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Critical VoIP Vulnerability in Grandstream GXP1600 Series (CVE-2026-2329)

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CVE-2026-2329 Remote Code Execution VoIP Phone Call Interception

Overview

A critical stack-based buffer overflow vulnerability exists in the Grandstream GXP1600 series VoIP phones' web-based API endpoint that enables unauthenticated remote code execution with root privileges. By sending a crafted HTTP request to the /cgi-bin/api.values.get interface without requiring credentials, attackers can trigger a buffer overflow and execute arbitrary code. With root access, attackers can extract local and SIP credentials, reconfigure the device's SIP settings to redirect through a malicious proxy, and silently intercept calls while the phone continues to display and function normally. This enables stealthy eavesdropping on confidential business, government, and personal communications.

Technical Specifications

Attribute Details
CVE ID CVE-2026-2329
Vulnerability Type Unauthenticated stack-based buffer overflow
CVSS Score 9.3 (Critical)
Product Grandstream GXP1600 series VoIP phones
Affected Component Web API endpoint /cgi-bin/api.values.get
Attack Vector Network (HTTP/HTTPS)
Authentication Required None
User Interaction None
Privileges Required None
Impact Remote Code Execution with root privileges
Affected Firmware ≤ v1.0.7.79
Patched Firmware ≥ v1.0.7.81

Affected Products

  • Grandstream GXP1600 series VoIP phones (all variants)
  • Firmware versions ≤ v1.0.7.79
  • Any device accessible via HTTP/HTTPS web interface
  • SIP/VoIP infrastructure using these phones
  • Status: Actively exploitable until firmware updated to v1.0.7.81+

Technical Details

Vulnerability Characteristics

  • Type: Stack-based buffer overflow
  • Attack Vector: HTTP POST/GET request
  • Endpoint: /cgi-bin/api.values.get web API
  • Authentication: None required (unauthenticated access)
  • Root Cause: Insufficient input validation and buffer bounds checking
  • Exploitation: No user interaction required

Vulnerable Endpoint

GET /cgi-bin/api.values.get?param=[MALICIOUS_PAYLOAD] HTTP/1.1
Host: target-phone-ip:port
Connection: close

The API endpoint processes parameters without proper bounds checking, allowing crafted payloads to overflow stack buffers.

Exploitation Technique

  1. Buffer Overflow Trigger: Crafted HTTP request with oversized parameter value
  2. Stack Memory Corruption: Payload overflows stack buffer, corrupting return addresses
  3. Code Execution Method: ROP chain or shellcode execution via corrupted return pointer
  4. Privilege Level: Execution occurs with root/system privileges (VoIP service account)
  5. No Constraints: No ASLR, DEP, or other modern protections on target device

Post-Exploitation Capabilities

  • Credential Extraction: Extract SIP credentials, local admin passwords from device memory
  • Configuration Modification: Alter SIP proxy settings and routing
  • Silent Reconfiguration: Changes made without user notification or alerting
  • Persistent Access: Install backdoors or persistence mechanisms
  • Call Interception: Redirect VoIP traffic through attacker infrastructure

Affected Firmware

  • Vulnerable: All firmware versions up to and including v1.0.7.79
  • Patched: Firmware version v1.0.7.81 and later
  • Status: Fix includes input validation and buffer overflow mitigation

Attack Scenario

  1. Target Discovery:

    • Attacker identifies Grandstream GXP1600 series phones on network
    • Scans for exposed web interfaces on VoIP phones
    • Identifies firmware versions vulnerable to CVE-2026-2329
    • May use public disclosure or mass scanning techniques

  2. Buffer Overflow Exploitation:

    • Attacker sends crafted HTTP request to /cgi-bin/api.values.get endpoint
    • Request contains oversized parameter designed to overflow stack buffer
    • Buffer overflow corrupts return address on stack
    • Code execution achieved with root/system privileges

  3. Credential Extraction:

    • Attacker executes code to extract stored credentials from device memory
    • Recovers local administrative passwords
    • Harvests SIP account credentials (username, password, authentication keys)
    • Captures device configuration including network settings

  4. SIP Configuration Hijacking:

    • Attacker modifies device's SIP proxy settings
    • Changes SIP registrar to attacker-controlled proxy server
    • Reconfiguration performed silently without user notification
    • Device continues to function normally (no error messages or alerts)

  5. Call Interception & Eavesdropping:

    • Incoming calls routed through attacker's proxy infrastructure
    • Attacker can record or monitor VoIP communications in real-time
    • Phone displays and operates normally—user unaware of interception
    • Calls appear to function as expected while being silently monitored

  6. Stealthy Persistence:

    • Attacker establishes persistent backdoor on compromised phone
    • Phone used as pivot point for internal network reconnaissance
    • Potential for lateral movement to other VoIP infrastructure
    • Minimal detection indicators from user perspective

Impact Assessment

  • Silent interception of all VoIP calls on compromised phone
  • Real-time eavesdropping on confidential business communications
  • Exposure of strategic decisions, negotiations, and contracts
  • Privacy violation for sensitive personal conversations
  • Extraction of SIP account credentials
  • Local administrative password theft
  • Potential for account impersonation and toll fraud
  • Lateral movement using stolen credentials
  • Access to additional VoIP infrastructure using compromised credentials
  • Stealthy network foothold in VoIP infrastructure
  • Undetectable call interception while device functions normally
  • Risk of business espionage and competitive intelligence theft
  • Regulatory and compliance exposure (call recording, privacy)
  • Reputational damage from communication breach
  • Potential legal liability for compromised communications
  • Compromised phones used as internal network pivot points
  • Potential for lateral movement to other organizational systems
  • VoIP infrastructure integrity undermined
  • Risk of further compromise and data exfiltration
  • Long-term persistent access via backdoor mechanisms

Mitigation Strategies

Immediate Actions

  • Firmware Update: Update all Grandstream GXP1600 phones to firmware v1.0.7.81 or later immediately
  • Inventory & Assessment: Audit all GXP1600 devices to identify current firmware versions
  • Prioritize Patching: Focus on phones in sensitive environments (executive, government, healthcare)
  • Temporary Isolation: If patching cannot be done immediately, restrict network access to phones
  • Credential Rotation: Reset SIP credentials and local admin passwords post-patch

Network Hardening

  • Restrict Web Access: Disable or restrict access to the web management interface
  • Firewall Rules: Block direct public internet access to VoIP phones
  • Network Segmentation: Place all VoIP phones on dedicated, isolated VLAN
  • Internal Access Control: Limit VoIP management access to specific subnets/admin networks
  • Disable Unused Services: Turn off unused web UI and API services where possible

Access Control & Authentication

  • Strong Credentials: Use strong, unique passwords for all VoIP phone admin accounts
  • MFA Implementation: Enable multi-factor authentication on phone management where available
  • Access Logging: Enable comprehensive logging of all administrative access
  • Principle of Least Privilege: Restrict admin access to only necessary personnel
  • Regular Audits: Audit and revoke unnecessary administrative access

Detection & Monitoring

  • API Traffic Monitoring: Monitor for suspicious HTTP requests to /cgi-bin/api.values.get endpoint
  • Buffer Overflow Detection: Alert on requests with unusually large parameter values
  • Configuration Change Monitoring: Track and alert on unexpected SIP configuration modifications
  • Credential Change Detection: Monitor for unauthorized credential modifications
  • Call Pattern Anomalies: Detect unusual call routing or proxy redirection changes

Resources and References

Incident Reports

Last Updated: February 23, 2026