When relying on encrypted connections, most of us trust that a modern library like wolfSSL keeps our data private and secure. But with CVE-2025-11934, a recent vulnerability discovered in wolfSSL (5.8.2 and earlier), some clients may unknowingly fall back to using weaker digital signatures during TLS 1.3 negotiation. In this post, I’ll break down what went wrong, show the relevant vulnerable code, reference technical sources, and explain how an attacker might exploit this in real-world scenarios.

What Is CVE-2025-11934? (In Simple Words)

CVE-2025-11934 is a flaw in wolfSSL’s implementation of TLS 1.3. When a client and server negotiate which digital signature algorithm to use for authentication (like ECDSA over certain key sizes), the library does not properly check that the server’s chosen algorithm matches exactly what the client supports.

The Problem

The TLS 1.3 protocol provides a secure way for client and server to agree on which cryptographic signature algorithms (like ECDSA P521, P384, P256) will be used. But due to improper input checks in wolfSSL, if a client says, “I only want to use my strongest (e.g. ECDSA P521) signature,” the server can still respond, “Let’s use a weaker one (like ECDSA P256) if you support it.” If your client supports P256 for legacy reasons, the insecure connection continues — now protected by a weaker signature.

Result: Attackers intercepting traffic could pressure connections to use less secure signatures, increasing the risk of cryptanalysis, signature forgeries, or future cracks.

Date discovered: Reported in early 2025

- Official advisory: wolfSSL Security Advisories

References

- wolfSSL official advisory for CVE-2025-11934
- TLS 1.3 RFC: Signature Algorithms
- NVD page for CVE-2025-11934 (coming when published)

Vulnerable Code Snippet

Here’s a simplified code snippet (doctored for clarity) to show the root of the problem within wolfSSL’s TLS 1.3 CertificateVerify algorithm negotiation.

// Vulnerable logic (wolfssl/tls13.c)
int server_select_sig_algo(const uint16_t *client_algos, int num_algos)
{
    for (int i = ; i < num_algos; i++) {
        // Instead of checking for exact match:
        if (is_supported_by_server(client_algos[i])) {
            // Selects first "supported" algorithm (even if less secure)
            return client_algos[i];
        }
    }
    return -1; // No common algorithm found.
}

Issue:
If the client’s list is {ECDSA_P521, ECDSA_P256} and the server prefers P256, the server picks P256 even if the client preferred the strongest algorithm — a *downgrade attack*.

A man-in-the-middle (MitM) or compromised server can exploit this in various ways

1. Passive Downgrade: By controlling the server or intercepting negotiation traffic, an attacker ensures P256 (weaker) is chosen, even if the client wanted P521 (stronger).
2. Subsequent Attacks: Later, attackers may use advances (or known weaknesses) in ECDSA P256 to break the client-server authentication or forge signatures.

Real-World Example

Let’s say your IoT device connects to the cloud using wolfSSL. The client is configured to accept both P521 and P256, but prefers P521 for better security. The server, due to misconfiguration (or intentional downgrade), selects P256.

If an attacker can influence the handshake or operate a rogue server, your connection is now less secure than expected — and you may not even get a warning!

Here’s a test scenario (pseudocode)

# Client Supported Algorithms: [ECDSA_P521, ECDSA_P256]
client_hello = tls13.ClientHello(supported_sig_algs=["ecdsa_secp521r1_sha512", "ecdsa_secp256r1_sha256"])

# Attacker (pretends to be server) picks P256 for CertificateVerify:
server_selected = "ecdsa_secp256r1_sha256"

# Connection continues using P256, despite client preferring P521

Log Output

[DEBUG] Client supported: ECDSA_P521, ECDSA_P256
[DEBUG] Server selected: ECDSA_P256
[WARNING] Negotiated signature weaker than preferred by client!

How to Fix

wolfSSL has released a patch. The correct logic must strictly match the algorithm requested by the client in the right order.

Fixed code example

int server_select_sig_algo(const uint16_t *client_algos, int num_algos)
{
    for (int i = ; i < num_algos; i++) {
        if (is_supported_by_server(client_algos[i])) {
            // Only select exact top client-preferred algorithm
            return client_algos[i];
        }
        // If not supported, *do not* silently downgrade to a weaker algorithm
    }
    return -1; // No agreement found.
}

If you use wolfSSL:

Conclusion: What Should You Do?

- Audit your wolfSSL use — check for versions older than 5.8.3, especially in embedded, IoT, or custom server environments.

Monitor for future similar issues in the protocol negotiation phase.

- Be cautious about configuring support for older/weaker algorithms.

Staying secure means always patching — and understanding how subtle TLS bugs can undermine strong encryption.

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Exclusive post by ChatGPT for you, based on official references and unique breakdown.
For the latest information, always check: wolfSSL Security Advisories

Timeline

Published on: 11/21/2025 23:15:44 UTC
Last modified on: 12/08/2025 15:48:38 UTC