CVE-2019-12706
📋 TL;DR
This vulnerability allows unauthenticated remote attackers to bypass email header filters on Cisco Email Security Appliances by sending specially crafted SPF packets. It affects Cisco AsyncOS Software for ESA devices, potentially allowing malicious content to pass through security controls. Organizations using vulnerable Cisco ESA devices with SPF functionality enabled are at risk.
💻 Affected Systems
- Cisco Email Security Appliance (ESA)
📦 What is this software?
⚠️ Risk & Real-World Impact
Worst Case
Attackers could deliver phishing emails, malware, or spam directly to user inboxes by bypassing all configured content filters, potentially leading to credential theft, malware infections, or data breaches.
Likely Case
Attackers bypass specific header-based filtering rules to deliver targeted malicious emails that would normally be blocked, increasing the success rate of phishing campaigns.
If Mitigated
With proper network segmentation and additional email security layers, the impact is limited to potential delivery of some malicious emails that other security controls might still catch.
🎯 Exploit Status
Exploitation requires sending specially crafted SPF packets to the vulnerable device. No authentication is required, making this easily exploitable.
🛠️ Fix & Mitigation
✅ Official Fix
Patch Version: Cisco AsyncOS Software for ESA version 13.0.0-392 and later
Vendor Advisory: https://tools.cisco.com/security/center/content/CiscoSecurityAdvisory/cisco-sa-20191002-esa-bypass
Restart Required: Yes
Instructions:
1. Download the appropriate AsyncOS update from Cisco. 2. Upload the update to the ESA device via web interface or CLI. 3. Apply the update following Cisco's upgrade procedures. 4. Reboot the device as required.
🔧 Temporary Workarounds
Disable SPF Checking
allTemporarily disable SPF functionality to prevent exploitation while planning for patching
Navigate to Mail Policies > HAT Overview > Edit HAT policy > Anti-Spam > SPF Verification > Disable
🧯 If You Can't Patch
- Implement additional email filtering layers (cloud-based or on-premise) to catch malicious emails that might bypass the ESA
- Monitor email traffic for unusual patterns and implement strict DMARC policies to reduce spoofing risk
🔍 How to Verify
Check if Vulnerable:
Check AsyncOS version via web interface (System Administration > System Software > Version) or CLI command: 'version'Check Version:
versionVerify Fix Applied:
Verify version is 13.0.0-392 or later and test SPF functionality with legitimate test emails📡 Detection & Monitoring
Log Indicators:
- Unusual SPF validation failures
- Emails bypassing header filters that normally would be blocked
- Increased volume of emails from suspicious sources
Network Indicators:
- Unusual SPF packet patterns to ESA devices
- Increased email traffic from single sources
SIEM Query:
source="cisco-esa" AND (spf_failure OR filter_bypass)