CVE-2025-53406: QNAP QTS Format String Vulnerability

CVE-2025-53406 Overview

CVE-2025-53406 is an externally-controlled format string vulnerability [CWE-134] affecting multiple versions of QNAP QTS and QuTS hero operating systems. An authenticated remote attacker holding an administrator account can exploit the flaw to read secret data from process memory or modify memory contents. QNAP disclosed the issue in advisory QSA-25-36 and released fixes in QTS 5.2.6.3195 build 20250715 and QuTS hero h5.2.6.3195 build 20250715.

Critical Impact
An authenticated administrator can trigger the format string flaw to disclose sensitive in-memory data or corrupt memory on affected QNAP network-attached storage (NAS) devices.

Affected Products

QNAP QTS versions from 5.2.0.2737 through 5.2.5.3145 (builds prior to 20250715)
QNAP QuTS hero versions from h5.2.0.2737 through h5.2.5.3138 (builds prior to 20250715)
QNAP NAS appliances running the affected QTS or QuTS hero firmware

Discovery Timeline

2025-10-03 - CVE-2025-53406 published to the National Vulnerability Database (NVD)
2026-06-17 - Last updated in NVD database

Technical Details for CVE-2025-53406

Vulnerability Analysis

The vulnerability is a format string flaw classified under [CWE-134]. Affected QTS and QuTS hero components pass externally-controlled input directly into a formatted output function without sanitization. When attacker-supplied data reaches a format string parameter, format specifiers such as %s, %x, or %n are interpreted by the runtime.

Exploitation requires administrator-level authentication over the network. A remote attacker with valid administrator credentials can craft input containing format specifiers to read stack memory, disclose secret values such as tokens or configuration data, and in some cases write to memory using %n-style specifiers to alter program state.

The flaw does not grant an initial foothold on its own. It amplifies the impact of a compromised or malicious administrator account by exposing sensitive process memory and enabling targeted memory modification within the affected service.

Root Cause

The root cause is improper handling of user-controlled strings passed as the format argument to a printf-family function. Instead of using a fixed format template with the untrusted value as a data argument (for example, printf("%s", user_input)), the vulnerable code path passes user-controlled data as the format string itself. This allows format specifiers embedded in attacker input to be parsed and executed against the stack or memory locations pointed to by argument registers.

Attack Vector

The attack vector is network-based and requires high privileges. An attacker authenticated as an administrator submits crafted input containing format specifiers to a vulnerable QTS or QuTS hero interface. The affected service processes the input through a formatted output function, resulting in memory disclosure or memory modification within the service context. No user interaction is required beyond the attacker's own authenticated session.

Refer to the QNAP Security Advisory QSA-25-36 for vendor guidance on affected interfaces.

Detection Methods for CVE-2025-53406

Indicators of Compromise

Administrator session activity containing unusual character sequences with format specifiers such as %s, %x, %p, or %n in request parameters
Unexpected process crashes, restarts, or segmentation faults in QTS or QuTS hero services following administrator requests
Anomalous administrator logins from unfamiliar IP addresses or geolocations preceding suspicious request patterns

Detection Strategies

Inspect QTS and QuTS hero web management and API logs for request parameters containing printf-style format tokens
Correlate administrator authentication events with subsequent service errors or memory-related faults on the NAS
Monitor firmware version banners across the fleet to identify appliances still running builds earlier than 20250715

Monitoring Recommendations

Forward QNAP system, access, and event logs to a centralized SIEM for retention and query
Alert on any administrator login originating from outside expected management networks
Track service restart counts on QTS and QuTS hero devices as a proxy for exploitation attempts against the format string flaw

How to Mitigate CVE-2025-53406

Immediate Actions Required

Upgrade QTS installations to 5.2.6.3195 build 20250715 or later without delay
Upgrade QuTS hero installations to h5.2.6.3195 build 20250715 or later without delay
Rotate all administrator credentials on affected NAS devices and enforce multi-factor authentication for administrator accounts
Restrict administrative access to trusted management networks only

Patch Information

QNAP has released fixed builds for both affected operating systems. Apply QTS 5.2.6.3195 build 20250715 or later, and QuTS hero h5.2.6.3195 build 20250715 or later. Refer to QNAP Security Advisory QSA-25-36 for full vendor guidance and download links.

Workarounds

Disable remote administrative access from untrusted networks by placing the NAS management interface behind a VPN or firewall access control list
Limit administrator account issuance and audit all existing administrator accounts for necessity
Enable QNAP account lockout and IP block policies to reduce credential-based access to administrator sessions

bash

# Verify installed firmware version on QNAP QTS or QuTS hero
getcfg System Version
getcfg System "Build Number"

# Confirm the running build is 20250715 or later before considering the device patched