SonicWall SonicOS Buffer Overflow Vulnerability (CVE-2020-5135) Technical Advisory
Executive & SecOps Summary
SECOPS ASSESSMENTSonicWall firewall appliances running SonicOS Gen 6 (prior to 6.5.4.7, 6.5.1.12, 6.0.5.3), SonicOSv (prior to 6.5.4.v), and Gen 7 (prior to 7.0.0.0)
An unauthenticated remote attacker can trigger a Denial of Service (DoS) or potentially execute arbitrary code with root privileges by sending a specially crafted HTTP request to the firewall
Immediately upgrade to the patched firmware versions provided by SonicWall and restrict external access to the management interface
Affected Products & Version Scope
Remediation & Live Workaround Commands
PATCH & WORKAROUND1. Official Patch & Version Upgrade Commands
Apply the latest firmware updates immediately via the MySonicWall portal. Ensure you back up your current configuration before proceeding with the upgrade.
# Verify current SonicOS version and prepare for upgrade via CLI
show version
# Export current configuration to a secure backup server
export current-config preferences ftp ftp://backup_server/sonicos_config_backup.exp2. Temporary Workaround & Hardening Configuration
If patching cannot be applied immediately, disable HTTP/HTTPS management access on all WAN-facing interfaces. Restrict management access exclusively to trusted internal LAN segments or dedicated management VPNs.
# Disable HTTP and HTTPS management access on the WAN interface (e.g., X1)
configure terminal
interface X1
no management-access http
no management-access https
commit
exit3. Log Audit & IoC Verification Commands
Monitor firewall system logs and web server logs for unexpected crashes, daemon restarts, or abnormally long HTTP request patterns.
# Search local logs for web management service crashes or unexpected restarts
show log view | grep -E 'Web management|restarted|crash|overflow'Stop Exposing VPN Ports to the Internet — Switch to Zero-Trust Access
Eliminate public-facing VPN concentrators and firewall management ports with identity-first Zero-Trust Network Access.
Technical Root Cause & Exploitation Mechanics
1. Vulnerability Overview & Root Cause
This vulnerability (CVE-2020-5135) is a critical stack-based buffer overflow (CWE-120) residing in the HTTP/HTTPS management interface of SonicWall SonicOS. The root cause lies in the protocol parsing component, which fails to properly validate the length of input data before copying it into a fixed-size memory buffer. This lack of bounds checking allows an attacker to overwrite adjacent stack memory.
2. Exploitation Mechanics & Attack Scenario
An attacker exploits this vulnerability by sending a malicious HTTP request containing an oversized header or parameter to the exposed SonicOS management port (typically TCP 443 or 80). When the vulnerable parser processes this request, the stack buffer overflows, allowing the attacker to overwrite the function return address. Successful exploitation can redirect the execution flow to attacker-controlled shellcode, leading to Remote Code Execution (RCE) under the context of the root user.
3. Infrastructure & Business Impact
With a CVSS score of 9.8 (Critical) and its inclusion in the CISA Known Exploited Vulnerabilities (KEV) catalog, this vulnerability poses an extreme risk to enterprise perimeters. Successful exploitation grants full control over the firewall, enabling attackers to bypass security controls, pivot into internal networks, hijack VPN sessions, or exfiltrate sensitive data. Alternatively, triggering a DoS can disrupt all inbound and outbound network traffic, causing severe business downtime.
Official Vendor Advisories & References
A buffer overflow vulnerability in SonicOS allows a remote attacker to cause Denial of Service (DoS) and potentially execute arbitrary code by sending a malicious request to the firewall. This vulnerability affected SonicOS Gen 6 version 6.5.4.7, 6.5.1.12, 6.0.5.3, SonicOSv 6.5.4.v and Gen 7 version 7.0.0.0.
Frequently Asked SecOps Questions (CVE-2020-5135)
FAQQ1. How critical is CVE-2020-5135 and is it actively exploited in the wild?
CVE-2020-5135 is rated CVSS 9.8 (CRITICAL) with a 30-day EPSS exploitation probability of 26.9%. It is listed in the CISA Known Exploited Vulnerabilities (KEV) catalog, requiring immediate emergency remediation.
Q2. Which vendors and products are affected by CVE-2020-5135?
Primary impacted vendors include sonicwall across sonicos, sonicosv. Scope summary: SonicWall hardware and virtual firewalls running SonicOS Gen 6 (6.5.4.7, 6.5.1.12, 6.0.5.3), SonicOSv (6.5.4.v), and Gen 7 (7.0.0.0)
Q3. What temporary mitigations or workarounds exist for CVE-2020-5135 before patching?
[Mitigation] Immediately upgrade to the patched firmware versions provided by SonicWall and restrict external access to the management interface Follow the CLI remediation commands and interactive SecOps checklist on this page to restrict exposure prior to your maintenance window.
Generate an Internal Engineering Notice or CISO Report for CVE-2020-5135
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