CVE-2020-5135: SonicWall SonicOS Buffer Overflow Vulnerability

CVE-2020-5135 Overview

CVE-2020-5135 is a stack-based buffer overflow vulnerability in SonicWall SonicOS that allows a remote, unauthenticated attacker to trigger denial of service and potentially execute arbitrary code on affected firewalls. The flaw resides in the HTTP/HTTPS service handling component used by the SSL VPN portal and management interface. An attacker can exploit it by sending a crafted HTTP request to the firewall, requiring no credentials or user interaction. CISA added CVE-2020-5135 to the Known Exploited Vulnerabilities (KEV) catalog, confirming in-the-wild abuse against perimeter devices.

Critical Impact

Unauthenticated remote attackers can crash SonicWall firewalls and potentially gain code execution on the perimeter device, bypassing network defenses entirely.

Affected Products

• SonicWall SonicOS Gen 6 versions 6.5.4.7, 6.5.1.12, and 6.0.5.3
• SonicWall SonicOSv 6.5.4.v (virtual appliance)
• SonicWall SonicOS Gen 7 version 7.0.0.0

Discovery Timeline

• 2020-10-12 - CVE-2020-5135 published to the National Vulnerability Database (NVD)
• 2025-10-31 - Last updated in NVD database

Technical Details for CVE-2020-5135

Vulnerability Analysis

The vulnerability is classified as a classic buffer overflow ([CWE-120]) affecting the SonicOS HTTP request handler. SonicOS processes incoming HTTP/HTTPS traffic on its management and SSL VPN interfaces, which are commonly exposed to the public internet. When the device parses a specially crafted request, it copies attacker-controlled data into a fixed-size stack buffer without enforcing length boundaries. This corrupts adjacent memory, including saved return addresses and control structures used by the parsing routine.

Because the affected service runs with high privileges on the firewall, successful exploitation can yield arbitrary code execution within the SonicOS environment. At minimum, the corruption triggers a crash and reboot of the device, producing a denial-of-service condition against the network perimeter. The EPSS percentile of 96 places this issue among the top tier of vulnerabilities most likely to be exploited.

Root Cause

The SonicOS HTTP request parser fails to validate the length of attacker-supplied data before copying it into a fixed-size stack buffer. The missing bounds check allows a long input field to overwrite memory beyond the destination buffer, including saved registers and return pointers on the call stack.

Attack Vector

Exploitation requires only network reachability to the SonicWall firewall's HTTP or HTTPS service. Many SonicWall appliances expose these ports for SSL VPN access, making the attack surface broadly internet-facing. The attacker sends a single crafted HTTP request containing an oversized parameter to trigger the overflow. No authentication, credentials, or user interaction are required.

No verified public exploit code is required to describe the mechanism. See the SonicWall PSIRT advisory SNWLID-2020-0010 for vendor-confirmed technical details.

Detection Methods for CVE-2020-5135

Indicators of Compromise

• Unexpected reboots, crashes, or core dumps reported by SonicOS on internet-facing firewalls.
• Anomalously long HTTP request URIs, headers, or POST parameters directed at the SonicWall management or SSL VPN ports.
• SSL VPN portal becoming intermittently unavailable without administrative action.
• Outbound connections from the firewall management plane to unknown external hosts following malformed inbound requests.

Detection Strategies

• Inspect web application firewall and IDS/IPS logs for HTTP requests targeting SonicWall appliances containing oversized fields or non-standard character sequences.
• Correlate SonicOS device reboot events with timestamps of inbound HTTP/HTTPS requests from external sources.
• Hunt for repeated failed connections followed by service restarts on firewalls running affected SonicOS versions.

Monitoring Recommendations

• Enable syslog forwarding from SonicWall devices into a centralized SIEM and alert on crash, watchdog, and reboot events.
• Monitor SSL VPN portal availability and authentication logs for gaps that correlate with suspicious inbound traffic.
• Track firmware versions across the SonicWall fleet and alert when any device runs an unpatched build of SonicOS 6.0, 6.5, or 7.0.0.0.

How to Mitigate CVE-2020-5135

Immediate Actions Required

• Upgrade SonicOS to the fixed versions listed in the SonicWall PSIRT advisory immediately, prioritizing internet-exposed appliances.
• Restrict access to SonicOS management interfaces to trusted administrative networks where business requirements permit.
• Review SSL VPN exposure and confirm whether public availability is still required for current operations.
• Audit firewall logs for crash and reboot events that may indicate prior exploitation attempts.

Patch Information

SonicWall released fixed firmware addressing CVE-2020-5135 in October 2020. Refer to the SonicWall PSIRT advisory SNWLID-2020-0010 for the specific patched build numbers per platform. The vulnerability is tracked in the CISA Known Exploited Vulnerabilities Catalog, which mandates federal agencies remediate affected devices.

Workarounds

• Disable the SSL VPN portal on affected devices until firmware can be upgraded if public exposure is not operationally required.
• Restrict HTTPS management access by source IP using SonicOS access rules to limit the reachable attack surface.
• Place upstream filtering devices in front of the firewall to drop malformed HTTP requests where architecturally feasible.

# Configuration example - restrict SonicOS management to trusted admin subnet
# (run from SonicOS CLI in config mode)
config
  address-object ipv4 AdminNet network 10.10.50.0 255.255.255.0
  access-rule from WAN to WAN action deny service HTTPS-Management source any
  access-rule from WAN to WAN action allow service HTTPS-Management source AdminNet
commit