Software Engineering Institute

Overview

Multiple Netgear devices contain a stack buffer overflow in the httpd web server's handling of upgrade_check.cgi, which may allow for unauthenticated remote code execution with root privileges.

Description

Many Netgear devices contain an embedded web server, which is provided by the httpd process, to provide administrative capabilities. On multiple Netgear devices, this code fails to properly validate the header size provided to the upgrade_check.cgi handler. Despite copying the header to a fixed-size buffer on the stack, the vulnerable code copies an attacker-provided count of bytes from attacker-provided data. This allows for remote code execution by way of stack buffer overflow. This vulnerability is exacerbated by a number of issues:

1. The httpd process runs with root privileges.
2. Stack cookies, which can help prevent exploitation of stack buffer overflows, are not universally used in Netgear devices.
3. Authentication is not required to reach the vulnerable code.
4. The vulnerability occurs before Cross-Site Request Forgery (CSRF) token checking occurs.
5. Target device fingerprinting can occur by visiting the /currentsetting.htm page on an affected device.

Exploit code that targets 79 different Netgear devices is publicly available.

Impact

By convincing a user to visit a malicious or compromised website, a remote, unauthenticated attacker may be able to execute arbitrary code on a vulnerable device with root privileges.

Solution

Apply an update

Netgear has provided updates for several vulnerable devices. Note that Netgear does not indicate when devices have reached an end of life (EOL) state. This may make it difficult to determine if a vulnerable device may receive an update in the future.

The CERT/CC has made a spreadsheet to more clearly indicate which devices have updates, and which devices may either be receiving an update in the future, or may possibly be unsupported.

As outlined in the blog post It's Time to Retire Your Unsupported Things, you should factor the vendor's support life span into purchasing decisions. Vendors that indicate how long products will be supported should be preferred over those that do not clearly indicate how long a device will be supported. Similarly, vendors that clearly indicate when a product has reached EOL state should be preferred over vendors that do not.

Acknowledgements

This vulnerability was publicly disclosed by ZDI, who in turn credit d4rkn3ss from VNPT ISC. Additional analysis was provided by GRIMM.

This document was written by Will Dormann.

Vendor Information

References

• https://www.zerodayinitiative.com/advisories/ZDI-20-712/
• https://blog.grimm-co.com/2020/06/soho-device-exploitation.html
• https://github.com/grimm-co/NotQuite0DayFriday/tree/master/2020.06.15-netgear
• https://docs.google.com/spreadsheets/d/1Tzq97rRisoZwKNQ1pUYE6phwl4LL7KnZxc828n-hXW0/