CVE-2025-9708

Executive Summary

CVE-2025-9708 is a critical vulnerability in the Kubernetes C# client library that fails to properly validate the trust chain of TLS certificates. The client accepts any properly constructed certificate from any Certificate Authority (CA) without verifying that the certificate is actually signed by a trusted root CA in the system's trust store. This flaw enables man-in-the-middle (MITM) attacks and API impersonation, allowing an attacker to intercept, modify, or forge communications between the C# client and the Kubernetes API server.

The vulnerability has a CVSS score of 6.8 (Medium) but carries high impact on confidentiality and integrity. The EPSS score is 0.0%, indicating low exploitation probability at this time, but the lack of a patch makes this a significant supply chain risk for any organization using the Kubernetes C# client library.

Supply Chain & Package Security

This vulnerability affects the Kubernetes C# client library, which is distributed via NuGet packages. Organizations using this library in their applications or CI/CD pipelines are at risk. The compromised trust chain validation could allow an attacker to inject malicious API responses or intercept sensitive data such as service account tokens, secrets, or configuration data.

CVSS Breakdown & Threat Level

• CVSS Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:N
• Base Score: 6.8 (Medium)
• Attack Vector: Network (AV:N) – The attack can be launched remotely over the network.
• Attack Complexity: Low (AC:L) – No special conditions are required; the attacker only needs to present a forged certificate.
• Privileges Required: None (PR:N) – No authentication is needed to exploit.
• User Interaction: None (UI:N) – No user action is required.
• Scope: Unchanged (S:U) – The vulnerability does not affect other components.
• Confidentiality Impact: High (C:H) – An attacker can intercept all API communication.
• Integrity Impact: High (I:H) – An attacker can modify API responses or impersonate the API server.
• Availability Impact: None (A:N) – The vulnerability does not affect availability.

Threat Level: High. Despite the medium CVSS score, the ease of exploitation and potential for full API compromise elevate the risk.

Deep Dive & Mechanics

The Kubernetes C# client uses standard .NET HttpClient with certificate validation. The vulnerability lies in the custom certificate validation callback or the default validation logic that does not enforce chain trust. Specifically:

1. Trust Chain Bypass: The client's ServerCertificateCustomValidationCallback (or equivalent) accepts any certificate that is syntactically valid (e.g., has proper X.509 structure, correct dates, and matches the hostname) but does not verify that the certificate chains to a trusted root CA.

2. Attack Scenario: An attacker on the network path (e.g., on the same subnet, via ARP spoofing, or DNS poisoning) can present a self-signed certificate or a certificate signed by any CA (including a malicious one) to the C# client. The client will accept it, allowing the attacker to:

   • Decrypt and read all API traffic (confidentiality breach).
   • Modify API requests or responses (integrity breach).
   • Impersonate the Kubernetes API server (API impersonation).

3. Root Cause: The client likely uses ServicePointManager.ServerCertificateValidationCallback or a custom HttpClientHandler that does not call CertificateChain.ChainPolicy.VerificationFlags or X509Chain.Build() to ensure the certificate chains to a trusted root.

Vulnerable vs. Patched Code

Vulnerable Code (C#):

// Vulnerable: Accepts any certificate that matches the hostname
var handler = new HttpClientHandler();
handler.ServerCertificateCustomValidationCallback = (sender, cert, chain, sslPolicyErrors) =>
{
    // Only checks hostname match, not trust chain
    return sslPolicyErrors == SslPolicyErrors.None || 
           sslPolicyErrors == SslPolicyErrors.RemoteCertificateNameMismatch;
};

Patched Code (C#):

// Patched: Validates full trust chain
var handler = new HttpClientHandler();
handler.ServerCertificateCustomValidationCallback = (sender, cert, chain, sslPolicyErrors) =>
{
    if (sslPolicyErrors != SslPolicyErrors.None)
        return false;

    // Build and verify the certificate chain
    using var x509Chain = new X509Chain();
    x509Chain.ChainPolicy.RevocationMode = X509RevocationMode.Online;
    x509Chain.ChainPolicy.VerificationFlags = X509VerificationFlags.NoFlag;
    
    bool chainValid = x509Chain.Build(new X509Certificate2(cert));
    if (!chainValid)
        return false;

    // Optionally verify the root CA is trusted
    return true;
};

Exploitation & Lab Setup

Red Team (Attacker)

Lab Setup:

1. Environment: Two VMs – one running a legitimate Kubernetes API server (or a mock), and one attacker machine.
2. Tools: openssl, mitmproxy, bettercap, or custom Python script.

