CVE-2020-5135

What is SonicWall SonicOS?

SonicWall SonicOS is the operating system powering SonicWall's network security appliances — firewalls, VPN gateways, and unified threat management devices deployed in enterprises, SMBs, and government organizations worldwide. SonicOS devices serve as network perimeter defense, handling VPN remote access, firewall policy enforcement, and network segmentation. The SSL-VPN portal component of SonicOS is typically internet-accessible on port 443 or 4433, enabling remote workers to authenticate and connect to corporate networks. Vulnerabilities in the SonicOS VPN portal processing layer are severe because they are reachable from the internet before any authentication occurs, making every internet-facing SonicWall device a potential attack target.

Overview

CVE-2020-5135 is a buffer overflow (CWE-120) in SonicWall SonicOS that allows a remote, unauthenticated attacker to cause denial of service or potentially execute arbitrary code by sending a malicious HTTP/HTTPS request to the firewall. SonicWall patched it in SNWLID-2020-0023 (October 2020). CISA added it to KEV in March 2022, reflecting confirmed exploitation of unpatched SonicWall appliances. The vulnerability affects SonicOS devices across multiple hardware generations, and SonicWall firewalls are widely deployed and internet-accessible by design, making this a high-exposure vulnerability.

Affected Versions

Technical Details

• Root cause: Buffer overflow (CWE-120) in SonicOS HTTP/HTTPS request processing — the SonicOS VPN portal processes incoming requests from the internet; a malformed HTTP request with an oversized field or crafted payload overflows a fixed-size buffer in the processing code, corrupting adjacent stack or heap memory; the severity of impact (DoS vs. code execution) depends on the specifics of the overflow and memory layout
• Pre-authentication exposure: The vulnerability is reachable before any authentication occurs — an attacker sends a single crafted HTTP/HTTPS request to the SonicOS management or VPN portal, triggering the overflow without providing credentials; this is the highest-severity exposure model for network appliance vulnerabilities
• SonicWall appliance attack surface: SonicWall firewalls are deployed at network perimeters with management/VPN interfaces deliberately exposed to the internet for remote access; unlike enterprise servers that may be behind additional firewall layers, security appliances are by design reachable from untrusted networks, eliminating network-level compensating controls
• Denial of service primary impact: At minimum, the vulnerability causes the SonicOS process to crash (DoS), forcing the appliance to restart or fail; for organizations relying on SonicWall for VPN connectivity, this disrupts all remote access; successful RCE would provide full control of the network security appliance
• Exploit development timeline: CISA's March 2022 KEV addition (17 months after the October 2020 patch) reflects that attackers developed working exploits and targeted unpatched appliances — common for network security device vulnerabilities where patch adoption is slow

Discovery

Identified by security researchers and reported to SonicWall. SonicWall patched CVE-2020-5135 in October 2020 alongside multiple other SonicOS vulnerabilities in SNWLID-2020-0023. CISA's March 2022 KEV addition confirmed active exploitation of unpatched SonicWall appliances in the wild.

Exploitation Context

SonicWall network security appliances are high-value targets for threat actors because they sit at network perimeters and, if compromised, provide an attacker with network-level access to the organization's internal network, ability to intercept VPN traffic and credentials, and potential for lateral movement. SonicWall experienced multiple severe vulnerabilities in 2020-2021 (including zero-day exploitation of SMA appliances), and threat actors actively scan for unpatched SonicWall devices using services like Shodan and Censys. The 17-month gap between the patch and CISA KEV addition demonstrates that a significant number of organizations failed to patch their perimeter security appliances in a timely manner.

Remediation

1. Apply SonicWall SNWLID-2020-0023 patches for all affected SonicOS versions immediately — check the SonicWall PSIRT advisory for the specific firmware version for your appliance model
2. If immediate patching is not possible, restrict management and VPN portal access to known IP ranges — do not allow internet access to SonicOS management interfaces from arbitrary source IPs
3. Enable SonicWall GMS (Global Management System) if available to centrally monitor firmware versions across all SonicWall appliances and identify unpatched devices
4. Review SonicOS logs for unusual HTTP/HTTPS requests or crash/restart events that may indicate exploitation attempts
5. Implement a regular firmware update schedule for all network security appliances — SonicWall publishes PSIRT advisories for all SonicOS vulnerabilities at psirt.global.sonicwall.com
6. For appliances that cannot be immediately patched, consider deploying an additional WAF or reverse proxy in front of the SonicOS VPN portal to filter malformed requests