CVE-2017-20201

CCleaner v5.33.6162 and CCleaner Cloud v1.07.3191 (32-bit builds) contained a malicious pre-entry-point loader that diverts execution from __scrt_common_main_seh into a custom loader. That loader decodes an embedded blob into shellcode, allocates executable heap memory, resolves Windows API functions at runtime, and transfers execution to an in-memory payload. The payload performs anti-analysis checks, gathers host telemetry, encodes the data with a two-stage obfuscation, and attempts HTTPS exfiltration to hard-coded C2 servers or month-based DGA domains. Potential impacts include remote data collection and exfiltration, stealthy in-memory execution and persistence, and potential lateral movement. CCleaner was developed by Piriform, which was acquired by Avast in July 2017; Avast later merged with NortonLifeLock to form the parent company now known as Gen Digital. According to vendor advisories, the compromised CCleaner build was released on August 15, 2017 and remediated on September 12, 2017 with v5.34; the compromised CCleaner Cloud build was released on August 24, 2017 and remediated on September 15, 2017 with v1.07.3214.

• CWE-506

• https://blog.avast.com/progress-on-ccleaner-investigation
• https://blog.avast.com/update-to-the-ccleaner-5.33.6162-security-incident
• https://blog.talosintelligence.com/avast-distributes-malware/
• https://www.ccleaner.com/
• https://www.ccleaner.com/knowledge/security-notification-ccleaner-v5336162-ccleaner-cloud-v1073191
• https://www.crowdstrike.com/en-us/blog/protecting-software-supply-chain-deep-insights-ccleaner-backdoor/
• https://www.morphisec.com/blog/morphisec-discovers-ccleaner-backdoor/
• https://www.vulncheck.com/advisories/ccleaner-and-ccleaner-cloud-malicious-backdoor-supply-chain-compromise
• https://www.crowdstrike.com/en-us/blog/protecting-software-supply-chain-deep-insights-ccleaner-backdoor/

Published

Oct 8, 2025

Last Modified

Jun 17, 2026

Status

Deferred