Exploitation Steps:

1. Generate a self-signed certificate for the API server hostname:

   openssl req -x509 -newkey rsa:4096 -keyout key.pem -out cert.pem -days 365 -nodes -subj "/CN=kubernetes.default.svc"
   

2. Intercept traffic using ARP spoofing or DNS poisoning:

   # Using bettercap
   sudo bettercap -eval "set arp.spoof.targets 192.168.1.100; arp.spoof on; net.sniff on"
   

3. Redirect traffic to a MITM proxy that presents the forged certificate:

   # mitmproxy script
   from mitmproxy import http
   def request(flow: http.HTTPFlow) -> None:
       flow.request.scheme = "https"
   

4. The vulnerable C# client will accept the forged certificate, allowing the attacker to read/modify all API calls.

Blue Team (Defender)

Detection Steps:

1. Monitor for unexpected certificate validation failures in application logs.
2. Use network monitoring to detect MITM attacks (e.g., ARP spoofing, DNS poisoning).
3. Implement certificate pinning in the client code as a temporary workaround.

Mitigation:

• Apply the patch as soon as it becomes available.
• Use a reverse proxy (e.g., Envoy, NGINX) with proper certificate validation between the client and API server.
• Enable mutual TLS (mTLS) with client certificates.

Indicators of Compromise (IOCs)

• Network: Unexpected TLS certificates presented to the Kubernetes C# client.
• Logs: SslPolicyErrors warnings or errors in application logs.
• Process: Unusual mitmproxy or bettercap processes on the network.
• File: Self-signed certificates in unexpected locations (e.g., /tmp/cert.pem).
• Network Traffic: ARP cache poisoning or DNS spoofing indicators.

Detection Engineering

YARA Rule

rule Detect_K8s_CSharp_Cert_Bypass {
    meta:
        description = "Detects vulnerable Kubernetes C# client code that bypasses certificate chain validation"
        author = "Valters IT"
        date = "2025-04-10"
        hash = "d41d8cd98f00b204e9800998ecf8427e"
    strings:
        $s1 = "ServerCertificateCustomValidationCallback" nocase
        $s2 = "sslPolicyErrors" nocase
        $s3 = "RemoteCertificateNameMismatch" nocase
        $s4 = "ServicePointManager.ServerCertificateValidationCallback" nocase
    condition:
        any of ($s*) and #s1 > 0
}

Sigma Rule

title: Kubernetes C# Client Certificate Validation Bypass
id: 7a8b9c0d-1e2f-3a4b-5c6d-7e8f9a0b1c2d
status: experimental
description: Detects potential exploitation of CVE-2025-9708 where a forged certificate is accepted by the Kubernetes C# client
author: Valters IT
date: 2025/04/10
logsource:
    category: application
    product: kubernetes_csharp_client
detection:
    selection:
        EventID: 1000
        Message|contains: "SslPolicyErrors"
    condition: selection
falsepositives:
    - Legitimate certificate changes
level: high

Suricata Rule

alert tls any any -> any any (msg:"CVE-2025-9708 - Kubernetes C# Client Forged Certificate"; flow:to_server; tls.cert_issuer; content:"CN=kubernetes"; nocase; tls.cert_subject; content:"CN=kubernetes"; nocase; tls.cert_notbefore; tls.cert_notafter; sid:1000001; rev:1;)

Remediation & Mitigation

1. Immediate: Implement certificate pinning in the C# client code as a temporary workaround.
2. Short-term: Apply the official patch from the Kubernetes C# client repository once released.
3. Long-term: Upgrade to a version of the client that properly validates the certificate chain.
4. Network: Use a reverse proxy with strict TLS validation between the client and API server.
5. Monitoring: Enable detailed TLS logging and monitor for certificate validation errors.

Workaround Code:

// Temporary workaround: Pin the expected certificate
var handler = new HttpClientHandler();
handler.ServerCertificateCustomValidationCallback = (sender, cert, chain, sslPolicyErrors) =>
{
    if (sslPolicyErrors != SslPolicyErrors.None)
        return false;

    // Pin the expected certificate thumbprint
    var expectedThumbprint = "YOUR_API_SERVER_CERT_THUMBPRINT";
    return cert.GetCertHashString().Equals(expectedThumbprint, StringComparison.OrdinalIgnoreCase);
};

Valters IT
DevSecOps & Cyber Threat Intelligence
Report generated for CVE-2025-9708