CVE-2025-40600: SonicWall SonicOS SSL VPN DoS Vulnerability

CVE-2025-40600 Overview

CVE-2025-40600 is a format string vulnerability [CWE-134] in the SonicWall SonicOS Secure Sockets Layer Virtual Private Network (SSL VPN) interface. A remote, unauthenticated attacker can send specially crafted input containing format specifiers to trigger service disruption on affected appliances. The flaw impacts a broad range of SonicWall firewalls, including the TZ, NSa, NSsp, and NSv series running affected SonicOS versions. SonicWall published advisory SNWLID-2025-0013 to address the issue.

Critical Impact

Unauthenticated network attackers can disrupt SSL VPN services on internet-exposed SonicWall firewalls without user interaction, affecting remote-access connectivity for the enterprise.

Affected Products

• SonicWall SonicOS on TZ series firewalls (TZ270, TZ270W, TZ370, TZ370W, TZ470, TZ470W, TZ570, TZ570P, TZ570W, TZ670)
• SonicWall SonicOS on NSa series firewalls (NSa 2700, NSa 3700, NSa 4700, NSa 5700, NSa 6700)
• SonicWall SonicOS on NSsp (NSsp 10700, NSsp 11700, NSsp 13700, NSsp 15700) and NSv (NSv 270, NSv 470, NSv 870) platforms

Discovery Timeline

• 2025-07-29 - CVE-2025-40600 published to the National Vulnerability Database (NVD)
• 2025-08-11 - Last updated in the NVD database

Technical Details for CVE-2025-40600

Vulnerability Analysis

The vulnerability resides in the SonicOS SSL VPN web interface, which processes attacker-supplied input as part of a format string. When a function such as printf, sprintf, or a logging routine consumes externally controlled data as the format argument, format specifiers like %s, %x, or %n are interpreted instead of being treated as literal text. This leads to invalid memory reads, corruption of the call stack, or process termination. Because the SSL VPN portal is reachable over the network and accepts unauthenticated requests, exploitation requires no credentials and no user interaction.

Root Cause

The root cause is improper handling of user-supplied strings passed directly into format-aware functions, classified under [CWE-134] Use of Externally-Controlled Format String. Input received through the SSL VPN interface is not sanitized to strip or escape format specifiers before being processed as a format argument.

Attack Vector

Exploitation occurs over the network against the SSL VPN listener on the firewall's WAN interface. An attacker crafts an HTTPS request containing format specifiers in a field consumed by a vulnerable formatter, then submits it to the SSL VPN endpoint. Successful exploitation results in service disruption of the SSL VPN process, breaking remote-access functionality. No public proof-of-concept exploit has been published, and the issue is not listed in the CISA Known Exploited Vulnerabilities (KEV) catalog at the time of writing.

// No verified exploit code is publicly available for CVE-2025-40600.
// The vulnerability class involves passing attacker-controlled input
// directly as the format argument to functions such as printf(user_input)
// rather than the safe form printf("%s", user_input).
// Refer to SNWLID-2025-0013 for vendor technical details.

Detection Methods for CVE-2025-40600

Indicators of Compromise

• Unexpected SSL VPN process crashes, restarts, or watchdog events in SonicOS system logs.
• Inbound HTTPS requests to the SSL VPN portal containing repeated format specifiers (%s, %x, %n, %p) in URL parameters, headers, or POST bodies.
• Loss of remote-access availability correlated with anomalous request patterns from a small set of source IP addresses.

Detection Strategies

• Inspect SonicOS event logs for SSL VPN service termination, segmentation faults, or repeated daemon restarts.
• Deploy network detection signatures on perimeter intrusion detection systems (IDS) to flag HTTPS requests to SSL VPN endpoints containing sequences of format specifiers.
• Correlate firewall management alerts with NetFlow or connection logs to identify scanning activity targeting SonicWall SSL VPN ports.

Monitoring Recommendations

• Forward SonicOS syslog and SSL VPN access logs to a centralized analytics platform for continuous review.
• Monitor availability of the SSL VPN service through synthetic health checks to detect disruption events quickly.
• Track inbound traffic to the SSL VPN portal from unfamiliar geographies or autonomous systems and alert on spikes.

How to Mitigate CVE-2025-40600

Immediate Actions Required

• Apply the SonicOS firmware update referenced in SonicWall advisory SNWLID-2025-0013 on all affected TZ, NSa, NSsp, and NSv appliances.
• Restrict access to the SSL VPN portal to known source IP ranges using WAN access rules until patching is complete.
• Audit SonicOS logs for prior service disruptions that may indicate exploitation attempts.

Patch Information

SonicWall has released fixed SonicOS firmware versions addressing CVE-2025-40600. Refer to the SonicWall Vulnerability Advisory SNWLID-2025-0013 for the exact patched build numbers per platform and follow the vendor's documented upgrade procedure.

Workarounds

• Disable the SSL VPN service on affected firewalls if it is not required for business operations.
• Place the SSL VPN portal behind an upstream reverse proxy or web application firewall (WAF) that strips or escapes format specifiers from request data.
• Limit SSL VPN reachability to trusted source addresses through geo-IP filtering and explicit allow lists at the WAN interface.

# Example: restrict SSL VPN access to a trusted management subnet
# using SonicOS CLI access rules (illustrative syntax)
config
  access-rule
    from WAN to WAN
    service SSLVPN
    source address-object "Trusted-Admin-Net"
    action allow
  exit
  access-rule
    from WAN to WAN
    service SSLVPN
    source any
    action deny
  exit
commit