Executive Summary

CVE-2026-7854 is a critical stack-based buffer overflow vulnerability found in the D-Link DI-8100 Gigabit Router firmware version 16.07.26A1. The flaw exists in the url_rule_asp handler function, which improperly uses the sprintf function to concatenate user-supplied HTTP POST parameters into a fixed-size stack buffer of only 8 bytes.

This improper handling allows remote attackers, either authenticated or unauthenticated, to corrupt stack memory by sending a specially crafted HTTP POST request to the /url_rule.asp endpoint. This can cause the router's httpd service to crash, resulting in a denial-of-service (DoS) condition and making the router's web interface inaccessible until a physical reboot.

If an attacker gains control over the instruction pointer through this overflow, it could lead to remote code execution (RCE), allowing full device compromise, traffic interception, or recruitment into a botnet.

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Impact Analysis

This vulnerability can severely impact the availability and security of your D-Link DI-8100 router. An attacker can cause a denial-of-service (DoS) by crashing the router's web interface, making it inaccessible until manually rebooted.

More critically, if exploited further, it could allow an attacker to execute arbitrary code remotely, leading to full device compromise. This could result in interception of your network traffic, unauthorized control over your device, or your router being used as part of a botnet.

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Detection Guidance

This vulnerability can be detected by monitoring for unusual HTTP POST requests sent to the /url_rule.asp endpoint of the D-Link DI-8100 router. Specifically, crafted POST requests with excessively large payloads in the 'name' parameter (such as 200,000 'A' characters) are indicative of an exploit attempt.

To detect potential exploitation attempts on your network, you can use network traffic inspection tools or web server logs to identify large or malformed POST requests targeting /url_rule.asp.

Example commands to detect such activity might include:

• Using tcpdump to capture HTTP POST requests to /url_rule.asp: tcpdump -A -s 0 'tcp port 80 and (((ip[2:2] - ((ip[0]&0xf)<<2)) - ((tcp[12]&0xf0)>>2)) != 0)' | grep 'POST /url_rule.asp'
• Using grep on web server logs to find large POST requests to /url_rule.asp: grep '/url_rule.asp' /var/log/httpd/access_log | awk '{print $0, length($0)}' | sort -kNF,NR -n
• Using a Python script or custom IDS rule to detect unusually large 'name' parameter values in POST requests.

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Mitigation Strategies

Immediate mitigation steps include restricting access to the router's web interface to trusted networks or IP addresses to prevent remote exploitation.

If possible, disable or restrict the URL filtering feature that uses the /url_rule.asp handler until a patch or firmware update is available.

Monitor the device for crashes or unavailability of the httpd service, which may indicate exploitation attempts.

Apply any available firmware updates or patches from the vendor addressing this vulnerability as soon as they are released.

Consider implementing network-level protections such as web application firewalls (WAF) or intrusion detection/prevention systems (IDS/IPS) to block malicious POST requests targeting /url_rule.asp.

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Compliance Impact

The vulnerability in the D-Link DI-8100 router allows remote attackers to cause denial-of-service or potentially execute remote code, leading to full device compromise. Such a compromise could result in unauthorized access to network traffic or device control, which may impact the confidentiality, integrity, and availability of data handled by the device.

While the provided information does not explicitly mention compliance with standards like GDPR or HIPAA, the potential for unauthorized access and disruption of service could lead to violations of these regulations, which require protection of personal data and system availability.

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