Cisco Secure Email Gateway(SEG/SMA) RCE

TL;DR In December 2025, Cisco published https://sec.cloudapps.cisco.com/security/center/content/CiscoSecurityAdvisory/cisco-sa-sma-attack-N9bf4 addressing CVE-2025-20393, a critical vulnerability (CVSS 10.0) affecting Cisco Secure Email Gateway and Secure Email and Web Manager. The advisory was notably sparse on technical details, describing only “Improper Input Validation” (CWE-20). We decided to dig deeper. Through reverse engineering and code analysis of AsyncOS 15.5.3, we uncovered the root cause: a single-byte integer overflow in the EUQ RPC protocol that bypasses authentication and chains into Python pickle deserialization — achieving unauthenticated remote code execution with a single HTTP request. (starlabs.sg)

TL;DR In December 2025, Cisco published https://sec.cloudapps.cisco.com/security/center/content/CiscoSecurityAdvisory/cisco-sa-sma-attack-N9bf4 addressing CVE-2025-20393, a critical vulnerability (CVSS 10.0) affecting Cisco Secure Email Gateway and Secure Email and Web Manager. The advisory was notably sparse on technical details, describing only “Improper Input Validation” (CWE-20). We decided to dig deeper. Through reverse engineering and code analysis of AsyncOS 15.5.3, we uncovered the root cause: a single-byte integer overflow in the EUQ RPC protocol that bypasses authentication and chains into Python pickle deserialization — achieving unauthenticated remote code execution with a single HTTP request. (starlabs.sg)