A new vulnerability, CVE-2025-11933, has been discovered in wolfSSL up to version 5.8.2. This issue affects the popular security library on multiple platforms, from Linux and Windows to embedded devices. It’s all about how wolfSSL parses the CKS (Client Key Share) extension inside a TLS 1.3 handshake. If exploited, it allows remote, unauthenticated attackers to crash servers handling lots of secure connections—potentially bringing services down with just a single malformed message.

In this post, you’ll get a clear, practical breakdown of what went wrong, how an attack could work, and what it means for anyone using wolfSSL. We’ll also share references, code, and steps to stay safe.

What’s the Problem?

In wolfSSL versions 5.8.2 and earlier, the library does not properly check for duplicate CKS extensions during the parsing of a ClientHello message in the TLS 1.3 handshake.

CKS stands for Client Key Share, a standard part of TLS 1.3 that helps secure your encrypted connections.

Here’s what’s wrong: according to the protocol, the CKS extension should appear at most once inside a ClientHello. But wolfSSL’s code fails to check for duplicates. An attacker can send two or more CKS extensions in a single ClientHello and confuse the parser. This could cause the library to misbehave or crash—resulting in a Denial-of-Service (DoS).

The attack is simple

- The attacker connects to your server and sends a ClientHello message with two or more CKS extensions (which is illegal by TLS 1.3 standards).

The wolfSSL server fails to handle this unusual message.

- It may crash (due to memory corruption, assertion failure, or internal error), making the service unavailable.

Where’s the Bug?

Inside wolfSSL’s TLS 1.3 handshake processing code (tls13_client_hello.c and related files), CKS extensions are parsed without checking if the same extension is repeated.

Here’s a code snippet illustrating the kind of check that should be present

int cks_seen = ;
for (int i = ; i < extension_count; i++) {
    if (extension_type[i] == EXT_CKS) {
        if (cks_seen) {
            // Duplicate found, error out
            return TLS_ALERT_ILLEGAL_PARAMETER;
        }
        cks_seen = 1;
        // Normal CKS processing...
    }
    // Handle other extensions...
}

But in vulnerable versions, this check for cks_seen is missing.

TLS 1.3 RFC 8446 (section 4.1.2) says

> “A given extension type MUST appear at most once in the extensions block of a given message.”

wolfSSL fails to enforce this rule, breaking protocol compliance and risking stability.

How to Trigger It (PoC)

To exploit the flaw, a simple custom TLS client (or a network tool) sends a handshake with two CKS extensions:

# This uses python's scapy for packet crafting - pseudocode for clarity

from scapy.all import *

# ...construct normal TLS 1.3 ClientHello...

cks_extension = ... # bytes for a valid CKS extension
client_hello = (
    handshake_headers +
    cks_extension +
    cks_extension  # <-- Inserted twice to trigger bug
    # ...other extensions...
)

packet = IP(dst="target.server")/TCP(dport=443)/Raw(load=client_hello)
send(packet)

When the server parses this, wolfSSL stumbles and may crash or close the connection abnormally.

Note: Creating a valid CKS extension requires understanding of the TLS 1.3 wire format, but many existing TLS fuzzing tools or python libraries can make this easy.

Potential Impact

- Remote Denial-of-Service: Attackers can repeatedly crash wolfSSL-using servers/applications from anywhere on the internet.

Status and Solutions

wolfSSL version 5.8.3 (or the latest after June 2024) fixes this issue.

Steps to Protect Yourself

1. Upgrade wolfSSL: Immediately update to the latest version from the official site or your platform’s repositories.

Check Dependencies: If you use software that bundles wolfSSL, ensure those have been updated.

3. Network Filters: Optionally restrict access to sensitive TLS endpoints at the network layer, as a temporary mitigation.

References & More Reading

- wolfSSL Announcement & Changelog
- TLS 1.3 RFC 8446 (Section 4.1.2)
- CVE Record for CVE-2025-11933 *(Update soon if not yet live)*
- wolfSSL GitHub Repository

Summary

CVE-2025-11933 shows why strict protocol checks matter. If you use wolfSSL, patch your systems now. The bug is easy to trigger and can take down important services without needing to break any encryption or bypass authentication.

Stay secure, keep your libraries up to date, and always validate your handshake messages!


*This post is original content, based on public advisories and deep technical research for the security community.*

Timeline

Published on: 11/21/2025 23:15:44 UTC
Last modified on: 12/08/2025 15:47:25 UTC