CVE-2021-34739
📋 TL;DR
This vulnerability allows unauthenticated remote attackers to replay intercepted session credentials to gain unauthorized administrative access to Cisco Small Business Series Switches web management interfaces. Attackers can exploit this via man-in-the-middle attacks to capture valid credentials and reuse them later. Organizations using affected Cisco Small Business switches with web management interfaces exposed are at risk.
💻 Affected Systems
- Cisco Small Business 250 Series Switches
- Cisco Small Business 350 Series Switches
- Cisco Small Business 350X Series Switches
- Cisco Small Business 550X Series Switches
📦 What is this software?
⚠️ Risk & Real-World Impact
Worst Case
Complete compromise of network switch with administrative privileges, enabling network traffic interception, configuration changes, VLAN manipulation, and potential lateral movement to other systems.
Likely Case
Unauthorized administrative access to switch management interface leading to network disruption, configuration changes, and potential credential harvesting from other systems.
If Mitigated
Limited impact if switches are patched, web management interfaces are not internet-facing, and proper network segmentation is implemented.
🎯 Exploit Status
Requires man-in-the-middle position to intercept credentials, but replay attack itself is straightforward once credentials are captured.
🛠️ Fix & Mitigation
✅ Official Fix
Patch Version: Firmware version 2.5.9.16 or later
Vendor Advisory: https://tools.cisco.com/security/center/content/CiscoSecurityAdvisory/cisco-sa-smb-switches-tokens-UzwpR4e5
Restart Required: Yes
Instructions:
1. Download firmware version 2.5.9.16 or later from Cisco website. 2. Backup current configuration. 3. Upload new firmware via web interface or TFTP. 4. Reboot switch. 5. Verify firmware version after reboot.
🔧 Temporary Workarounds
Disable web management interface
allDisable HTTP/HTTPS web interface and use SSH/CLI for management instead
no ip http server
no ip http secure-server
Restrict web interface access
allLimit web interface access to specific management networks using ACLs
ip http access-class MANAGEMENT-ACL
ip http secure-server access-class MANAGEMENT-ACL
🧯 If You Can't Patch
- Disable web-based management interface and use SSH/CLI exclusively
- Implement strict network segmentation to isolate switch management interfaces from untrusted networks
🔍 How to Verify
Check if Vulnerable:
Check firmware version via web interface or CLI command: show versionCheck Version:
show versionVerify Fix Applied:
Verify firmware version is 2.5.9.16 or later using: show version📡 Detection & Monitoring
Log Indicators:
- Multiple successful admin logins from same IP with same session token
- Admin logins from unusual IP addresses
- Session token reuse across different IPs
Network Indicators:
- HTTP/HTTPS traffic to switch management interface from unexpected sources
- Man-in-the-middle activity on switch management network
SIEM Query:
source="switch_logs" AND (event_type="admin_login" AND ip_count>1) OR (event_type="session_token_reuse")