Plone 4.1.3 and earlier computes hash values for form parameters without restricting the ability to trigger hash collisions predictably, which allows remote attackers to cause a denial of service (CPU consumption) by sending many crafted parameters.
Plone CMS versions 4.1.3 and earlier are vulnerable to a denial-of-service (DoS) attack due to a flaw in how they handle form parameter hashing. Attackers can exploit this by sending a large number of crafted parameters, causing excessive CPU usage and rendering the server unresponsive. This vulnerability poses a significant risk to the availability of Plone-based websites and applications.
Step 1: Payload Generation: The attacker crafts a malicious HTTP POST request. This request includes a large number of form parameters, each designed to hash to the same or a small set of hash values, causing collisions.
Step 2: Request Submission: The attacker sends the crafted HTTP POST request to a vulnerable Plone instance.
Step 3: Parameter Processing: The Plone server receives the request and begins processing the form parameters.
Step 4: Hash Calculation and Collision Detection: The server calculates the hash values for each parameter. Due to the crafted payload, a significant number of hash collisions occur.
Step 5: CPU Exhaustion: The server spends a large amount of CPU time comparing the colliding values, attempting to resolve the collisions. This process consumes significant resources.
Step 6: Denial of Service: The CPU exhaustion leads to a denial of service, making the Plone instance unresponsive to legitimate user requests.
The vulnerability stems from a weakness in Plone's handling of form parameters. Specifically, the software uses a hashing algorithm (likely a simple one or one with known weaknesses) to process form data. The lack of proper collision resistance in the hashing function allows attackers to craft inputs that generate a large number of hash collisions. When these collisions occur, the server spends excessive CPU cycles comparing the colliding values, leading to a denial-of-service (DoS) condition. The root cause is the predictable nature of the hash collisions, allowing attackers to pre-calculate and generate malicious payloads. This is a classic example of a hash collision DoS attack.