Inconsistent handling of exceptional input

 This vulnerability demonstrates a case of inconsistent handling of exceptional input, where different components of the application process unusually large input values differently, leading to a mismatch in how user identity is stored, validated, and later trusted for authorization.

The attack begins by discovering a hidden administrative endpoint (/admin) through content discovery tools. Direct access to this endpoint is blocked, but the response reveals an important condition: only users with a specific corporate email domain are allowed access. This indicates that administrative authorization is based on email address matching rather than a proper role-based access control system.

The registration process suggests that employees should use a company email address, but it does not strictly enforce this rule. At this stage, the system already relies on email-based assumptions for identity. However, the key vulnerability is revealed when testing how the system handles exceptionally long input values during registration.

By submitting an email address that exceeds typical length constraints (over 200 characters), the application still accepts the input and processes account creation. The user then receives a confirmation email through the lab’s email system, confirming that the backend successfully handled the oversized input without rejecting it. However, when viewing the account details, the system truncates the stored email address to a maximum length of 255 characters. This indicates that different parts of the system handle input length differently: the registration process accepts long input, but the storage layer enforces a fixed limit through truncation rather than validation.

This inconsistency becomes exploitable in the second step. A new account is created using a carefully crafted long email address that includes the domain dontwannacry.com placed within a larger string. Due to the 255-character truncation limit, the backend cuts off the email at a fixed boundary. Because of how the string is structured, the truncated result still leaves the visible stored email appearing as if it belongs to the trusted @dontwannacry.com domain.

This means that although the user never legitimately owns or controls a real corporate email, the system mistakenly interprets the truncated version as valid. The registration confirmation email is still delivered successfully, which further reinforces the illusion that the account is legitimate. This inconsistency between validation, storage, and display allows the attacker to bypass domain-based access restrictions.

After logging in with the manipulated account, the application incorrectly grants access to the admin panel because it relies on the stored email value for authorization decisions. Since the stored (truncated) email appears to match the trusted domain condition, the user is treated as an internal employee. This grants access to administrative functionality, including the ability to delete other users, such as “carlos,” completing the lab.

The root cause of this vulnerability is inconsistent enforcement of input validation and storage constraints across different system components. The application fails to properly validate email structure at the time of registration and instead relies on truncation behavior at the database or application layer. This creates a mismatch between what the user submits, what is stored, and what is later used for security decisions.

A secure implementation would enforce strict validation before storage, ensuring that email addresses conform to expected formats and length limits without relying on truncation. Additionally, authorization should never depend on string matching of user-controlled fields like email addresses. Instead, access control should be based on server-side roles or permissions that cannot be influenced by input manipulation.

This vulnerability highlights how edge cases in input handling—especially oversized or unexpected data—can lead to serious security failures when different layers of an application apply inconsistent rules.