CVE-2026-18187 Overview
CVE-2026-18187 is a format string vulnerability [CWE-134] in the Internal Backup component of Asustor Data Master (ADM). User-controlled task input reaches an error response path that passes attacker-supplied data into an unsafe format string operation. An authenticated attacker can leverage the flaw to disclose process memory or trigger a denial of service condition in the affected CGI process.
Affected versions span ADM 4.1.0 through ADM 4.3.3.RUN1, and ADM 5.0.0 through ADM 5.1.3.RI81.
Critical Impact
Authenticated attackers can read sensitive memory contents from the CGI process or crash the Internal Backup service, disrupting NAS backup operations.
Affected Products
- Asustor ADM 4.1.0 through 4.3.3.RUN1
- Asustor ADM 5.0.0 through 5.1.3.RI81
- Asustor NAS appliances running the Internal Backup component
Discovery Timeline
- 2026-07-30 - CVE-2026-18187 published to NVD
- 2026-07-30 - Last updated in NVD database
Technical Details for CVE-2026-18187
Vulnerability Analysis
The flaw resides in the Internal Backup functionality of ADM. When a backup task produces an error, the affected CGI handler builds an error response that incorporates user-controlled task input. That input is then passed as the format string argument to a printf-family function rather than as a data argument, satisfying the classic conditions for a format string vulnerability [CWE-134].
An authenticated attacker who can create or modify a backup task can embed format specifiers such as %x, %s, or %n in task fields. The CGI process interprets these specifiers when generating the error response, causing it to read from the process stack or invalid memory locations. This produces memory disclosure or a segmentation fault that terminates the CGI worker.
Root Cause
The root cause is the direct use of untrusted task input as the format string parameter in a formatted output function. Proper handling requires passing user data as an argument to a fixed format string, for example printf("%s", user_input) instead of printf(user_input). The Internal Backup error path omits this separation, allowing format specifiers in the input to control interpretation of the output buffer and adjacent memory.
Attack Vector
Exploitation requires network access to the ADM web interface and valid credentials on the appliance. The attacker submits crafted task data containing format specifiers through the Internal Backup CGI endpoint. When the backend generates an error response referencing the task, the malicious specifiers execute against the CGI process memory. Repeated requests can enumerate stack contents or reliably crash the process, denying backup service availability. Technical specifics are described in the Asustor Security Advisory #67.
Detection Methods for CVE-2026-18187
Indicators of Compromise
- Repeated CGI request entries in ADM access logs referencing Internal Backup endpoints with task parameters containing % sequences such as %x, %s, %p, or %n.
- Unexpected termination or restart events for the Internal Backup CGI process recorded in system logs.
- Backup tasks created or modified by non-administrative accounts with unusually formatted names or parameters.
Detection Strategies
- Inspect HTTP request bodies and query strings destined for Internal Backup CGI handlers for occurrences of format specifiers in user-supplied task fields.
- Correlate authenticated web sessions with CGI crash events and error responses that leak numeric or hexadecimal fragments consistent with memory disclosure.
- Baseline normal backup task creation patterns and alert on anomalous frequency or malformed input from a single account.
Monitoring Recommendations
- Forward ADM web and system logs to a centralized log platform and retain them for correlation with authentication events.
- Alert on repeated 5xx responses or process restarts tied to the Internal Backup service.
- Monitor administrative account creation and privilege changes on the NAS to reduce the pool of accounts capable of exploiting the flaw.
How to Mitigate CVE-2026-18187
Immediate Actions Required
- Upgrade ADM to a fixed release beyond 4.3.3.RUN1 in the 4.x branch or beyond 5.1.3.RI81 in the 5.x branch as published in the Asustor Security Advisory #67.
- Restrict ADM web management access to trusted management networks and block exposure to the public internet.
- Rotate credentials for all ADM accounts and remove unused or shared accounts that could be abused to reach the authenticated attack surface.
Patch Information
Asustor addresses CVE-2026-18187 in the vendor-supplied ADM updates referenced in Asustor Security Advisory #67. Administrators should apply the vendor-published ADM version that supersedes the affected 4.1.0–4.3.3.RUN1 and 5.0.0–5.1.3.RI81 ranges. Verify the installed ADM build after upgrade to confirm the patched version is running.
Workarounds
- Limit access to the Internal Backup feature to a minimal set of administrative accounts protected by strong, unique passwords and two-factor authentication where supported.
- Place the NAS management interface behind a VPN or firewall access control list until patches are deployed.
- Disable or pause Internal Backup task creation for non-essential users if the appliance cannot be updated immediately.
Disclaimer: This content was generated using AI. While we strive for accuracy, please verify critical information with official sources.
