CVE-2026-18188 Overview
CVE-2026-18188 is a format string vulnerability [CWE-134] affecting the Rsync Backup component of ASUSTOR Data Master (ADM). The flaw occurs because user-controlled rsync backup configuration or log data flows into an unsafe format string operation. An authenticated attacker can exploit this issue to disclose memory contents or trigger denial of service against the affected backup component.
Affected releases span ADM 4.1.0 through ADM 4.3.3.RUN1 and ADM 5.0.0 through ADM 5.1.3.RI81. ASUSTOR published details in Asustor Security Advisory #67.
Critical Impact
An authenticated attacker on the network can leak memory information from the Rsync Backup component or crash it, disrupting scheduled backup operations on affected ADM-based NAS devices.
Affected Products
- ASUSTOR ADM 4.1.0 through 4.3.3.RUN1
- ASUSTOR ADM 5.0.0 through 5.1.3.RI81
- Rsync Backup component on ADM
Discovery Timeline
- 2026-07-30 - CVE-2026-18188 published to NVD
- 2026-07-30 - Last updated in NVD database
Technical Details for CVE-2026-18188
Vulnerability Analysis
The vulnerability resides in the Rsync Backup functionality of ASUSTOR ADM. Rsync backup configuration data and associated log entries include values supplied by an authenticated user. Instead of being passed as arguments to a formatted output function, these values are passed directly as a format string.
When format specifiers such as %s, %x, or %n appear in attacker-controlled input, the underlying printf-family function interprets them against the process stack. This behavior enables memory disclosure and, in some cases, memory corruption that terminates the backup process.
Root Cause
The root cause is a classic CWE-134 pattern: user-supplied strings reach a formatted output function without a fixed format specifier. Backup job names, share paths, remote host fields, or log message content originating from the user are treated as the format argument itself. Any format specifier embedded in the input is expanded at runtime.
Attack Vector
Exploitation requires authenticated access to the ADM web interface or an interface capable of configuring rsync backup jobs. The attacker submits crafted values containing format specifiers through backup configuration fields. When the Rsync Backup component processes the configuration or writes log data, the specifiers trigger unintended reads of stack memory or produce a process crash. No user interaction beyond the attacker's own actions is needed.
No verified public exploit code is available. The vulnerability is described in prose in the Asustor Security Advisory #67.
Detection Methods for CVE-2026-18188
Indicators of Compromise
- Rsync backup job definitions containing format specifiers such as %s, %x, %p, or %n in name, path, or host fields.
- Unexpected termination or repeated restarts of the Rsync Backup service on ADM devices.
- Log entries from the backup component containing garbled output, pointer-like hexadecimal values, or truncated messages.
Detection Strategies
- Audit rsync backup configuration exports for user-supplied fields containing % character sequences that are not part of legitimate paths.
- Correlate backup service crash events with recent authenticated web-UI or API activity to identify the originating account.
- Monitor ADM system logs for abnormal printf-style output patterns emitted by the backup daemon.
Monitoring Recommendations
- Forward ADM syslog output to a centralized logging platform and alert on Rsync Backup process restarts.
- Track authenticated administrative sessions that modify backup jobs, especially from unusual source addresses.
- Baseline normal backup job configuration values so anomalous format-specifier content stands out.
How to Mitigate CVE-2026-18188
Immediate Actions Required
- Upgrade ADM to the fixed release identified in Asustor Security Advisory #67.
- Restrict administrative access to the ADM web interface to trusted management networks only.
- Review existing rsync backup jobs and remove or sanitize any entries containing format specifiers.
Patch Information
ASUSTOR addresses CVE-2026-18188 in updates published for ADM 4.x and ADM 5.x branches. Refer to Asustor Security Advisory #67 for the exact fixed versions and download instructions. Apply the vendor update to all NAS units running affected ADM builds.
Workarounds
- Disable the Rsync Backup service until the ADM firmware update can be applied.
- Limit accounts permitted to create or modify backup jobs to a minimal set of trusted administrators.
- Place the NAS management interface behind a VPN or firewall rule that blocks untrusted networks.
Disclaimer: This content was generated using AI. While we strive for accuracy, please verify critical information with official sources.
