CVE-2025-32818 - Null Pointer Dereference in SonicOS SSLVPN Virtual Office – Remote Crash & DoS Explained

In early 2025, a significant security vulnerability was found in SonicWall SonicOS, affecting its SSLVPN Virtual Office interface. Identified as CVE-2025-32818, this flaw lets any remote, unauthenticated attacker crash the firewall, causing a Denial-of-Service (DoS) that interrupts VPN connectivity for all users. Let’s break down what happened, why it’s dangerous, and how someone might go about exploiting it.

The Vulnerability in Simple Terms

CVE-2025-32818 is a Null Pointer Dereference vulnerability. In simple words, the SonicOS firewall’s SSLVPN Virtual Office portal does not properly handle certain kinds of requests. When it tries to use information it expects to be there but is actually missing (null), it “dereferences” a pointer to nothing, which crashes the firewall software.

The result? The firewall stops working until it’s rebooted or restarted—a classic Denial-of-Service (DoS) situation. For organizations relying on SonicWall firewalls for secure remote access, this is bad news because their users can’t reach critical resources.

Product: SonicWall SonicOS (SSLVPN Virtual Office)

- Versions: Confirmed on versions prior to the patched release (consult vendor advisory for your exact version)

Attackers: Anyone over the Internet — authentication is *not* required

Reference:
- SonicWall Security Advisory *(example link)*
- NVD Entry for CVE-2025-32818

How the Exploit Works

When the SonicOS SSLVPN Virtual Office interface receives a malformed or incomplete HTTP request, it tries to process missing data. The function expects a value (pointer), but instead, it gets nothing (null). Since the code does not check for null pointers, it attempts to use that value. This instantly crashes the process, knocking the SSLVPN service (and sometimes the whole firewall) offline.

Here’s a simplified version of the kind of mistake that causes this problem

void handle_request(Request *req) {
    User *user = req->get_user();
    // No NULL pointer check here!
    int session_id = user->get_session_id(); // Crash if user is NULL
    // process session...
}

A correct implementation would first check if user is not NULL before using it.

Exploit Details & Example

Goal: Crash the SonicWall firewall remotely (DoS)

Requirements: No login needed! Just reach the SSLVPN Virtual Office portal (default: https://:4433/ or often just https:///)

Send Malformed Request:

The exploit involves sending a specially crafted HTTP request to a vulnerable endpoint. For example, omitting required POST parameters or sending a request with missing cookies/headers can trigger the bug.

Firewall Crashes:

As the firewall attempts to process your invalid request, the SSLVPN process dereferences a null pointer and crashes. Active VPN sessions drop, and new logins fail.

Example Exploit Using curl

*(Warning: For educational purposes only. Do NOT try this on systems you don’t own or have permission to test!)*

Suppose the vulnerable endpoint is /cgi-bin/handle_vpn. You can send a minimal, malformed POST request with missing body parameters:

curl -k -X POST "https://firewall.example.com/cgi-bin/handle_vpn";

Or purposely omit a required header

curl -k -H "User-Agent:" "https://firewall.example.com/cgi-bin/handle_vpn";

With some custom tweaking (as demonstrated in Proof-of-Concept on Exploit-DB if available), this simple request could be enough to cause the crash.

SonicWall has released patched versions. Update immediately.

Restrict Access:

If you can’t update yet, limit access to the SSLVPN portal using firewall rules or put it behind a trusted VPN.

Monitor for Exploit Attempts:

Watch your logs for unusual HTTP requests to the SSLVPN URLs, as repeated null dereference crashes may indicate an active attack.

Vendor Fix:
Official SonicWall Patch and Advisory

References

- NVD Entry for CVE-2025-32818
- SonicWall Advisory for CVE-2025-32818
- General Description of Null Pointer Dereference

Summary

CVE-2025-32818 is a serious, remotely exploitable bug in SonicOS’s SSLVPN Virtual Office feature, allowing attackers to crash firewalls at will. If you’re still running vulnerable versions, patch immediately and implement mitigations to avoid business disruption.

Timeline

Published on: 04/23/2025 20:15:43 UTC