D-Link DI-8100 CVE-2026-7248: Critical Buffer Overflow in tgfile.htm

In late April 2026, CVE-2026-7248 (CVSS 9.8) was published against the D-Link DI-8100 router on firmware 16.07.26A1. The tgfile_htm function in the tgfile.htm endpoint suffers from a remotely exploitable buffer overflow without authentication via the fn parameter. A public PoC has been released, immediately exposing every deployed and reachable unit.

Technical Details

The CGI endpoint tgfile.htm handles internal file manipulation operations on the router. The tgfile_htm function copies the fn HTTP parameter into a fixed-size stack buffer without bounds checking. By sending a sufficiently long string in fn, an attacker overwrites the stack and can hijack execution flow.

Depending on the protections compiled into the firmware (often absent or partial on consumer routers — no stack canary, partial ASLR on some MIPS/ARM architectures), the attack can lead to:

• A denial of service (CGI process crash, router reboot)
• Arbitrary code execution as root if the attacker controls the return path via ROP

Characteristics

Affected Products and Versions

Other DI-8100 firmwares should also be treated as potentially affected until an official advisory rules them out. D-Link has not released a patch at the time of writing.

Exploitation and Impact

Attack vector

A single HTTP request with an fn parameter containing a long payload is enough. If the admin panel is exposed to the internet, the attack can be launched from any IP.

Impact

• Denial of service: router reboots in a loop, internet connectivity loss for the LAN
• Root RCE: if a working ROP exploit is built for the targeted firmware, full router takeover
• LAN pivot: every device behind the router becomes reachable
• Botnet recruitment: Mirai and variants actively scan for D-Link devices

Public PoC

# Generic example — exact length depends on compiled version
curl "http://<router-ip>/tgfile.htm?fn=$(python3 -c 'print("A"*2048)')"

Detection and IOCs

Network-side

• HTTP requests with abnormally long fn parameters (> 256 chars) to /tgfile.htm
• Repeated crashes / reboots of the router (visible in SNMP logs or your network monitoring)

Snort/Suricata rule

alert http any any -> any any (msg:"D-Link DI-8100 tgfile.htm buffer overflow attempt"; \
  http.uri; content:"/tgfile.htm"; http.uri; content:"fn="; \
  pcre:"/fn=[^&]{256,}/"; sid:2026007248; rev:1;)

Post-exploitation indicators

• Unusual outbound traffic
• DNS configuration changes
• Unexpected admin accounts in router config

Mitigation

Immediate action

1. Disable internet access to the admin interface
2. Restrict LAN access to the panel via a strict ACL (single management IP)
3. Block long requests to /tgfile.htm at an upstream firewall

Upstream workaround

On an upstream firewall or WAF, add a rule limiting HTTP parameter length to the router. On Nginx as a reverse proxy for example:

location /tgfile.htm {
    if ($arg_fn ~ ".{256,}") {
        return 403;
    }
    proxy_pass http://router;
}

Long-term solution

• Update firmware as soon as D-Link releases a patch
• Replace the router if end-of-life is announced
• Consider third-party firmware (OpenWrt) on supported models

Why Continuous Monitoring Matters

Routers are almost never included in traditional vulnerability inventories — yet they are prime gateways for attackers. A CVE like CVE-2026-7248, with public PoC and no patch, can compromise thousands of organizations that are still unaware one of their devices is affected.

With cveo.tech, inventory your routers alongside your servers and get automatic alerts the moment a critical CVE hits one of your exact versions — no delay, no manual watch.