CVE-2020-3436
📋 TL;DR
This vulnerability allows unauthenticated remote attackers to upload arbitrarily large files to specific folders on Cisco ASA and Firepower Threat Defense devices, potentially causing a denial of service through device reboots. It affects Cisco ASA and FTD software with vulnerable web services interfaces. Organizations using these devices with internet-facing web services are at risk.
💻 Affected Systems
- Cisco Adaptive Security Appliance (ASA)
- Cisco Firepower Threat Defense (FTD)
📦 What is this software?
Adaptive Security Appliance Software by Cisco
View all CVEs affecting Adaptive Security Appliance Software →
Adaptive Security Appliance Software by Cisco
View all CVEs affecting Adaptive Security Appliance Software →
Adaptive Security Appliance Software by Cisco
View all CVEs affecting Adaptive Security Appliance Software →
Adaptive Security Appliance Software by Cisco
View all CVEs affecting Adaptive Security Appliance Software →
Adaptive Security Appliance Software by Cisco
View all CVEs affecting Adaptive Security Appliance Software →
⚠️ Risk & Real-World Impact
Worst Case
Complete denial of service through repeated device reboots, disrupting network security and connectivity for extended periods.
Likely Case
Intermittent device reboots causing service disruption and potential security monitoring gaps during downtime.
If Mitigated
Limited impact with proper network segmentation and access controls preventing exploitation attempts.
🎯 Exploit Status
Simple file upload mechanism makes exploitation straightforward. Public exploit code exists.
🛠️ Fix & Mitigation
✅ Official Fix
Patch Version: Multiple fixed versions available - see Cisco advisory for specific version mappings
Vendor Advisory: https://tools.cisco.com/security/center/content/CiscoSecurityAdvisory/cisco-sa-asaftd-fileup-dos-zvC7wtys
Restart Required: Yes
Instructions:
1. Review Cisco advisory for fixed versions matching your hardware/software. 2. Download appropriate firmware from Cisco. 3. Backup configuration. 4. Apply update following Cisco upgrade procedures. 5. Verify successful update and functionality.
🔧 Temporary Workarounds
Disable Web Services Interface
allDisable the vulnerable web services interface if not required for operations.
no http server enable
no http server enable <port>
Restrict Access with ACLs
allApply access control lists to limit web services access to trusted sources only.
access-list WEB-ACCESS extended permit tcp <trusted-networks> any eq <web-port>
access-group WEB-ACCESS in interface <interface-name>
🧯 If You Can't Patch
- Implement strict network segmentation to isolate vulnerable devices from untrusted networks
- Deploy intrusion prevention systems to detect and block exploitation attempts
🔍 How to Verify
Check if Vulnerable:
Check ASA/FTD version against Cisco advisory. Verify if web services are enabled with 'show running-config | include http server'.Check Version:
show version | include VersionVerify Fix Applied:
Verify updated version with 'show version' and confirm it matches fixed versions in Cisco advisory.📡 Detection & Monitoring
Log Indicators:
- Unexpected device reloads
- Large file uploads to web services
- Watchdog timeout messages
Network Indicators:
- HTTP POST requests with large payloads to ASA/FTD web interfaces
- Unusual traffic patterns preceding device reboots
SIEM Query:
source="asa_ftd_logs" AND ("reload" OR "watchdog" OR "file upload")