Source: cve@mitre.org
Vulnerability in Cisco routers versions 8.2 through 9.1 allows remote attackers to bypass access control lists when extended IP access lists are used on certain interfaces, the IP route cache is enabled, and the access list uses the "established" keyword.
Cisco routers running versions 8.2 through 9.1 are vulnerable to an access control bypass, allowing remote attackers to circumvent firewall rules and potentially gain unauthorized access to internal networks. This vulnerability arises when using extended IP access lists with the 'established' keyword and the IP route cache enabled, potentially leading to data exfiltration or network compromise.
Step 1: Target Identification: The attacker identifies a Cisco router within the vulnerable version range (8.2-9.1) that uses extended IP access lists with the 'established' keyword and has IP route caching enabled.
Step 2: Connection Establishment (Spoofing): The attacker initiates a TCP connection to a service on the target router, potentially using a legitimate source IP address or spoofing one. This is done to create an entry in the route cache.
Step 3: Packet Crafting: The attacker crafts malicious packets that appear to be part of the established connection, but are designed to bypass the access control list rules. These packets are crafted to exploit the vulnerability.
Step 4: Packet Injection: The attacker sends the crafted packets to the target router. These packets are designed to trigger the vulnerability.
Step 5: Access Control Bypass: Because the router's route cache is enabled and the 'established' keyword is used, the router incorrectly applies the access control rules to the cached route, allowing the malicious packets to bypass the access control list.
Step 6: Exploitation: The attacker leverages the access control bypass to gain unauthorized access, potentially leading to data exfiltration, network reconnaissance, or further exploitation.
The vulnerability stems from a flaw in how Cisco routers handle packets that match the 'established' keyword within extended IP access lists when the IP route cache is enabled. The router's logic fails to correctly apply the access control rules to cached routes, allowing packets that should be denied to pass through. Specifically, the route cache bypasses the access list checks for established connections, assuming they are legitimate. This oversight allows attackers to craft packets that appear to be part of an established connection, thereby bypassing the intended security restrictions. The root cause is a logic error in the packet processing flow, where the cached route lookup prioritizes speed over security, leading to an incorrect access control decision. This is not a buffer overflow or memory corruption issue, but a design flaw in the access control implementation.
While no specific APTs are directly linked to this CVE, the vulnerability's nature makes it attractive to any threat actor seeking to bypass network security controls. The vulnerability's age and potential for widespread impact make it a likely target for opportunistic attacks. This CVE is not listed in CISA KEV as of the current date.
Monitor network traffic for unusual patterns, such as packets bypassing expected access control rules.
Analyze router logs for access control violations, especially those related to established connections.
Implement intrusion detection systems (IDS) with signatures that detect attempts to exploit this vulnerability.
Examine network traffic for packets that do not adhere to established connection rules, especially those that appear to be part of an established connection but are malicious.
Use network monitoring tools to identify traffic that should be blocked by the access control lists but is still reaching internal resources.
Upgrade to a patched version of Cisco IOS that addresses the vulnerability. This is the primary and most effective remediation step.
If upgrading is not immediately possible, disable the IP route cache on the affected interfaces. This will mitigate the vulnerability but may impact network performance.
Review and harden access control lists to ensure they are correctly configured and effectively block unauthorized traffic.
Implement a defense-in-depth strategy, including firewalls, intrusion detection systems, and regular security audits.
Segment the network to limit the impact of a potential compromise.
Monitor network traffic for suspicious activity, especially traffic that bypasses access control lists.