CVE-2025-7921: Askey Modem Stack Buffer Overflow RCE Flaw

CVE-2025-7921 Overview

Certain modem models developed by Askey contain a Stack-based Buffer Overflow vulnerability (CWE-121) that allows unauthenticated remote attackers to control the program's execution flow and potentially execute arbitrary code. This critical firmware vulnerability in network infrastructure devices poses significant risks to organizations and consumers relying on affected Askey modems for internet connectivity.

Critical Impact

Unauthenticated remote attackers can exploit this stack-based buffer overflow to gain complete control over affected Askey modem devices, potentially executing arbitrary code without any user interaction or authentication requirements.

Affected Products

• Certain Askey modem models (specific models not disclosed)
• Network infrastructure devices running vulnerable Askey firmware

Discovery Timeline

• July 21, 2025 - CVE-2025-7921 published to NVD
• July 22, 2025 - Last updated in NVD database

Technical Details for CVE-2025-7921

Vulnerability Analysis

This vulnerability is classified as CWE-121 (Stack-based Buffer Overflow), a memory corruption issue where data written to a stack-allocated buffer exceeds its allocated size, overwriting adjacent memory on the stack. In the context of Askey modems, this flaw enables attackers to corrupt critical stack data structures including saved return addresses and function pointers.

The attack can be conducted over the network without requiring any authentication, making it particularly dangerous for internet-facing devices. Successful exploitation grants attackers complete control over the device's execution flow, allowing them to redirect program execution to attacker-controlled code.

Root Cause

The vulnerability stems from improper bounds checking when processing network input on affected Askey modem firmware. When the modem receives specially crafted input data, the application fails to validate the length of the data before copying it into a fixed-size stack buffer. This allows an attacker to write beyond the buffer boundaries, corrupting adjacent stack memory including return addresses and other critical control structures.

Attack Vector

This is a network-based attack vector that requires no authentication or user interaction. An attacker can remotely send malicious network packets to vulnerable Askey modems. The attack exploits the stack-based buffer overflow by:

1. Sending oversized input data to a vulnerable network service on the modem
2. Overflowing the stack buffer to overwrite the saved return address
3. Redirecting program execution to attacker-controlled shellcode or gadgets
4. Achieving arbitrary code execution with the privileges of the vulnerable process

The vulnerability mechanism involves improper input validation in the modem's network-facing services. When processing incoming data, the firmware fails to properly check buffer boundaries before memory copy operations, allowing attackers to corrupt the execution stack. For detailed technical information, refer to the TW CERT Security Advisory.

Detection Methods for CVE-2025-7921

Indicators of Compromise

• Unexpected network traffic patterns to/from Askey modem devices on non-standard ports
• Modem device crashes or unexpected reboots indicating exploitation attempts
• Anomalous outbound connections from modem devices to unknown external IP addresses
• Modified firmware checksums or unexpected configuration changes on modem devices

Detection Strategies

• Deploy network intrusion detection systems (NIDS) to monitor for oversized or malformed packets targeting modem management interfaces
• Implement network segmentation to isolate modem devices and monitor traffic crossing segment boundaries
• Use SentinelOne Singularity Platform to detect anomalous network behavior and potential exploitation attempts targeting embedded devices
• Monitor for unusual memory access patterns or process crashes on modem devices through centralized logging

Monitoring Recommendations

• Enable comprehensive logging on network perimeter devices to capture traffic to/from Askey modems
• Implement real-time alerting for connection attempts to modem management interfaces from untrusted sources
• Deploy honeypots mimicking vulnerable Askey modem configurations to detect active exploitation attempts
• Regularly audit modem firmware versions and compare against vendor security advisories

How to Mitigate CVE-2025-7921

Immediate Actions Required

• Restrict network access to Askey modem management interfaces using firewall rules and access control lists
• Disable remote management features on affected modems until patches are available
• Implement network segmentation to isolate vulnerable modem devices from critical infrastructure
• Contact Askey or your ISP to inquire about firmware updates addressing this vulnerability

Patch Information

Organizations should consult the TW CERT Security Advisory and TW CERT Incident Report for the latest patch information from Askey. Given the critical severity of this vulnerability, immediate coordination with your device vendor or ISP is strongly recommended to obtain and apply security updates as soon as they become available.

Workarounds

• Place affected modems behind a properly configured firewall that blocks untrusted inbound connections to management interfaces
• Disable any unnecessary network services running on the modem to reduce the attack surface
• Use VPN or other secure tunneling solutions to protect modem management traffic when remote access is required
• Consider replacing affected devices with models from vendors who have confirmed patches available

# Example firewall rules to restrict modem access (adjust IPs and ports as needed)
# Block external access to common modem management ports
iptables -A INPUT -p tcp --dport 80 -s ! 192.168.1.0/24 -j DROP
iptables -A INPUT -p tcp --dport 443 -s ! 192.168.1.0/24 -j DROP
iptables -A INPUT -p tcp --dport 8080 -s ! 192.168.1.0/24 -j DROP
# Log blocked connection attempts for monitoring
iptables -A INPUT -p tcp --dport 80 -j LOG --log-prefix "Modem-Access-Blocked: "