DrayTek Vigor Routers Unauthenticated Remote Code Execution Vulnerability Analysis and Mitigation Guide (CVE-2020-8515)
Executive & SecOps Summary
SECOPS ASSESSMENTDrayTek Vigor2960 (1.3.1_Beta and prior), Vigor3900 (1.4.4_Beta and prior), and Vigor300B (1.3.3_Beta, 1.4.2.1_Beta, 1.4.4_Beta and prior)
Unauthenticated remote attackers can execute arbitrary system commands with root privileges via the web management interface, leading to complete device compromise
Immediately upgrade the device firmware to version v1.5.1 or higher, and disable WAN-side web management access
Affected Products & Version Scope
Remediation & Live Workaround Commands
PATCH & WORKAROUND1. Official Patch & Version Upgrade Commands
Apply the official firmware update provided by the vendor to resolve this vulnerability.
# Download and apply the firmware update from the official DrayTek portal
# Target Version: Upgrade Vigor3900 / Vigor2960 / Vigor300B to v1.5.1 or later
# Note: Ensure to back up the current configuration before applying the update2. Temporary Workaround & Hardening Configuration
If immediate patching is not feasible, disable remote management from the WAN side and restrict access to trusted IP addresses only.
# Disable 'Allow Management from Internet' in the router's Access Control settings
# Block external access to HTTP/HTTPS management ports (default 80, 443) via firewall rules3. Log Audit & IoC Verification Commands
Inspect web server access logs for suspicious requests targeting the vulnerable CGI script.
# Search access logs for exploitation attempts containing shell metacharacters
grep -Ei 'mainfunction.cgi' /var/log/httpd/access_log | grep -E '(%3b|%60|%7c|;|`|\|)'
# Check for unauthorized active shell processes on the system
ps aux | grep -E '(sh|bash|ash) -c'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 exists in the cgi-bin/mainfunction.cgi URI of the web management interface on DrayTek Vigor routers. Due to insufficient validation and sanitization of user-supplied input, the application is susceptible to an OS Command Injection vulnerability (CWE-78) when handling shell metacharacters passed through HTTP requests.
2. Exploitation Mechanics & Attack Scenario
An unauthenticated attacker can send a specially crafted HTTP POST request to the vulnerable cgi-bin/mainfunction.cgi endpoint. By embedding shell metacharacters (such as ;, ` `, or |) within request parameters (e.g., keyPath), the attacker injects arbitrary commands. The web server executes these commands directly via system shells with root` privileges, bypassing any authentication mechanisms.
3. Infrastructure & Business Impact
With a CVSS score of 9.8 and active exploitation documented in CISA's KEV catalog, this vulnerability poses a severe threat. Successful exploitation allows attackers to establish persistent access, capture VPN traffic, manipulate routing tables, alter DNS settings, and pivot into the internal corporate network, causing catastrophic business disruption.
Official Vendor Advisories & References
DrayTek Vigor2960 1.3.1_Beta, Vigor3900 1.4.4_Beta, and Vigor300B 1.3.3_Beta, 1.4.2.1_Beta, and 1.4.4_Beta devices allow remote code execution as root (without authentication) via shell metacharacters to the cgi-bin/mainfunction.cgi URI. This issue has been fixed in Vigor3900/2960/300B v1.5.1.
Frequently Asked SecOps Questions (CVE-2020-8515)
FAQQ1. How critical is CVE-2020-8515 and is it actively exploited in the wild?
CVE-2020-8515 is rated CVSS 9.8 (CRITICAL) with a 30-day EPSS exploitation probability of 100.0%. It is listed in the CISA Known Exploited Vulnerabilities (KEV) catalog, requiring immediate emergency remediation.
Q2. Which vendors and products are affected by CVE-2020-8515?
Primary impacted vendors include draytek across vigor2960, vigor3900, vigor300b. Scope summary: DrayTek Vigor2960 (v1.3.1_Beta and prior), Vigor3900 (v1.4.4_Beta and prior), Vigor300B (v1.3.3_Beta, v1.4.2.1_Beta, v1.4.4_Beta and prior)
Q3. What temporary mitigations or workarounds exist for CVE-2020-8515 before patching?
[Mitigation] Immediately upgrade the device firmware to version v1.5.1 or higher, and disable WAN-side web management access 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-2020-8515
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