CVE-2026-24858: FortiOS SSO Zero-Day Exploited in the Wild - SOC Prime

Why This Matters Now

Why This Matters Now: The recent exploitation of CVE-2026-24858 in FortiOS SSO has compromised several high-profile organizations. This zero-day vulnerability allows attackers to bypass authentication mechanisms, leading to unauthorized access to internal systems and sensitive data. If you’re running FortiOS, this is urgent.

Timeline of Events

Understanding CVE-2026-24858

CVE-2026-24858 is a zero-day vulnerability in the Single Sign-On (SSO) feature of FortiOS, a popular network security operating system used by many organizations. This vulnerability allows attackers to bypass authentication checks and gain unauthorized access to the SSO system.

Vulnerability Details

The core issue lies in how FortiOS handles certain SSO requests. Attackers can craft malicious requests that exploit this weakness to authenticate without valid credentials.

Impact

If exploited, attackers can:

• Access internal networks and resources.
• Steal sensitive data.
• Deploy malware within the network.
• Conduct further attacks.

How Attackers Exploit CVE-2026-24858

Attackers exploit CVE-2026-24858 by sending specially crafted HTTP requests to the FortiOS SSO endpoint. These requests are designed to bypass the authentication mechanism, granting unauthorized access.

Example Malicious Request

POST /sso/login HTTP/1.1
Host: fortios.example.com
Content-Type: application/x-www-form-urlencoded

username=admin&password=invalid&bypass_auth=true

Detection

Signs of exploitation include:

• Unauthorized login attempts.
• Unusual traffic patterns.
• Suspicious access logs.

Mitigation Strategies

Immediate action is crucial to mitigate the risk posed by CVE-2026-24858.

Apply the Patch

Fortinet released a patch on December 14, 2024, addressing this vulnerability. Apply the patch as soon as possible.

Disable Unnecessary Features

Disable any SSO features that are not in use to reduce the attack surface.

Implement Network Segmentation

Segment your network to limit the impact of a potential breach. Ensure that SSO systems are isolated from other critical infrastructure.

Enable Logging and Monitoring

Enable detailed logging and set up monitoring to detect suspicious activities early.

Review Access Controls

Regularly review and update access controls to ensure that only authorized users and systems have access to SSO.

Technical Analysis

Vulnerable Code Example

Here’s a simplified example of vulnerable code that could be exploited:

def authenticate_user(username, password, bypass_auth=False):
    if bypass_auth:
        return True  # Vulnerable line
    user = User.objects.get(username=username)
    if user.check_password(password):
        return True
    return False

Secure Code Example

The secure version removes the bypass option:

def authenticate_user(username, password):
    user = User.objects.get(username=username)
    if user.check_password(password):
        return True
    return False

Error Handling

Ensure that error messages do not reveal sensitive information.

Incorrect Error Handling

try:
    user = User.objects.get(username=username)
except User.DoesNotExist:
    return "User does not exist"

Correct Error Handling

try:
    user = User.objects.get(username=username)
except User.DoesNotExist:
    return "Invalid username or password"

Common Mistakes to Avoid

Not Patching Promptly

Delaying patches can leave your system vulnerable to attacks. Always apply security patches as soon as they are available.

Misconfiguring SSO

Improper configuration can introduce vulnerabilities. Follow best practices for setting up SSO.

Ignoring Logs

Neglecting to monitor logs can prevent you from detecting and responding to breaches promptly. Set up comprehensive logging and monitoring solutions.

Case Study: Real-World Impact

A large financial institution fell victim to CVE-2026-24858 due to delayed patching. Attackers gained unauthorized access to the SSO system, leading to a data breach affecting thousands of customers.

Conclusion

CVE-2026-24858 is a critical vulnerability in FortiOS SSO that requires immediate attention. By applying the patch, disabling unnecessary features, implementing network segmentation, enabling logging, reviewing access controls, and avoiding common mistakes, you can significantly reduce the risk of exploitation.

• Check if you're affected
• Update your FortiOS installation
• Review SSO configurations
• Implement network segmentation
• Enable logging and monitoring