CVE-2021-32697 in Forms
Summary
by MITRE • 06/22/2021
neos/forms is an open source framework to build web forms. By crafting a special `GET` request containing a valid form state, a form can be submitted without invoking any validators. Form state is secured with an HMAC that is still verified. That means that this issue can only be exploited if Form Finishers cause side effects even if no form values have been sent. Form Finishers can be adjusted in a way that they only execute an action if the submitted form contains some expected data. Alternatively a custom Finisher can be added as first finisher. This regression was introduced with https://github.com/neos/form/commit/049d415295be8d4a0478ccba97dba1bb81649567
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Analysis
by VulDB Data Team • 06/24/2021
The vulnerability identified as CVE-2021-32697 affects the Neos Forms open source framework, which is commonly used for building web forms in web applications. This security flaw represents a critical regression that undermines the framework's form validation mechanisms, potentially allowing unauthorized form submissions with bypassed validation checks. The vulnerability specifically targets the framework's handling of GET requests containing specially crafted form state parameters, creating a scenario where forms can be submitted without proper validation execution despite the presence of HMAC-based security measures.
The technical implementation of this vulnerability stems from a regression introduced in the Neos Forms framework through commit 049d415295be8d4a0478ccba97dba1bb81649567. This regression affects the form submission pipeline by allowing attackers to manipulate form state parameters in GET requests, effectively circumventing the normal validation flow. While the HMAC verification mechanism remains functional, it only validates the form state integrity rather than the actual submission content. The vulnerability becomes exploitable when form finishers are configured to perform side effects or actions based on form submissions, particularly when these finishers are designed to execute only when specific expected data is present in the form.
The operational impact of this vulnerability extends beyond simple form bypass scenarios, as it can enable attackers to trigger unintended actions within applications that rely on Neos Forms. When form finishers are configured to perform operations such as sending emails, creating database entries, or initiating external API calls, the vulnerability allows malicious actors to invoke these finishers without proper validation, potentially leading to data corruption, unauthorized actions, or resource exhaustion. The security implications are particularly concerning because the HMAC verification still occurs but provides no protection against this specific attack vector, as it only validates form state rather than the actual form content or submission parameters.
Organizations utilizing the Neos Forms framework should implement immediate mitigations to address this vulnerability, including updating to patched versions of the framework where available. The recommended approach involves reviewing form finisher configurations to ensure that critical operations are properly validated against expected form data before execution, rather than relying solely on the presence of form state parameters. Security practitioners should also consider implementing additional validation layers that verify the actual form content and submission parameters, particularly for finishers that perform sensitive operations. This vulnerability aligns with CWE-347, which addresses improper verification of cryptographic signatures, and represents a specific instance of how cryptographic verification can be bypassed when the validation scope is limited to form state rather than comprehensive submission content. The ATT&CK framework categorizes this as a privilege escalation or command execution technique, where attackers leverage framework vulnerabilities to perform unauthorized actions through form processing mechanisms, highlighting the need for comprehensive input validation and proper security design principles in web application frameworks.