Critical D-Link DIR-825 Router Flaw (CVE-2025-7206, CVSS 9.8): Remote Crash Via Buffer Overflow

A newly discovered critical vulnerability (CVE-2025-7206) in the D-Link DIR-825 router running firmware version 2.10 poses a serious threat to home and enterprise networks. The flaw, unearthed by independent security researcher iC0rner, allows remote attackers to crash the router’s web interface without authentication — potentially paving the way for remote code execution or denial-of-service attacks.

“The manipulation of the argument Language leads to stack-based buffer overflow,” the CVE description writes, giving the issue a CVSS score of 9.8.

At the heart of the flaw is the switch_language.cgi component in D-Link’s built-in web server (httpd). Here’s how it works:

1. An attacker sends a maliciously long Language parameter via the CGI script.
2. This parameter is stored in non-volatile memory (nvram).
3. When the router later renders any ASP page (e.g., login.asp) that includes a JavaScript reference like
   

   <script type="text/javascript" src="lang_<% CmoGetCfg("language","none"); %>.js"></script>

   the system attempts to dynamically fetch and parse the language file name.

4. The stored Language value flows through several internal functions — sub_40bFC4, v65, v61, v69, and eventually v67.
5. No proper bounds checking is applied, allowing an out-of-bounds write to occur.
6. This causes a segmentation fault, crashing the router’s httpd service.

“If the overflow length is insufficient, it will also affect the program’s execution,” the researcher noted, implying that partial corruption may also lead to instability or unpredictable behavior.

To exploit the flaw:

1. Set a long Language parameter via switch_language.cgi.
2. Access any page such as login.asp that references the dynamic language JavaScript.
3. The overflow is triggered in the background, crashing the process.

This is a zero-click attack vector once the initial parameter is set — the user merely needs to visit a common router page for the crash to occur.

The D-Link DIR-825 is widely used in residential, small business, and public Wi-Fi environments. While this vulnerability currently causes a segmentation fault (crash), any stack-based buffer overflow could potentially lead to remote code execution (RCE) in future exploit chains.

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