CVE-2022-31793: Muhttpd Path Traversal Vulnerability

CVE-2022-31793 Overview

CVE-2022-31793 is a path traversal vulnerability in the do_request function within request.c of muhttpd before version 1.1.7. This flaw allows remote attackers to read arbitrary files from the filesystem by constructing a malicious URL with a single character prepended to the desired path. The vulnerability stems from a critical coding error where the HTTP server implementation skips over the first character when serving files, effectively bypassing path validation mechanisms.

This vulnerability has significant real-world impact as muhttpd is embedded in numerous widely-deployed Arris network devices, including consumer and ISP-provided routers. The affected devices include Arris NVG443, NVG599, NVG589, NVG510, as well as Arris-derived BGW210 and BGW320 devices commonly used by major telecommunications providers.

Critical Impact

Remote attackers can read sensitive configuration files, credentials, and other confidential data from millions of vulnerable Arris routers without authentication, potentially exposing entire networks to further compromise.

Affected Products

• Inglorion muhttpd (versions before 1.1.7)
• Arris NVG443 (firmware)
• Arris NVG599 (firmware)
• Arris NVG589 (firmware)
• Arris NVG510 (firmware)
• Arris BGW210 (firmware)
• Arris BGW320 (firmware)

Discovery Timeline

• 2022-08-04 - CVE-2022-31793 published to NVD
• 2024-11-21 - Last updated in NVD database

Technical Details for CVE-2022-31793

Vulnerability Analysis

The vulnerability resides in the do_request function within request.c of the muhttpd lightweight HTTP server. When processing incoming HTTP requests, the server incorrectly skips the first character of the requested path before serving files. This off-by-one error in path handling creates a trivial path traversal condition that allows attackers to escape the web root directory.

By prepending any single character (such as / or .) to a relative path like ../etc/passwd, an attacker can effectively bypass the server's path normalization logic. The server strips the first character, leaving a path that traverses outside the intended web root and into the underlying filesystem.

This vulnerability is particularly severe because muhttpd is typically embedded in router firmware with root-level access to the filesystem. Sensitive files such as /etc/passwd, /etc/shadow, configuration files containing WiFi credentials, VPN configurations, and administrative passwords become accessible to unauthenticated remote attackers.

Root Cause

The root cause is an off-by-one programming error in the do_request function within request.c. The code incorrectly increments a pointer or index before processing the requested file path, causing the first character to be skipped during path resolution. This fundamentally breaks the path validation and sanitization logic that would otherwise prevent directory traversal attacks.

The vulnerability is classified under CWE-22 (Improper Limitation of a Pathname to a Restricted Directory), commonly known as "Path Traversal." This class of vulnerability occurs when user-controllable input is used to construct file paths without proper validation, allowing attackers to access files outside the intended directory structure.

Attack Vector

The attack is network-based and requires no authentication or user interaction. An attacker can exploit this vulnerability remotely over the network by sending a specially crafted HTTP GET request to the vulnerable muhttpd server.

The exploitation is straightforward: an attacker constructs a URL where the path begins with any arbitrary character followed by a directory traversal sequence. For example, a request to GET /x../../../etc/passwd HTTP/1.1 would result in the server stripping the leading x character, then attempting to serve ../../../etc/passwd relative to the web root, ultimately returning the contents of /etc/passwd.

This attack can be performed using standard tools like curl or a web browser, making it trivially exploitable at scale. The Malwarebytes Vulnerability Analysis notes that millions of Arris routers deployed by ISPs are potentially vulnerable.

Detection Methods for CVE-2022-31793

Indicators of Compromise

• HTTP access logs showing requests with unusual path patterns containing single-character prefixes followed by ../ sequences
• Requests targeting sensitive system files such as /etc/passwd, /etc/shadow, or device-specific configuration paths
• Network traffic showing HTTP responses containing system file contents from router management interfaces
• Unexpected external connections to router web interfaces on ports 80, 443, or custom management ports

Detection Strategies

• Deploy network intrusion detection rules to identify HTTP requests containing path traversal patterns with single-character prefixes (e.g., /x../, /.../, /a../)
• Monitor HTTP response sizes from router management interfaces for anomalously large responses that may indicate file disclosure
• Implement web application firewall (WAF) rules to block requests containing ../ sequences regardless of prefix characters
• Enable verbose logging on network perimeter devices to capture and analyze suspicious HTTP request patterns

Monitoring Recommendations

• Configure SIEM correlation rules to alert on multiple path traversal attempts against router management interfaces
• Establish baseline network behavior for router management traffic and alert on deviations
• Monitor for reconnaissance activity targeting router web interfaces from external IP addresses
• Review access logs from router management interfaces regularly for signs of exploitation attempts

How to Mitigate CVE-2022-31793

Immediate Actions Required

• Update muhttpd to version 1.1.7 or later where the vulnerability has been patched
• Contact your ISP or router vendor to determine if firmware updates are available for affected Arris devices
• Restrict access to router management interfaces to trusted internal networks only
• Implement network segmentation to limit exposure of vulnerable devices to the internet
• Deploy firewall rules to block external access to router HTTP/HTTPS management ports

Patch Information

The muhttpd project has addressed this vulnerability in version 1.1.7. Users running the standalone muhttpd server should upgrade to this version or later immediately. Information about the muhttpd project and updates is available from the Inglorion Software Overview.

For Arris router users, firmware updates must be obtained from Arris or the ISP providing the device. The CERT Vulnerability Notification #495801 provides additional guidance and coordination information. Users of ISP-provided equipment (such as AT&T customers with BGW210 or BGW320 gateways) should contact their service provider to request patched firmware.

The Derek Abdine Advisory on Arris provides detailed technical information about the vulnerability and affected device versions.

Workarounds

• Disable the web management interface on affected routers if not required for operations
• Place affected devices behind a firewall that blocks external access to HTTP/HTTPS management ports
• Use VPN or other secure access methods to reach router management interfaces rather than exposing them directly
• If possible, replace vulnerable devices with equipment running patched or unaffected firmware

# Example firewall rules to restrict access to router management interface
# Block external access to common management ports (adjust for your environment)
iptables -A INPUT -i eth0 -p tcp --dport 80 -j DROP
iptables -A INPUT -i eth0 -p tcp --dport 443 -j DROP
iptables -A INPUT -i eth0 -p tcp --dport 8080 -j DROP

# Allow management access only from trusted internal subnet
iptables -A INPUT -s 192.168.1.0/24 -p tcp --dport 80 -j ACCEPT
iptables -A INPUT -s 192.168.1.0/24 -p tcp --dport 443 -j ACCEPT