CVE-2026-38821 in openNDS
Summary
by MITRE • 08/28/2026
A heap-based buffer overflow vulnerability exists in openNDS before 11.0.0 that allows an unauthenticated attacker on the captive portal network to crash the openNDS daemon (denial of service) and potentially achieve remote code execution. This is in http_microhttpd.c.
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Analysis
by VulDB Data Team • 08/28/2026
The identified vulnerability represents a critical security flaw within the openNDS software, specifically affecting versions prior to 11.0.0. As an open-source captive portal solution widely deployed for managing network access and authentication on public Wi-Fi networks, openNDS processes numerous HTTP requests from clients attempting to connect. The core of this issue resides in the http_microhttpd.c module, which handles the underlying HTTP communication layer using the MicroHTTPD library. This component is responsible for parsing incoming client data to facilitate login pages, redirections, and session management. When an unauthenticated attacker interacts with the captive portal interface by sending specially crafted HTTP requests containing malformed or excessively long input fields, they can trigger a heap-based buffer overflow condition within this module.
A heap-based buffer overflow occurs when a program writes more data to a dynamically allocated memory block on the heap than it was intended to hold. In the context of openNDS, improper bounds checking during the processing of HTTP request parameters allows an attacker to overwrite adjacent memory structures. This corruption can lead to immediate instability in the application's execution flow. The most direct and observable consequence is a denial of service condition where the openNDS daemon crashes due to segmentation faults or other fatal errors triggered by the corrupted heap metadata. For network administrators relying on captive portals for access control, such crashes result in significant downtime, preventing legitimate users from authenticating and accessing the network resources.
Beyond simple availability disruption, this vulnerability carries severe implications for confidentiality and integrity through potential remote code execution. By carefully crafting the overflow payload, an attacker can overwrite function pointers or return addresses stored within the heap structure. This manipulation allows the injection of arbitrary shellcode that executes with the privileges of the openNDS process. If the daemon is running as root, which is common in network infrastructure components to handle low-level networking tasks, successful exploitation grants the attacker full control over the underlying system. This can lead to further lateral movement within the internal network, data exfiltration from connected devices, or the establishment of persistent backdoors for future attacks.
From a classification perspective, this vulnerability aligns with CWE-120, which denotes Buffer Copy without Checking Size of Input, specifically manifesting as a heap-based overflow under CWE-787. The attack vector is categorized under ATT&CK technique T1190, Exploit Public-Facing Application, as it targets an internet-facing or network-accessible service that does not require prior authentication. Furthermore, the ability to crash the service relates to availability impacts described in various risk frameworks, while potential code execution maps to privilege escalation and command injection vectors if combined with other local vulnerabilities.
Mitigation strategies must prioritize immediate version upgrades for all deployments of openNDS. Administrators should upgrade to version 11.0.0 or later where this heap overflow has been patched through improved input validation and bounds checking in the http_microhttpd.c module. In environments where upgrading is not immediately feasible, network segmentation can provide a temporary layer of defense by restricting access to the captive portal interface only from trusted subnets or implementing strict rate limiting on HTTP requests to mitigate exploitation attempts. Additionally, deploying intrusion detection systems configured with signatures for buffer overflow patterns in web server traffic may help detect and block attempted exploits before they succeed. Regular security audits and code reviews focusing on memory management functions are essential to prevent similar vulnerabilities in future updates.