CVE-2022-32548: Draytek Vigor3910 Buffer Overflow Flaw

CVE-2022-32548 Overview

CVE-2022-32548 is a critical buffer overflow vulnerability affecting the web management interface of numerous DrayTek Vigor router models. The vulnerability exists in the /cgi-bin/wlogin.cgi endpoint, which processes authentication requests. By sending specially crafted username or password data to the aa or ab fields, an unauthenticated remote attacker can trigger a buffer overflow condition that may lead to complete device compromise.

This vulnerability is particularly dangerous because it requires no authentication and can be exploited remotely over the network. DrayTek Vigor routers are widely deployed in small and medium business (SMB) environments, making this a significant threat to organizations relying on these devices for network connectivity and security.

Critical Impact
Unauthenticated remote attackers can achieve complete device compromise, potentially leading to remote code execution, data theft, network interception, and using the router as a pivot point for further attacks on internal networks.

Affected Products

DrayTek Vigor3910 (firmware versions before 4.3.1.1)
DrayTek Vigor2962 and Vigor2962P series
DrayTek Vigor2927 series (including AX, AC, VAC, L, LAC variants)
DrayTek Vigor2926 series (including N, AC, VAC, L, LN, LAC variants)
DrayTek Vigor2865 and Vigor2866 series
DrayTek Vigor2862 series
DrayTek Vigor2765 and Vigor2766 series
DrayTek Vigor2762 series
DrayTek Vigor2133 and Vigor2135 series
DrayTek Vigor1000B, Vigor165, Vigor166, Vigor2620L, Vigor2832, Vigor2915, Vigor2952, Vigor3220, and VigorLTE 200n

Discovery Timeline

August 29, 2022 - CVE-2022-32548 published to NVD
November 21, 2024 - Last updated in NVD database

Technical Details for CVE-2022-32548

Vulnerability Analysis

The vulnerability resides in the login processing mechanism of DrayTek Vigor routers' web management interface. When a user submits authentication credentials through the /cgi-bin/wlogin.cgi endpoint, the CGI handler processes the username and password values from the aa and ab form fields respectively. Due to insufficient bounds checking, overly long input values can overflow a fixed-size stack buffer, corrupting adjacent memory and potentially overwriting the return address on the stack.

This classic stack-based buffer overflow vulnerability allows attackers to hijack program execution flow. Since the web management interface often runs with elevated privileges on embedded devices, successful exploitation can grant attackers root-level access to the router. The attack can be executed without any prior authentication, requiring only network access to the device's management interface, whether exposed on the LAN or, in misconfigured environments, directly to the internet.

Root Cause

The root cause is a classic CWE-120 (Buffer Copy without Checking Size of Input) vulnerability. The wlogin.cgi binary uses fixed-size buffers to store username and password data but fails to validate that the incoming data does not exceed these buffer boundaries before copying it into memory. This allows attackers to supply input exceeding the allocated buffer size, resulting in adjacent memory corruption.

Attack Vector

The attack can be performed remotely over the network. An attacker crafts a malicious HTTP POST request to the /cgi-bin/wlogin.cgi endpoint containing an oversized payload in either the aa (username) or ab (password) field. When the vulnerable CGI handler processes this request, the buffer overflow occurs during the copy operation of the user-supplied data.

The exploitation scenario involves:

Identifying a vulnerable DrayTek Vigor router accessible via its web management interface
Crafting an HTTP POST request with an oversized aa or ab parameter
Sending the malicious request to trigger the buffer overflow
Achieving code execution or denial of service depending on the payload construction

Technical analysis from Trellix demonstrates that this vulnerability enables remote code execution without authentication. For detailed technical information, see the Trellix RCE Vulnerability Report.

Detection Methods for CVE-2022-32548

Indicators of Compromise

Unusual HTTP POST requests to /cgi-bin/wlogin.cgi with abnormally large payload sizes in the aa or ab parameters
Router crashes, unexpected reboots, or instability following network activity on the management interface
Unauthorized configuration changes or new administrative accounts appearing on the device
Unexpected outbound connections from the router to unknown IP addresses

Detection Strategies

Monitor web server logs for HTTP POST requests to /cgi-bin/wlogin.cgi containing payloads exceeding normal credential lengths (typically over 256 bytes)
Deploy network intrusion detection systems (IDS) with rules to detect buffer overflow patterns in HTTP traffic destined for DrayTek management interfaces
Implement firmware version auditing across all network devices to identify unpatched DrayTek routers
Use vulnerability scanners to identify exposed DrayTek management interfaces and their firmware versions

Monitoring Recommendations

Enable and centralize logging from DrayTek devices to a SIEM for anomaly detection
Set up alerts for management interface access from unexpected source IP addresses or geographic locations
Monitor for signs of lateral movement or reconnaissance activity originating from router IP addresses
Regularly audit device configurations to detect unauthorized changes that may indicate prior compromise

How to Mitigate CVE-2022-32548

Immediate Actions Required

Update all affected DrayTek Vigor routers to firmware versions released after July 2022 (Vigor3910 requires version 4.3.1.1 or later)
Disable remote management access to the web interface if not strictly required
Restrict management interface access to trusted IP addresses using access control lists (ACLs)
Place management interfaces on isolated network segments inaccessible from untrusted networks

Patch Information

DrayTek released patched firmware versions in July 2022 addressing this vulnerability. For the Vigor3910, firmware version 4.3.1.1 or later contains the fix. Administrators should visit the official DrayTek support website to download the latest firmware for their specific router model. The firmware update addresses the buffer overflow by implementing proper input length validation for the authentication parameters.

For additional context on the vulnerability's impact on SMBs, refer to the SecurityWeek Vulnerability Analysis.

Workarounds

If immediate patching is not possible, disable the web-based management interface entirely and use alternative management methods such as SSH or serial console
Configure firewall rules to block external access to TCP ports 80 and 443 on the router's management interface
Enable strong access control lists restricting management interface access to specific trusted administrator IP addresses only
Consider deploying a VPN requirement for any remote management access to network infrastructure devices

bash

# Example: Restrict management access via firewall (implementation varies by environment)
# Block external access to router management ports
iptables -A INPUT -p tcp --dport 80 -s ! 192.168.1.0/24 -j DROP
iptables -A INPUT -p tcp --dport 443 -s ! 192.168.1.0/24 -j DROP