SonicWall firewall VPN vulnerability (CVE-2020-5135): Overview and technical walkthrough

Pedro Tavares
February 24, 2021 by
Pedro Tavares

A critical stack-based buffer overflow vulnerability was discovered in SonicWall Network Security Appliance (NSA) VPN. In this article, we will address the problem explored by this flaw, its impact worldwide and mitigation measures to fix the problem and avoid cyber-incidents using this specific vector as an entry point on the internal infrastructure.

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CVE-2020-5135: Overview

The flaw can be triggered by an unauthenticated HTTP request involving a custom protocol handler. The vulnerability leverages the HTTP/HTTPS service used for product management as well as SSL VPN remote access. This service can be exploited to cause a denial-of-service condition and possibly remote code execution.

Nikita Abramov of Positive Technologies and Craig Young of Tripwire’s Vulnerability and Exposures Research Team (VERT) discovered CVE-2020-5135. This flaw affects the following versions of SonicOS:

SonicOS and earlier

SonicOS and earlier

SonicOS and earlier

SonicOSv and earlier


According to the researchers, the vulnerability resides in a pre-authentication and in a component SSLVPN, which is often exposed to the public internet. This opens the doors to possible attacks in the wild.

Impacted devices by number

The number of exposed devices on the internet is huge, and nearly 800,000 hosts may be vulnerable. This number is based on a Shodan search for the HTTP banner of the SonicWall firewall.

product:"SonicWALL firewall http config"

product:"SonicWALL SSL-VPN http proxy"

The Tenable team doesn’t confirm that the hosts found on Shodan were affected by this particular vulnerability.

“The hosts discovered with our Shodan queries are indicative that they are internet facing SonicWall servers, their respective versions could not be determined and thus it is unclear if they are vulnerable.”

With this set of information in place, criminals could abuse a DoS condition — which is easy to obtain, as the attack only requires a successful connection with the affected device. The malicious payload can be used to trigger the flaw on the SSL VPN portal.

Looking at the last year, we can find a set of vulnerabilities present and explored by criminals in SSL VPN solutions. As these kinds of devices are the edge of the internal network with the public internet, they are an enticing target for criminals. As VPNs take an increasingly important role amidst the rise in working remotely, exploitation of these devices can allow criminals to pivot to an internal network and begin targeting the entire ecosystem.

As observed below, some notable vulnerabilities were found in VPN devices from several vendors last few years, including:

Mitigation measures

With CVE-2020-5135 in place, attackers potentially have another SSL VPN vulnerability in scope to target vulnerable systems. In this way, patching the affected versions is mandatory in order to fix the problem.

In total, SonicWall patched 11 vulnerabilities on October 12th, 2020. The following table lists the remaining 10 vulnerabilities that were patched:

All of the vulnerabilities were discovered by security researcher Nikita Abramov of Positive Technologies Offensive Team. Abramov is credited with discovering CVE-2020-5135, along with Craig Young of VERT.

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At the moment of writing this article, no public exploit or PoC was published online.

More details about this vulnerability can be found here:



SonicWall VPN Portal Critical Flaw (CVE-2020-5135), Tripwire

Vulnerability List, SonicWall

If you want to practice writing exploits and worms, there's a big hijacking hole in SonicWall firewall VPNs, The Register

CVE-2020-5135: Critical SonicWall VPN Portal Stack-based Buffer Overflow Vulnerability, Tenable

Pedro Tavares
Pedro Tavares

Pedro Tavares is a professional in the field of information security working as an Ethical Hacker, Malware Analyst and a Security Evangelist. He is also Editor-in-Chief of the security computer blog

In recent years, he has invested in the field of information security, exploring and analyzing a wide range of topics, such as malware, reverse engineering, pentesting (Kali Linux), hacking/red teaming, mobile, cryptography, IoT, and security in computer networks. He is also a Freelance Writer.