Implement robust backup strategies: Regularly back up critical data to offline media or immutable cloud storage. Test restore procedures to ensure data can be recovered without paying ransom.
• Enforce least privilege and segmentation: Limit user permissions and isolate sensitive systems to prevent lateral movement. Use network segmentation to contain the spread of ransomware.
• Deploy endpoint detection and response (EDR): EDR solutions that can detect file encryption activity, process creation, and anomalous network connections will provide early warning.
• Apply timely patches: Keep operating systems, applications, and firmware up to date to close known vulnerabilities that could be exploited to gain initial foothold.
• Educate users on phishing: Conduct regular training sessions and simulated phishing campaigns to reduce the likelihood of credential compromise.
• Use threat intelligence feeds: Subscribe to reputable threat intelligence services that provide indicators of compromise for Yurei and related malware families.
• Configure application whitelisting: Allow only approved binaries to run, especially on critical servers and endpoints.
• Establish an incident response plan: Define clear roles, communication channels, and containment procedures to respond quickly when an attack is detected.
• Monitor file integrity: Implement file integrity monitoring to detect unexpected changes in critical system files and directories.
• Secure configuration of encryption services: Ensure that any legitimate encryption tools used within the organization are properly configured and monitored to avoid misuse by attackers.

In addition to these controls, organizations should consider a layered approach that combines technical defenses with procedural safeguards. For example, combining EDR with network segmentation can stop ransomware from moving laterally, while user training reduces the chances of initial compromise via phishing. Regular tabletop exercises that simulate a Yurei infection can help teams refine their response and identify gaps in their defenses.

In summary, the AlienVault report provides a comprehensive view of Yurei’s encryption methodology, operational tactics, and the threat actors’ motivations. By understanding the technical details of the encryption pipeline and the attack lifecycle, security analysts can develop targeted detection rules, strengthen defensive controls, and reduce the likelihood of a successful ransomware deployment. The recommendations outlined above are designed to be actionable for organizations of all sizes and can be integrated into existing security frameworks to enhance overall resilience against this evolving threat.