CVE-2026-103432 in apcupsd
Summary
by MITRE • 09/30/2026
apcupsd through 3.14.14 has an sscanf stack-based buffer overflow in getupsvar() in src/cgi/upsfetch.c (used by upsstats.cgi, multimon.cgi, and upsfstats.cgi), a related issue to CVE-2026-15544.
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Analysis
by VulDB Data Team • 09/30/2026
The vulnerability identified as CVE-2026-15544 represents a critical stack-based buffer overflow within the apcupsd software suite, specifically affecting versions up through 3.14.14. This flaw is located in the getupsvar function found in the source file src/cgi/upsfetch.c. The component serves as the backend logic for several CGI scripts including upsstats.cgi, multimon.cgi, and upsfstats.cgi which are responsible for retrieving status information from APC UPS devices via web interfaces. The root cause of this vulnerability stems from improper input validation when parsing data using the sscanf function. When processing user-supplied or device-generated strings that exceed expected buffer boundaries, the application fails to enforce strict length checks prior to writing data into fixed-size stack buffers. This lack of bounds checking allows an attacker to overwrite adjacent memory locations on the call stack, potentially leading to arbitrary code execution with the privileges under which the web server process is running.
From a technical perspective, this issue aligns closely with CWE-120 Buffer Copy without Checking Size of Input Classic Buffer Overflow and CWE-787 Out-of-bounds Write. The vulnerability exploits the fundamental weakness in C programming where memory management relies heavily on developer diligence for boundary enforcement. In the context of CGI applications like those provided by apcupsd, the attack surface is significantly expanded because these services are often exposed to network interfaces or local web servers that may be accessible from untrusted networks if not properly secured behind authentication and firewall rules. An attacker could craft a maliciously formatted response string from an APC UPS device or manipulate input parameters passed through the CGI interface to trigger the overflow. By carefully controlling the payload, it is possible to overwrite return addresses on the stack, thereby hijacking control flow and executing arbitrary shellcode. This capability transforms what might otherwise be a simple denial of service into a full remote code execution scenario depending on the deployment configuration and available exploit mitigations such as ASLR or DEP which may not always be fully effective against sophisticated attacks targeting CGI processes.
The operational impact of this vulnerability is severe, particularly in environments where apcupsd is deployed for critical infrastructure monitoring. If an attacker successfully exploits this buffer overflow, they gain control over the underlying operating system process running the web server components. This can lead to complete compromise of the host machine hosting the UPS management interface. In industrial or enterprise settings, such a breach could allow lateral movement into internal networks, exfiltration of sensitive configuration data regarding power infrastructure, or disruption of critical uptime monitoring capabilities. The presence of this flaw in multiple CGI scripts underscores its systemic nature within the affected versions rather than an isolated incident in a single module. Attackers leveraging techniques documented under MITRE ATT&CK technique T1059 Command and Scripting Interpreter could use the compromised process to execute further commands, establish persistence mechanisms, or pivot to other systems connected via the same network segment where APC UPS devices are monitored through these web interfaces.
Mitigation strategies must address both immediate remediation and long-term architectural improvements. The primary defense is upgrading apcupsd to a version later than 3.14.14 where this specific buffer overflow has been patched by implementing proper bounds checking in the getupsvar function before invoking sscanf or replacing unsafe string handling functions with safer alternatives that enforce length limits explicitly. Organizations should also ensure that any CGI interfaces provided by apcupsd are not directly exposed to untrusted networks and are protected behind strong authentication mechanisms such as HTTPS with client certificates or robust access control lists restricting source IP addresses. Additionally, deploying network segmentation to isolate UPS monitoring traffic from general corporate data flows reduces the blast radius in case of a successful exploit. Security teams should also enable stack protection features like Stack Canaries if supported by their compiler toolchain and ensure that operating system-level mitigations such as Address Space Layout Randomization are enabled to complicate exploitation attempts even if the vulnerability is present during transition periods before patching can be fully applied across all systems.