CVE-2025-7912: Totolink T6 Buffer Overflow Vulnerability

CVE-2025-7912 Overview

CVE-2025-7912 is a buffer overflow vulnerability in the TOTOLINK T6 router firmware version 4.1.5cu.748_B20211015. The flaw resides in the recvSlaveUpgstatus function within the MQTT Service component. Attackers can trigger the overflow by manipulating the s argument, leading to memory corruption on the affected device. The vulnerability is remotely exploitable and a public proof-of-concept has been disclosed, increasing risk for exposed devices. The weakness is categorized under [CWE-119], improper restriction of operations within the bounds of a memory buffer.

Critical Impact
Remote attackers can corrupt memory in the MQTT Service to potentially execute arbitrary code or crash the router, compromising network integrity.

Affected Products

TOTOLINK T6 router (hardware revision 3)
TOTOLINK T6 firmware version v4.1.5cu.748_b20211015
MQTT Service component handling recvSlaveUpgstatus

Discovery Timeline

2025-07-20 - CVE-2025-7912 published to NVD
2025-07-23 - Last updated in NVD database

Technical Details for CVE-2025-7912

Vulnerability Analysis

The vulnerability exists in the recvSlaveUpgstatus function of the MQTT Service running on TOTOLINK T6 devices. The function processes the s parameter without enforcing proper bounds checks on its length or content. When an attacker supplies oversized input, the data overflows a fixed-size buffer in memory. This memory corruption can overwrite adjacent stack or heap structures, including return addresses and function pointers.

The MQTT (Message Queuing Telemetry Transport) Service typically listens on a network-accessible port to coordinate device upgrades and telemetry. Because the attack vector is network-based with low complexity, an authenticated attacker on the same network segment can deliver the malicious payload directly to the service.

Root Cause

The root cause is the absence of input length validation in recvSlaveUpgstatus before copying the s argument into a fixed-size buffer. This classic [CWE-119] pattern is common in embedded firmware where developers use unsafe string handling primitives such as strcpy or sprintf without size constraints. The MQTT service trusts that incoming message fields conform to expected lengths.

Attack Vector

An attacker with network access to the MQTT Service sends a crafted message containing an oversized s parameter targeting recvSlaveUpgstatus. The overflow corrupts adjacent memory, which can be leveraged to crash the service or hijack execution flow. The publicly disclosed proof-of-concept reduces the skill required to weaponize the flaw. See the GitHub PoC Repository and VulDB Entry #317027 for technical details.

Detection Methods for CVE-2025-7912

Indicators of Compromise

Unexpected restarts or crashes of the MQTT Service process on TOTOLINK T6 devices
Abnormally large or malformed MQTT messages targeting the slave upgrade status handler
Unusual outbound connections originating from the router following inbound MQTT traffic
Configuration changes or new administrative accounts appearing without authorization

Detection Strategies

Inspect MQTT traffic for oversized payloads in fields handled by recvSlaveUpgstatus
Monitor router system logs for segmentation faults or service watchdog restarts
Deploy network IDS signatures that flag malformed MQTT control packets targeting embedded devices
Baseline normal MQTT traffic patterns and alert on deviations in message length distributions

Monitoring Recommendations

Capture and review firmware version inventory to identify exposed TOTOLINK T6 units
Centralize router logs in a SIEM and alert on repeated MQTT Service failures
Monitor for scanning activity probing the MQTT port range on the WAN interface
Track threat intelligence feeds for active exploitation reports referencing CVE-2025-7912

How to Mitigate CVE-2025-7912

Immediate Actions Required

Block external access to the MQTT Service port on the WAN interface using upstream firewall rules
Restrict LAN-side access to the MQTT Service to known management hosts only
Disable the MQTT Service entirely if it is not required for device operation
Inventory all TOTOLINK T6 devices running firmware 4.1.5cu.748_B20211015 and prioritize them for remediation

Patch Information

No official vendor patch is referenced in the current advisory data. Administrators should monitor the Totolink Official Website for firmware updates addressing CVE-2025-7912. Until a fixed firmware release is available, network-level mitigations are required.

Workarounds

Place affected routers behind a perimeter firewall that denies inbound MQTT traffic
Segment IoT and management networks to limit lateral access to the MQTT Service
Replace the affected device with a supported router if the vendor does not release a patch
Apply strict ACLs on the router management plane to limit administrative reachability

bash

# Example iptables rule to block external MQTT traffic on the router uplink
iptables -A INPUT -i wan0 -p tcp --dport 1883 -j DROP
iptables -A INPUT -i wan0 -p tcp --dport 8883 -j DROP