CVE-2024-20465 Overview
A vulnerability in the access control list (ACL) programming of Cisco IOS Software running on Cisco Industrial Ethernet 4000, 4010, and 5000 Series Switches could allow an unauthenticated, remote attacker to bypass a configured ACL. This Authorization Bypass vulnerability stems from the incorrect handling of IPv4 ACLs on switched virtual interfaces (SVIs) when an administrator enables and disables Resilient Ethernet Protocol (REP).
Critical Impact
An unauthenticated remote attacker can bypass ACL protections on affected Cisco Industrial Ethernet switches, potentially gaining unauthorized network access to protected segments.
Affected Products
- Cisco IOS Software version 15.2(8)E2
- Cisco IOS Software version 15.2(8)E3
- Cisco IOS Software version 15.2(8)E4
- Cisco IOS Software version 15.2(8)E5
- Cisco Industrial Ethernet 4000, 4010, and 5000 Series Switches
Discovery Timeline
- September 25, 2024 - CVE CVE-2024-20465 published to NVD
- October 24, 2024 - Last updated in NVD database
Technical Details for CVE-2024-20465
Vulnerability Analysis
This vulnerability is classified as CWE-284 (Improper Access Control), affecting the ACL programming logic within Cisco IOS Software. The flaw resides in how the switch handles IPv4 ACLs applied to switched virtual interfaces (SVIs) during specific operational state changes of the Resilient Ethernet Protocol (REP).
When an administrator enables and subsequently disables REP on an affected switch, the ACL configuration on SVIs enters an inconsistent state. The underlying ACL enforcement mechanism fails to properly reinitialize or maintain the configured access rules, creating a window where traffic that should be denied by the ACL is permitted to traverse the switch.
This vulnerability is particularly concerning in industrial environments where Cisco Industrial Ethernet switches are deployed to segment and protect operational technology (OT) networks from unauthorized access.
Root Cause
The root cause lies in the incorrect handling of IPv4 ACL state during REP configuration changes. When REP is toggled (enabled then disabled), the ACL programming on SVIs is not properly restored to its intended configuration. This results in a race condition or state synchronization issue between the REP protocol handler and the ACL enforcement engine, leaving the switch in a permissive state that does not reflect the administrator's intended security policy.
Attack Vector
An attacker can exploit this vulnerability remotely over the network without authentication. The exploitation scenario involves:
- The attacker identifies a Cisco Industrial Ethernet 4000, 4010, or 5000 Series Switch running a vulnerable version of Cisco IOS
- The attacker monitors or infers that REP has been toggled on the device (potentially through network reconnaissance or after administrative maintenance windows)
- The attacker sends traffic through the affected switch that would normally be blocked by the configured ACL
- Due to the ACL bypass condition, the traffic is permitted, allowing unauthorized access to protected network segments
The vulnerability affects network traffic filtering, which means attackers could potentially reach sensitive industrial control systems, management interfaces, or other protected resources behind the switch.
Detection Methods for CVE-2024-20465
Indicators of Compromise
- Unexpected network traffic traversing SVIs that should be blocked by configured ACLs
- Log entries indicating REP configuration changes followed by anomalous traffic patterns
- Network connections to protected segments from unauthorized sources
- ACL hit counters not incrementing for traffic that should be matched and denied
Detection Strategies
- Implement network traffic monitoring on segments protected by Cisco Industrial Ethernet switches to identify unauthorized access attempts
- Review switch logs for REP enable/disable events and correlate with any subsequent unauthorized traffic
- Deploy network intrusion detection systems (IDS) to monitor for traffic patterns that violate expected ACL policies
- Conduct periodic ACL configuration audits to verify enforcement matches intended policy
Monitoring Recommendations
- Enable syslog forwarding from affected switches to a centralized SIEM for real-time monitoring of REP state changes
- Configure SNMP traps for ACL violations and configuration changes on industrial Ethernet switches
- Implement NetFlow or equivalent traffic analysis on upstream devices to detect anomalous traffic patterns through protected segments
- Establish baseline traffic patterns for industrial networks and alert on deviations
How to Mitigate CVE-2024-20465
Immediate Actions Required
- Review the Cisco Security Advisory for detailed guidance and fixed software versions
- Identify all Cisco Industrial Ethernet 4000, 4010, and 5000 Series Switches in your environment running vulnerable IOS versions
- Plan and schedule software upgrades to a fixed version as recommended by Cisco
- Avoid toggling REP on affected switches until the vulnerability is remediated
Patch Information
Cisco has released a security advisory addressing this vulnerability. Administrators should consult the Cisco Security Advisory cisco-sa-repacl-9eXgnBpD for specific fixed software versions and upgrade instructions. The affected IOS versions include 15.2(8)E2, 15.2(8)E3, 15.2(8)E4, and 15.2(8)E5.
Workarounds
- Avoid enabling or disabling REP on affected switches until the software is upgraded to a fixed version
- If REP must be toggled, manually verify ACL enforcement is functioning correctly afterward by inspecting ACL hit counters and testing traffic filtering
- Implement compensating controls such as additional ACLs on upstream devices or network segmentation through other means
- Consider deploying network access control (NAC) solutions to provide an additional layer of authorization
# Verify ACL configuration and hit counters on affected switch
show ip access-lists
show interfaces <interface-name> | include access-group
# After any REP configuration change, verify ACL is still enforcing
show running-config | section access-list
Disclaimer: This content was generated using AI. While we strive for accuracy, please verify critical information with official sources.
