A critical stack-based buffer overflow in the D-Link DIR-825 Rev.B 2.10 router firmware allows unauthenticated, zero-click remote attackers to crash the device’s HTTP server.

Tracked as CVE-2025-7206, the flaw resides in the router’s httpd binary and stems from improper handling of the language parameter in the switch_language.cgi endpoint.

Exploitation requires no valid credentials or user interaction, meaning an adversary only needs network access to the target device’s management interface to trigger a denial-of-service condition.

Key Takeaways
1. Unauthenticated stack overflow in DIR-825 Rev.B 2.10 crashes the HTTP server.
2. Oversized language parameter in switch_language.cgi stored in NVRAM triggers the overflow.
3. Zero-click DoS disrupts VPNs, guest Wi-Fi, and IoT device management.
4. Apply firmware patch, limit web-UI access, and flag unusually long language posts.

D-Link 0-Click Vulnerability (CVE-2025-7206)

According to the security researcher iC0rner, the flaw lies in the sub_410DDC function within the httpd executable, which directly accepts attacker-controlled input without length checks.

Specifically, when an HTTP POST is made to switch_language.cgi, the language parameter is written into NVRAM for persistent storage.

This occurs before any sanitization, meaning a maliciously long string can overflow the local stack frame:

Once the overflowed NVRAM entry is saved, any subsequent request to a front-end ASP page (e.g., login.asp) triggers dynamic loading of a corresponding language JavaScript file. The page includes:

Upon parsing, httpd enters the do_ebd_js path, eventually calling nvram_get(“language”).

The returned string is passed through a convoluted series of internal functions—sub_40bFC4—where another unsafe concatenation writes beyond the intended buffer, ultimately causing a segmentation fault and crashing the service.

Risk Factors	Details
Affected Products	D-Link DIR-825 Rev.B 2.10
Impact	Stack-based buffer overflow
Exploit Prerequisites	Network access to router’s web management interface (typically port 80/443); no authentication required
CVSS 3.1 Score	9.8 (Critical)

Proof-of-Concept Exploit

A minimal proof-of-concept (PoC) demonstrates the crash in two steps. First, set the oversized language value via switch_language.cgi:

Immediately following, a simple GET of any ASP page triggers the overflow:

The overly long NVRAM entry corrupts the stack during sub_40bFC4, leading to an immediate crash of the httpd process without any authentication or explicit user action.

Mitigations

Organizations relying on the DIR-825 Rev.B 2.10 for VPN termination, guest networks, or IoT segmentation should consider the following mitigations:

Apply Firmware Update: D-Link must release a patched firmware version that enforces strict input validation on the language parameter and ensures proper bounds checking in sub_40bFC4.

Network Access Controls: Block access to the router’s web management interface from untrusted networks or the internet at large.

Intrusion Detection: Monitor for anomalous HTTP POST requests to switch_language.cgi containing abnormally long language values and flag or block these at the perimeter firewall.

By implementing these measures, administrators can guard against CVE-2025-7206 and restore confidence in their network edge security.

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