CVE-2026-17251 in TL-MR6400
Summary
by MITRE • 08/21/2026
A NULL pointer dereference vulnerability exists in the HTTP request parsing functionality of TL-MR6400 v7. An unauthenticated remote attacker can trigger the vulnerability by sending a specially crafted HTTP request containing a malformed session cookie header.
Successful exploitation may cause the HTTP service process to crash, resulting in a denial-of-service condition and temporary loss of management or CGI functionality until service recovery.
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Analysis
by VulDB Data Team • 08/21/2026
The TL-MR6400 v7 router contains a critical NULL pointer dereference vulnerability within its HTTP request parsing subsystem, specifically affecting the handling of session cookie headers. This flaw arises from insufficient validation logic when processing incoming HTTP requests that contain malformed or improperly formatted session cookies. In C-based embedded systems common in networking equipment like this TP-Link device, memory management relies heavily on explicit checks to ensure pointers are valid before dereferencing them. When an attacker sends a specially crafted HTTP request with a maliciously constructed cookie header, the parser fails to verify whether the resulting pointer is NULL prior to accessing it. This lack of defensive programming allows the execution flow to proceed into invalid memory addresses, triggering a segmentation fault that crashes the associated service process.
From a technical perspective, this vulnerability aligns closely with CWE-476, which defines a NULL Pointer Dereference as an error where code attempts to dereference a pointer that is expected to be valid but is actually null. The root cause lies in the application logic failing to handle edge cases during string parsing or memory allocation for session data. In many embedded web servers, such as those based on BusyBox httpd or similar lightweight frameworks often found in consumer routers, error handling paths are sometimes overlooked during development cycles focused on feature delivery rather than robustness. The specific trigger vector involves the HTTP header field designated for cookies, which is typically parsed to establish user sessions and maintain state across requests. By manipulating this input, an attacker can force the parser into a state where it expects data that does not exist or cannot be allocated, leading directly to the crash condition.
The operational impact of exploiting this vulnerability is primarily focused on availability rather than confidentiality or integrity in its initial stage. A successful exploitation results in the immediate termination of the HTTP service process running on the router. This causes a denial-of-service condition where the device becomes unreachable via its web management interface and any CGI-based administrative functions cease to operate temporarily. For network administrators, this manifests as an inability to configure firewall rules, monitor traffic, or manage connected devices through the standard GUI. While some embedded systems may have watchdog timers that automatically restart crashed services after a short interval, this introduces intermittent instability and potential gaps in security monitoring during the downtime period. Furthermore, if the crash occurs during critical configuration updates, it could lead to corrupted state files requiring manual intervention to restore full functionality.
This vulnerability is categorized under MITRE ATT&CK technique T1498, specifically Network Denial of Service via service disruption or resource exhaustion. Although this specific instance does not involve network flooding but rather a single crafted packet causing a crash, the effect is identical in terms of impact on availability. The attack vector is remote and unauthenticated, meaning no prior access credentials are required to trigger the flaw. This significantly lowers the barrier for entry for malicious actors who may seek to disrupt small business or home networks by rendering management interfaces inaccessible. It also highlights the risks associated with exposing administrative interfaces directly to untrusted networks without adequate input sanitization mechanisms in place at the application layer.
Mitigation strategies must address both immediate remediation and long-term architectural improvements. The primary defense is for users to apply vendor-provided firmware updates that patch this specific parsing logic error, ensuring proper NULL checks are implemented before any pointer dereference operations occur within the HTTP parser. In environments where updating firmware immediately is not feasible, network-level controls should be employed to restrict access to the router's management interface exclusively from trusted internal subnets using firewall rules or VLAN segmentation. Additionally, implementing rate limiting on incoming HTTP requests can help mitigate potential abuse of this flaw by automated scanning tools that might attempt to trigger the crash repeatedly. Security teams should also monitor for unusual spikes in error logs related to web server crashes as an indicator of active exploitation attempts against similar vulnerable devices in their infrastructure.