CVE-2026-17251 Overview
CVE-2026-17251 is a NULL pointer dereference vulnerability [CWE-476] in the HTTP request parsing functionality of the TP-Link TL-MR6400 v7 4G LTE router. An unauthenticated attacker on an adjacent network can send a specially crafted HTTP request containing a malformed session cookie header to trigger the flaw. Successful exploitation crashes the HTTP service process, producing a denial-of-service condition. The device loses management interface and CGI functionality until the service recovers.
Critical Impact
Unauthenticated adjacent-network attackers can crash the router's HTTP management service through a malformed cookie header, disabling web-based administration and CGI endpoints.
Affected Products
- TP-Link TL-MR6400 v7 (4G LTE Wireless N Router)
- TL-MR6400 v7 firmware with vulnerable HTTP request parser
- Web management service and CGI handlers on the device
Discovery Timeline
- 2026-08-21 - CVE-2026-17251 published to NVD
- 2026-08-21 - Last updated in NVD database
Technical Details for CVE-2026-17251
Vulnerability Analysis
The flaw resides in the HTTP request parsing routine of the TL-MR6400 v7 web management service. When the parser processes an incoming request, it inspects the session cookie header without adequately validating pointer state before dereference. A malformed cookie value causes the parser to operate on a NULL pointer, terminating the HTTP daemon.
Because the vulnerability affects the router's management HTTP service, exploitation removes the primary administrative surface. CGI endpoints that depend on the same process become unreachable until the service watchdog restarts the daemon or the device is rebooted. Repeated triggering enables sustained denial of service against local network administrators.
The issue does not disclose memory contents or permit code execution. Confidentiality and integrity remain unaffected. Impact is limited to availability of the HTTP management plane.
Root Cause
The root cause is a missing NULL check in the cookie header parsing path, classified under [CWE-476] NULL Pointer Dereference. When the parser encounters a malformed Cookie: header structure, an internal lookup or tokenization function returns a NULL pointer that downstream code dereferences without validation.
Attack Vector
Exploitation requires network reachability to the router's HTTP management interface, typically restricted to the LAN or Wi-Fi segment. The attacker sends a single crafted HTTP request with a malformed session cookie. No authentication or user interaction is required. The malformed cookie header triggers the dereference and crashes the HTTP process. Detailed technical write-ups are not published; refer to the TP-Link TL-MR6400 FAQ for vendor guidance.
Detection Methods for CVE-2026-17251
Indicators of Compromise
- Unexpected restarts or crashes of the router's HTTP management daemon in system logs
- Loss of access to the web administration console or CGI endpoints from LAN clients
- HTTP requests containing malformed or truncated Cookie: headers directed at the router management IP
- Repeated short-lived TCP connections to the router's management port followed by service unavailability
Detection Strategies
- Monitor router syslog output for HTTP daemon segmentation faults or watchdog-initiated service restarts
- Inspect HTTP traffic to router management interfaces for anomalous or malformed session cookie headers
- Baseline availability of the web management interface and alert on repeated unreachability events
Monitoring Recommendations
- Forward router logs to a centralized logging platform for correlation with network traffic
- Track connection attempts to the management interface from unusual internal hosts
- Alert on repeated management interface outages within a short time window, indicating potential exploitation
How to Mitigate CVE-2026-17251
Immediate Actions Required
- Restrict access to the router's HTTP management interface to trusted administrative hosts and VLANs
- Disable remote management on the WAN interface if it is not required
- Apply firmware updates from TP-Link as soon as a patched release is available for TL-MR6400 v7
- Reboot the device if the management interface becomes unresponsive to restore service
Patch Information
TP-Link publishes firmware releases for this model on the vendor download portals. Check the TP-Link TL-MR6400 Firmware page and the regional firmware mirror for versions addressing CVE-2026-17251. Apply the latest firmware that references remediation for the HTTP cookie parsing flaw.
Workarounds
- Segment the router management interface onto a dedicated administrative VLAN with access control lists
- Block untrusted client subnets from reaching the router's HTTP management port at an upstream firewall
- Prefer HTTPS management where available and disable unused CGI endpoints
- Monitor for repeated management outages and treat sustained unavailability as a potential exploitation event
# Example: restrict router management port access with an upstream firewall rule
iptables -A FORWARD -p tcp --dport 80 -d <router_mgmt_ip> -s <trusted_admin_subnet> -j ACCEPT
iptables -A FORWARD -p tcp --dport 80 -d <router_mgmt_ip> -j DROP
Disclaimer: This content was generated using AI. While we strive for accuracy, please verify critical information with official sources.
