CVE-2020-5135: SonicWall SonicOS RCE Vulnerability

CVE-2020-5135 Overview

A critical buffer overflow vulnerability exists in SonicWall SonicOS that allows remote attackers to cause Denial of Service (DoS) conditions and potentially execute arbitrary code by sending a malicious request to the firewall. This vulnerability affects multiple versions of SonicOS Gen 6, SonicOSv, and Gen 7, representing a significant security risk for organizations relying on SonicWall firewall appliances for network perimeter defense.

Critical Impact

This vulnerability is actively exploited in the wild and is listed in the CISA Known Exploited Vulnerabilities (KEV) catalog. Remote unauthenticated attackers can crash firewall appliances or potentially gain complete control over affected devices, compromising network security boundaries.

Affected Products

• SonicWall SonicOS Gen 6 version 6.5.4.7
• SonicWall SonicOS Gen 6 version 6.5.1.12
• SonicWall SonicOS Gen 6 version 6.0.5.3
• SonicWall SonicOSv version 6.5.4.v
• SonicWall SonicOS Gen 7 version 7.0.0.0

Discovery Timeline

• October 12, 2020 - CVE-2020-5135 published to NVD
• October 31, 2025 - Last updated in NVD database

Technical Details for CVE-2020-5135

Vulnerability Analysis

CVE-2020-5135 is classified as CWE-120 (Buffer Copy without Checking Size of Input), a classic buffer overflow vulnerability. The flaw exists in the network request handling component of SonicOS, where insufficient bounds checking allows an attacker to overflow a buffer by sending specially crafted network packets to the firewall interface.

The vulnerability is particularly dangerous because it can be exploited remotely without authentication. When a malicious request is processed, the buffer overflow can corrupt adjacent memory, leading to service disruption or, in more sophisticated attacks, arbitrary code execution with the privileges of the firewall process. Given that SonicWall firewalls typically operate at the network perimeter with elevated privileges, successful exploitation could allow attackers to pivot into internal networks or disable critical security controls.

Root Cause

The root cause of this vulnerability is improper input validation in the SonicOS request handling mechanism. When processing incoming network requests, the affected SonicOS versions fail to properly validate the size of input data before copying it into a fixed-size buffer. This classic buffer overflow condition (CWE-120) occurs because the code does not verify that the source data length is smaller than or equal to the destination buffer size, allowing memory corruption when oversized data is received.

Attack Vector

The attack vector is network-based and requires no authentication or user interaction. An attacker can exploit this vulnerability by:

1. Identifying an exposed SonicWall firewall running a vulnerable SonicOS version
2. Crafting a malicious network request with oversized payload data
3. Sending the malicious request to the firewall's network interface
4. The overflow corrupts memory, causing either a denial of service (crash) or enabling code execution

The network-accessible nature of firewalls combined with the unauthenticated exploitation path makes this vulnerability particularly attractive to threat actors. Attackers can scan for vulnerable SonicWall devices and launch attacks without requiring any prior access or credentials.

Detection Methods for CVE-2020-5135

Indicators of Compromise

• Unexpected firewall service crashes or restarts without administrative action
• Anomalous network traffic patterns targeting SonicWall management interfaces
• Memory corruption errors or core dumps on SonicWall appliances
• Unusual outbound connections from the firewall to unknown destinations

Detection Strategies

• Deploy network intrusion detection signatures to identify malformed requests targeting SonicWall devices
• Monitor SonicWall system logs for crash events, memory errors, or unexpected service restarts
• Implement anomaly detection for traffic volumes and patterns directed at firewall interfaces
• Utilize SentinelOne Singularity to detect post-exploitation behavior if attackers pivot from compromised firewalls

Monitoring Recommendations

• Enable verbose logging on SonicWall appliances to capture detailed request information
• Configure alerts for SonicOS service restarts or availability issues
• Monitor network traffic for scanning activity targeting known SonicWall ports
• Review CISA KEV alerts and threat intelligence feeds for active exploitation campaigns

How to Mitigate CVE-2020-5135

Immediate Actions Required

• Verify all SonicWall appliances are running patched firmware versions
• Review firewall access controls to limit management interface exposure
• Implement network segmentation to isolate SonicWall management interfaces
• Monitor for exploitation attempts using IDS/IPS signatures

Patch Information

SonicWall has released security patches to address this vulnerability. Organizations should apply the latest firmware updates immediately. Detailed patch information is available in the SonicWall Vulnerability Detail SNWLID-2020-0010. Due to active exploitation documented in the CISA Known Exploited Vulnerabilities Catalog, patching should be prioritized as critical.

Workarounds

• Restrict access to SonicWall management interfaces to trusted IP addresses only
• Place SonicWall management interfaces behind VPN or additional authentication layers
• Implement network-level filtering to block potentially malicious traffic patterns
• Consider deploying Web Application Firewall (WAF) rules in front of management interfaces if direct patching is delayed