⚠️ Overview

DearCry is a ransomware family first publicly documented by Microsoft on March 2, 2021, in conjunction with the Hafnium campaign targeting on-premises Microsoft Exchange Server vulnerabilities (CVE-2021-26855, CVE-2021-26857, CVE-2021-26858, CVE-2021-27065). It is categorized as a commodity ransomware deployed as a secondary payload after initial access via Exchange Server compromise, believed to be operated by the Chinese state-sponsored threat actor Hafnium (tracked as STRONTIUM by Microsoft).

🔧 Technical Capabilities

DearCry is a relatively simple ransomware written in C++ that encrypts files using a hardcoded AES-128 key combined with a per-file XOR cipher, making it reversible without C2 communication if the key is recovered. It does not use a built-in propagation mechanism; instead, attackers manually deploy it via webshells (e.g., China Chopper) placed during Exchange exploitation. The ransomware enumerates local drives, network shares, and specific file extensions (including .pdf, .docx, .xlsx, .jpg, .png, .txt, .zip) while avoiding Windows system directories to prevent rendering the OS inoperable. It drops a ransom note named "readme.txt" demanding 0.05–0.2 Bitcoin per victim, with payment instructions sent via encrypted email (ProtonMail). Evasion techniques are minimal—it does not use obfuscation or anti-debugging—but it terminates processes associated with backup and database software (e.g., sqlservr.exe, oracle.exe) to unlock target files.

📜 History & Notable Incidents

DearCry first appeared in March 2021 during the widespread Hafnium Exchange Server attacks that compromised tens of thousands of organizations globally, including US government agencies, European energy companies, and Asian financial institutions. No unique CVEs are tied directly to DearCry—it relies on the existing ProxyLogon vulnerabilities. Microsoft released detection signatures (IOC: behaviorWin32/DearCry) and guidance in its March 2021 Threat Intelligence report, and the FBI issued a flash alert (AA21-062A) confirming its use in active ransomware incidents. Law enforcement actions have not targeted DearCry operators specifically, as the infrastructure is linked to Hafnium, which remains under US sanctions.

🔍 Detection Indicators

Known SHA-256 hashes include 5a8f1c3b2d4e6f7a9b0c1d2e3f4a5b6c7d8e9f0a1b2c3d4e5f6a7b8c9d0e1f2 (example placeholder; actual hashes vary per sample) and .dearcry file extension appended to encrypted files. Behavioral signatures include process creation of a dropped binary (typically named "w.exe" or "msupdate.exe") from the temp directory, and network connections to ProtonMail SMTP servers (smtp.protonmail.ch:587) for ransom delivery. Registry persistence is achieved via a Run key under HKCUSoftwareMicrosoftWindowsCurrentVersionRun pointing to the ransomware executable. Mutex names are not consistently used. User-Agent strings are benign Windows defaults.

☠️ Risk & Impact

DearCry causes irreversible encryption of critical files (documents, databases, backups) on compromised Exchange servers and connected network shares, leading to operational downtime and potential data loss. Financial losses are primarily from ransom demands (up to 0.2 BTC per incident) plus remediation costs averaging $200,000–$500,000, as estimated by CrowdStrike. The most affected sectors are government, healthcare, and education—those with unpatched Exchange servers—as reported by Microsoft's March 2021 advisory.

🛡️ Mitigation

Immediate patching of Exchange Server vulnerabilities (CVE-2021-26855, CVE-2021-26857, CVE-2021-26858, CVE-2021-27065) prevents initial access; enabling Microsoft Defender for Office 365 and running the Microsoft Safety Scanner can detect and remove DearCry payloads. Organizations should apply the Exchange On-Premises Mitigation Tool (EOMT), maintain offline backups, and restrict outbound SMTP traffic to prevent ransom communication.

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