CVE-2021-20090: Buffalo WSR Path Traversal Vulnerability

CVE-2021-20090 Overview

CVE-2021-20090 is a critical path traversal vulnerability affecting Buffalo wireless router firmware. The vulnerability exists in the web interfaces of Buffalo WSR-2533DHPL2 firmware version 1.02 and earlier, as well as WSR-2533DHP3 firmware version 1.24 and earlier. This flaw allows unauthenticated remote attackers to bypass authentication mechanisms entirely, potentially gaining full administrative control over affected devices.

Critical Impact

This vulnerability enables unauthenticated remote attackers to bypass authentication on affected Buffalo routers, potentially leading to complete device compromise, network infiltration, and malicious configuration changes. This vulnerability is listed in CISA's Known Exploited Vulnerabilities catalog, indicating active exploitation in the wild.

Affected Products

• Buffalo WSR-2533DHPL2-BK firmware version <= 1.02
• Buffalo WSR-2533DHPL2-BK hardware devices
• Buffalo WSR-2533DHP3-BK firmware version <= 1.24
• Buffalo WSR-2533DHP3-BK hardware devices

Discovery Timeline

• 2021-04-29 - CVE-2021-20090 published to NVD
• 2025-11-03 - Last updated in NVD database

Technical Details for CVE-2021-20090

Vulnerability Analysis

This path traversal vulnerability (CWE-22) resides in the web management interface of affected Buffalo routers. The vulnerability allows attackers to craft malicious HTTP requests that traverse directory paths to access restricted resources without proper authentication. By exploiting improper input validation in URL path handling, attackers can bypass authentication checks and access administrative functions.

The flaw is particularly dangerous because it requires no authentication and can be exploited remotely over the network. Attackers can leverage this vulnerability to access sensitive configuration files, modify router settings, inject malicious configurations, or pivot deeper into the network. The vulnerability has been observed under active exploitation, prompting CISA to add it to their Known Exploited Vulnerabilities catalog.

Root Cause

The root cause of CVE-2021-20090 is improper input validation in the web interface's URL path handling mechanism. The affected firmware fails to properly sanitize user-supplied path components in HTTP requests, allowing specially crafted directory traversal sequences (such as ../) to bypass authentication controls. This enables attackers to access authenticated endpoints without providing valid credentials.

Attack Vector

The attack vector for CVE-2021-20090 is network-based, requiring no privileges or user interaction. An attacker can exploit this vulnerability by sending specially crafted HTTP requests to the router's web management interface. The path traversal sequences in the request allow the attacker to navigate outside intended directories and access authentication-protected resources.

The attack flow typically involves:

1. Identifying a vulnerable Buffalo router with an exposed web management interface
2. Crafting HTTP requests containing path traversal sequences
3. Bypassing authentication checks to access administrative endpoints
4. Extracting sensitive configuration data or modifying router settings

This vulnerability is documented in Tenable Security Research TRA-2021-13 and CERT Vulnerability Report #914124. Additional context on active exploitation can be found in the SecPod Blog on Exploitation.

Detection Methods for CVE-2021-20090

Indicators of Compromise

• Unusual HTTP requests to the router's web interface containing path traversal sequences such as ../ or encoded variants
• Unexpected configuration changes on affected Buffalo routers
• Authentication log anomalies showing access to administrative functions without proper login
• Network traffic patterns indicating reconnaissance or exploitation attempts targeting router management ports

Detection Strategies

• Monitor network traffic for HTTP requests containing directory traversal patterns directed at Buffalo router management interfaces
• Implement web application firewall rules to detect and block path traversal attempts
• Review router access logs for unauthenticated access to privileged endpoints
• Deploy intrusion detection system (IDS) signatures for CVE-2021-20090 exploitation patterns
• Conduct regular vulnerability scans to identify unpatched Buffalo devices on the network

Monitoring Recommendations

• Enable verbose logging on affected Buffalo routers to capture detailed access attempts
• Configure network monitoring to alert on suspicious traffic to router management ports (typically ports 80 and 443)
• Establish baseline router configuration and monitor for unauthorized changes
• Implement network segmentation to limit exposure of router management interfaces

How to Mitigate CVE-2021-20090

Immediate Actions Required

• Update affected Buffalo routers to the latest firmware version immediately
• Restrict access to router web management interfaces to trusted internal networks only
• Implement firewall rules to block external access to router management ports
• Monitor affected devices for signs of compromise until patches can be applied
• Consider replacing end-of-life devices that cannot receive security updates

Patch Information

Buffalo has released firmware updates to address this vulnerability. Users should visit the Buffalo support website to download and install the latest firmware for their affected devices. For WSR-2533DHPL2 devices, firmware versions after 1.02 include the security fix. For WSR-2533DHP3 devices, firmware versions after 1.24 address this vulnerability.

Additional guidance is available from CISA's Known Exploited Vulnerabilities catalog entry for CVE-2021-20090.

Workarounds

• Disable remote management features on affected routers until firmware can be updated
• Place affected routers behind a firewall that blocks external access to management interfaces
• Implement network access control lists (ACLs) to limit management interface access to specific trusted IP addresses
• Consider using a VPN for remote administration instead of exposing the web interface directly

# Example: Block external access to router management interface
# Add firewall rules to restrict management port access
iptables -A INPUT -p tcp --dport 80 -s 192.168.1.0/24 -j ACCEPT
iptables -A INPUT -p tcp --dport 443 -s 192.168.1.0/24 -j ACCEPT
iptables -A INPUT -p tcp --dport 80 -j DROP
iptables -A INPUT -p tcp --dport 443 -j DROP