CVE-2021-31802: Netgear R7000 Buffer Overflow Vulnerability

CVE-2021-31802 Overview

CVE-2021-31802 is a heap-based buffer overflow vulnerability affecting NETGEAR R7000 routers running firmware version 1.0.11.116. This vulnerability is exploitable from the local network without authentication, allowing attackers to execute arbitrary code with root privileges. The flaw exists within the handling of HTTP requests to the backup.cgi endpoint, where a user-provided length value is improperly trusted during file upload operations.

Critical Impact

Unauthenticated attackers on the local network can achieve remote code execution with root privileges by exploiting improper validation of Content-Length headers in HTTP requests.

Affected Products

• NETGEAR R7000 Firmware version 1.0.11.116
• NETGEAR R7000 Hardware

Discovery Timeline

• 2021-04-26 - CVE-2021-31802 published to NVD
• 2024-11-21 - Last updated in NVD database

Technical Details for CVE-2021-31802

Vulnerability Analysis

This heap-based buffer overflow vulnerability (CWE-787: Out-of-bounds Write) stems from the router's HTTP daemon (httpd) improperly handling file upload requests to the backup.cgi endpoint. The vulnerability allows unauthenticated attackers on the adjacent network to achieve complete system compromise with root-level access.

The attack requires no user interaction and can be executed from the local network segment. Given the router's position as a network gateway device, successful exploitation grants attackers control over all traffic passing through the device, enabling man-in-the-middle attacks, DNS hijacking, and lateral movement to other networked devices.

Root Cause

The root cause of this vulnerability lies in the improper validation of user-supplied input during HTTP request processing. Specifically, the backup.cgi handler trusts the Content-Length header value provided by the user without proper bounds checking. When an attacker supplies a malformed request with a newline character (\n) before the Content-Length header, the server mishandles the length calculation, leading to a heap-based buffer overflow condition.

Attack Vector

The attack is executed from an adjacent network position, meaning the attacker must have access to the same local network as the target router. The exploitation technique involves:

1. Crafting a malicious HTTP POST request targeting the backup.cgi endpoint
2. Inserting a newline character (\n) before the Content-Length header
3. Providing a manipulated length value that triggers the heap overflow
4. Embedding shellcode or ROP chain to achieve code execution

The vulnerability does not require any authentication, making it particularly dangerous for environments where untrusted devices share the same network segment as the router. For detailed technical analysis and proof-of-concept information, refer to the SSD Disclosure Advisory.

Detection Methods for CVE-2021-31802

Indicators of Compromise

• Unusual HTTP POST requests to /backup.cgi endpoint with malformed headers
• Anomalous Content-Length header values in router access logs
• Unexpected root-level processes spawned by the httpd service
• Router configuration changes without administrator authorization

Detection Strategies

• Monitor network traffic for HTTP requests to backup.cgi containing newline characters in header fields
• Implement intrusion detection rules to identify malformed HTTP headers targeting NETGEAR devices
• Review router logs for repeated access attempts to backup or configuration endpoints
• Deploy network behavior analysis to detect anomalous router activity

Monitoring Recommendations

• Enable verbose logging on NETGEAR R7000 devices if supported
• Implement network segmentation to isolate IoT and router management interfaces
• Use network monitoring tools to baseline and alert on abnormal traffic patterns to router endpoints
• Consider deploying a dedicated IDS/IPS solution to inspect local network traffic

How to Mitigate CVE-2021-31802

Immediate Actions Required

• Update NETGEAR R7000 firmware to the latest available version from the official NETGEAR support site
• Restrict access to the router's web management interface to trusted administrative workstations only
• Implement network segmentation to limit which devices can reach the router's management interface
• Disable remote management features if not required

Patch Information

NETGEAR has addressed this vulnerability in firmware updates. Users should visit the Netgear Security Information page to download the latest firmware for the R7000 router. Always verify firmware integrity before installation and ensure the update process completes successfully.

Workarounds

• Configure firewall rules to block access to the backup.cgi endpoint from untrusted network segments
• Implement VLAN segmentation to isolate the router's management interface from general network traffic
• Use a dedicated management network for router administration
• Consider replacing end-of-life devices that no longer receive security updates