CVE-2021-1539
📋 TL;DR
This vulnerability allows authenticated remote attackers to bypass authorization controls on Cisco ASR 5000 Series devices running StarOS software. Attackers can execute a subset of CLI commands they shouldn't have access to, potentially compromising network devices. Organizations using affected Cisco ASR 5000 Series routers are at risk.
💻 Affected Systems
- Cisco ASR 5000 Series
📦 What is this software?
Staros by Cisco
Staros by Cisco
Staros by Cisco
Staros by Cisco
Staros by Cisco
Staros by Cisco
⚠️ Risk & Real-World Impact
Worst Case
An attacker gains administrative control over critical network infrastructure, enabling network disruption, data interception, or lateral movement to other systems.
Likely Case
Attackers with existing low-privilege access escalate privileges to execute unauthorized commands, potentially modifying configurations or accessing sensitive network information.
If Mitigated
With proper network segmentation and access controls, impact is limited to the affected device only, preventing lateral movement to other systems.
🎯 Exploit Status
Exploitation requires existing authenticated access to the device CLI
🛠️ Fix & Mitigation
✅ Official Fix
Patch Version: Refer to Cisco advisory for specific fixed versions
Vendor Advisory: https://tools.cisco.com/security/center/content/CiscoSecurityAdvisory/cisco-sa-asr5k-autho-bypass-mJDF5S7n
Restart Required: Yes
Instructions:
1. Review Cisco advisory for affected versions. 2. Download and install appropriate fixed StarOS software version. 3. Reboot device to apply update. 4. Verify update was successful.
🔧 Temporary Workarounds
Restrict CLI Access
allLimit CLI access to only trusted administrators using strict access controls
Network Segmentation
allIsolate ASR 5000 management interfaces from untrusted networks
🧯 If You Can't Patch
- Implement strict network segmentation to isolate affected devices
- Enforce least privilege access controls and monitor all CLI sessions
🔍 How to Verify
Check if Vulnerable:
Check StarOS version against affected versions listed in Cisco advisoryCheck Version:
show versionVerify Fix Applied:
Verify installed StarOS version matches or exceeds fixed versions in advisory📡 Detection & Monitoring
Log Indicators:
- Unauthorized CLI command execution attempts
- Privilege escalation patterns in system logs
Network Indicators:
- Unusual CLI access patterns from unexpected sources
SIEM Query:
Search for failed authorization attempts followed by successful command execution on ASR 5000 devices