Grandstream VoIP Phones Hit by Critical, Unauthenticated Hack – What You Need to Know
A serious security flaw has been discovered in Grandstream GXP1600 series VoIP phones, allowing attackers to gain complete control of vulnerable devices without needing a username or password. The vulnerability, tracked as CVE-2026-2329, poses a significant risk to businesses, schools, and other organizations relying on these phones for communication.
The Vulnerability: A Stack Overflow with Serious Consequences
The core of the problem lies in a stack-based buffer overflow within the phones’ web-based API service (/cgi-bin/api.values.get). This service, accessible by default, doesn’t properly check the length of data it receives. Attackers can exploit this by sending a specially crafted request with excessive data, overwriting critical memory areas and gaining root-level access to the device. Rapid7 researchers discovered the vulnerability on January 6, 2026.
Which Grandstream Phones Are Affected?
The following Grandstream GXP1600 series models are vulnerable if running firmware versions prior to 1.0.7.81:
- GXP1610
- GXP1615
- GXP1620
- GXP1625
- GXP1628
- GXP1630
This impacts a wide range of users, as the GXP product line is popular among small and medium-sized businesses, educational institutions, hotels, and Internet Telephony Service Providers (ITSPs).
What Can Attackers Do With This Access?
Successful exploitation of CVE-2026-2329 grants attackers a frightening level of control. They can:
- Execute arbitrary commands on the device.
- Steal sensitive credentials, including local user accounts and SIP account details.
- Reconfigure the phone to redirect calls through a malicious SIP proxy, enabling eavesdropping on communications.
Even devices not directly exposed to the internet are at risk, as attackers can pivot through other compromised systems on the network.
How Was the Vulnerability Exploited?
Rapid7 researchers developed a Metasploit module to demonstrate the exploit. The challenge was bypassing a restriction that only allowed writing a single null byte during the overflow. They overcame this by using multiple colon-separated identifiers in the request, triggering the overflow repeatedly and writing multiple null bytes to construct a return-oriented programming (ROP) chain.
Source: Rapid7
The Patch and What You Need to Do
Grandstream released firmware version 1.0.7.81 on February 3, 2026, to address this critical vulnerability. It is crucial that all users of affected Grandstream GXP1600 series phones update their firmware immediately.
Beyond Grandstream: The Growing Threat to VoIP Security
This incident highlights a broader trend: the increasing vulnerability of VoIP systems. As more organizations rely on VoIP for critical communications, they become attractive targets for attackers. The ease with which this unauthenticated access was achieved underscores the need for robust security practices, including regular firmware updates, strong password policies, and network segmentation.
Did you know? VoIP systems are increasingly targeted by cybercriminals due to the sensitive information they handle, including call recordings and user credentials.
FAQ
Q: What is a stack-based buffer overflow?
A: It’s a type of security vulnerability where a program writes more data to a memory buffer than it can hold, overwriting adjacent memory and potentially allowing attackers to execute malicious code.
Q: Is my phone system safe if it’s behind a firewall?
A: While a firewall adds a layer of security, it doesn’t guarantee protection. Attackers can still potentially pivot through other compromised systems on your network to reach vulnerable devices.
Q: Where can I find the latest firmware for my Grandstream phone?
A: Visit the Grandstream support website: https://www.grandstream.com/support/firmware
Q: What is a Metasploit module?
A: Metasploit is a penetration testing framework. Modules within Metasploit are pre-built tools that exploit known vulnerabilities, like CVE-2026-2329.
Pro Tip: Regularly check your VoIP phone manufacturer’s website for security updates and apply them promptly.
Stay informed about the latest security threats and capture proactive steps to protect your communication systems. Explore additional resources on VoIP security best practices to ensure your organization remains resilient against evolving cyberattacks.
