CVE-2021-1251 in Small Business RV Series Router
Summary
by MITRE • 04/08/2021
Multiple vulnerabilities exist in the Link Layer Discovery Protocol (LLDP) implementation for Cisco Small Business RV Series Routers. An unauthenticated, adjacent attacker could execute arbitrary code or cause an affected router to leak system memory or reload. A memory leak or device reload would cause a denial of service (DoS) condition on an affected device. For more information about these vulnerabilities, see the Details section of this advisory. Note: LLDP is a Layer 2 protocol. To exploit these vulnerabilities, an attacker must be in the same broadcast domain as the affected device (Layer 2 adjacent).
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Analysis
by VulDB Data Team • 04/11/2021
The vulnerability identified as CVE-2021-1251 affects the Link Layer Discovery Protocol implementation within Cisco Small Business RV Series Routers, representing a significant security weakness that could be exploited by adjacent attackers. This vulnerability resides in the Layer 2 protocol stack, specifically targeting the LLDP functionality that routers use to advertise their identity and capabilities to neighboring network devices. The affected devices operate within the small business networking segment, where LLDP is commonly enabled to facilitate network discovery and management. The vulnerability's exploitation requires an attacker to exist within the same broadcast domain, meaning they must be physically or logically adjacent to the target device at Layer 2, which limits the attack surface but does not eliminate the risk in environments where network segmentation is insufficient.
The technical flaw in this vulnerability stems from improper handling of LLDP packets received from neighboring devices, potentially allowing for buffer overflows, memory corruption, or other memory management issues that could result in arbitrary code execution. The vulnerability's nature suggests that the router's LLDP implementation lacks adequate input validation and bounds checking when processing received LLDP frames. This type of flaw commonly maps to CWE-121, which describes heap-based buffer overflow conditions, or CWE-122, which addresses stack-based buffer overflow scenarios. The implementation error likely occurs during parsing of LLDP TLV (Type-Length-Value) structures where the router fails to properly validate the length fields or enforce proper memory allocation limits, allowing crafted malicious packets to overwrite adjacent memory locations.
The operational impact of CVE-2021-1251 extends beyond simple denial of service conditions to potentially enable complete system compromise through arbitrary code execution. When exploited, the vulnerability could cause the affected router to crash and reload, creating a denial of service condition that disrupts network connectivity for all devices relying on that router for network access. The memory leak aspect of the vulnerability could gradually consume system resources, leading to performance degradation and eventual system instability. In environments where routers serve as critical network infrastructure components, such as branch office connections or small business networks, these disruptions could severely impact business operations and network availability. The vulnerability's potential for remote code execution means that an attacker could gain control of the router's operating system, potentially allowing for persistent access, data exfiltration, or further network reconnaissance and lateral movement.
Mitigation strategies for CVE-2021-1251 should focus on both immediate defensive measures and long-term architectural improvements. Network administrators should first ensure that LLDP is disabled on affected routers if it is not required for network operations, as this immediately removes the attack surface. When LLDP must remain enabled, implementing proper network segmentation through VLANs and access control lists can limit the broadcast domain and reduce the likelihood of adjacent attacks. Cisco has released security patches and firmware updates addressing this vulnerability, which should be applied immediately to all affected devices. Network monitoring should be enhanced to detect unusual LLDP traffic patterns or malformed packets that could indicate exploitation attempts. The vulnerability's classification under the ATT&CK framework would place it within the T1046 technique for Network Service Scanning, as attackers might probe for vulnerable LLDP implementations, and potentially the T1059 technique for Command and Scripting Interpreter if code execution is achieved. Organizations should also consider implementing network access controls and firewall rules to limit Layer 2 broadcast domains and reduce the attack surface for adjacent network-based attacks. Regular vulnerability assessments and network audits should be conducted to identify and remediate similar issues in other network infrastructure components.