Adobe Experience Manager (AEM) is a comprehensive content management solution for building websites, mobile apps, and forms. However, like any powerful tool, it can be vulnerable to specific security issues if not properly configured. This blog post discusses a vulnerability related to the AEM instance's cache invalidation mechanism, how attackers can exploit it, and how to secure your AEM instance.

What is the Issue?

An improperly configured AEM instance can allow unauthorized users to invalidate cached pages. This vulnerability stems from the absence of the /allowedClients property in the dispatcher configuration, which should restrict which clients can flush the cache. Without this restriction, attackers can repeatedly flush the cache, severely impacting site performance.

How to Discover the Vulnerability

The vulnerability can be identified by accessing a specific URL that triggers cache invalidation. For example, visiting https://example.com/dispatcher/invalidate.cache can show whether the cache invalidation endpoint is exposed and unprotected.

Using a simple curl command, an attacker can exploit the vulnerability to flush the cache:

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curl -H "CQ-Handle: /content" -H "CQ-Path: /content" https://example.com/dispatcher/invalidate.cache

Why is This a Problem?

Allowing unauthorized users to invalidate the cache can lead to significant performance issues:

• Performance Degradation: Frequent cache invalidation forces the server to regenerate content dynamically, increasing load and response times.
• Denial of Service (DoS): Continuous cache invalidation requests can overwhelm the server, leading to downtime or severely degraded performance.
• Resource Exhaustion: Increased server resource usage due to repeated cache regeneration can lead to exhaustion of server resources.

How to Fix It

To mitigate this vulnerability, it is essential to properly configure the dispatcher settings to restrict cache invalidation to authorized clients only. This involves setting the /allowedClients property in the dispatcher configuration.

Example Configuration

Add the /allowedClients property to the dispatcher configuration to specify which clients are allowed to perform cache invalidation:

/allowedClients {

    /0000 {

        /glob "*"

        /type "deny"

    }

    /0001 {

        /glob "127.0.0.1"

        /type "allow"

    }

    /0002 {

        /glob "<trusted_ip_range>"

        /type "allow"

    }

}

References

• AEM Security on Twitter

Impact

Without proper configuration, unauthorized attackers can remotely invalidate the dispatcher cache without any rate limiting. This can be exploited to:

• Disrupt Site Performance: Frequent cache invalidation can significantly degrade site performance.
• Potential Denial of Service (DoS): Repeated exploitation can lead to server overload, causing downtime or reduced availability.
• Increased Server Load: Regenerating content dynamically for every request can lead to high server load and resource exhaustion.

Conclusion

Ensuring the security of your AEM instance involves properly configuring the dispatcher to restrict cache invalidation access. By defining the /allowedClients property, you can prevent unauthorized users from exploiting this vulnerability and safeguard your site’s performance and availability. Regularly reviewing and updating your security configurations is crucial to maintaining a secure and efficient AEM environment.

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