Password Reset Vulnerability to Full Account Takeover

Introduction:

I found a full unauthenticated Account Takeover (ATO) on a live booking/CRM platform. To maintain responsible disclosure for remaining adjacent issues, the target will be referred to throughout this write-up as xyz.com.

The vulnerability stems from a critical logic flaw: the password reset endpoint returned a newly generated temporary password straight back to the client within a POST field named password_to_print. Consequently, any attacker capable of triggering a reset for a known target email could extract the plaintext credentials directly out of the server's response stream—completely bypassing the need for inbox access.

Upon reporting, the process proved challenging. It required navigating two rejected triage submissions and tracking down a direct corporate email channel before the flaw was silently resolved without acknowledgment or credit.

Vulnerability Definition:

Account Takeover via Client-Side Credential Generation in a Password Reset Flow. The underlying bug is not a typical server misconfiguration. It represents a fundamental architectural anti-pattern: executing password-hashing procedures inside the client browser rather than securely on the server. Because the server expects a pre-computed hash from the client, the plaintext temporary credential must exist within the client-side state at some point in the execution flow. Once a sensitive cryptographic secret enters an execution space fully controlled by an external actor (such as an intercepting proxy or browser DOM), it must be considered compromised.

Root Cause Analysis:

The application's password reset form located at /mobile/pwdrst.php routes submission actions to /login/process_login.php. During this sequence, the interface loads a client-side execution script at /login/sha512.js. This script calculates the target cryptographic hash and passes the data into a custom execution block:

<input type='hidden' name='password' id='password' class='login_input_text'/>
<input type='submit' value='Login' onclick='formhash(this.form, this.form.password);' class='login_button_input' />

<input type='hidden' name='password' id='password' class='login_input_text'/>
<input type='submit' value='Login' onclick='formhash(this.form, this.form.password);' class='login_button_input' />

Use code with caution.

Because the temporary password is not generated, hashed securely server-side, and dispatched purely out-of-band, the backend passes the raw value to the client framework to handle local hashing steps. This architecture forces the plaintext string to sit inside a hidden document object model input element (password_to_print), leaking it clearly into the outgoing response and subsequent POST request headers.

This highlights a common security mistake: treating the user's browser web browser as a verified, trusted execution environment. Client-side processing does not obfuscate secrets from the entity supervising the endpoint framework.

Detection Methodology:

Identifying this flaw requires no advanced scanning instrumentation:

1. Trigger the standard password reset sequence for an identified target email address.
2. Intercept the resulting downstream network response data using a proxy like Burp Suite, or inspect the direct rendering document source (view-source:).
3. Scan the parameters for hidden inputs or script-calculated values operating parallel to the standard password submission parameters (e.g., password_to_print, formhash(), _hash, _temp).

Exploitation Vector:

1. Identify a valid target email address (or leverage administrative account naming conventions).
2. Dispatch a standard reset request targeting the designated account.
3. Capture the downstream response package to isolate and extract the plaintext entry inside the password_to_print variable parameter.
4. Immediately authenticate using the intercepted temporary credential string, completing a full account takeover before the legitimate account owner accesses their email notification.

Remediation Blueprint:

1. Identify a valid target email address (or leverage administrative account naming conventions).
2. Dispatch a standard reset request targeting the designated account.
3. Capture the downstream response package to isolate and extract the plaintext entry inside the password_to_print variable parameter.
4. Immediately authenticate using the intercepted temporary credential string, completing a full account takeover before the legitimate account owner accesses their email notification.

Approximately two months following direct escalation, the platform modified its application flow to implement an email-delivered link design matching these recommendations.

The Disclosure Timeline:

• Initial Report: Submitted comprehensive technical analysis to a third-party bug bounty triage platform. Closed as Incapable of Reproduction.
• Secondary Appeal: Resubmitted with a refined proof-of-concept walkthrough. Closed under the same status.
• Direct Escalation: Bypassed the platform to message the vendor's primary engineering support queue with raw logs, a clear replication manual, and remediation guidance. Received no direct reply.
• Verification Audit: Re-tested the target endpoint approximately 60 days later to confirm the legacy logic workflow had been replaced by a link-validation architecture.

The Canary: Evaluating Patch Thoroughness:

During initial target scoping, a secondary user enumeration vulnerability was observed via highly verbose, differential authentication error responses on the login page. When testing the application's interface, an invalid user entry returned a distinct warning explicitly stating: Your Email is not Registered - Please Register.

I judged this finding as a lower-severity business logic issue at the time and intentionally withheld it from the original disclosure report to serve as a telemetry control marker.

That choice turned out to be incredibly useful. Since I had never notified them about it, it became a clean test of whether the eventual fix came from a thorough, comprehensive security review or a narrow patch applied directly to the specific endpoint I had exposed.

To confirm whether the user enumeration bug still functioned — and to validate the concept practically — I followed a standard credential-guessing convention based on public domain infrastructure. Just as large enterprise platforms often utilize predictable aliases like support@github.com, most modern business portals spin up localized administrative routes using standard prefixes attached to their unique web host domain.

By analyzing a localized alert string on the layout stating Cookies must be on to use [domain], I inferred the structure and tested a common target alias format: admin@xyz.com.

Upon submission, the authentication module shifted its state and returned a different message: Wrong Password. This verbose handling confirmed that the guessed account infrastructure was valid and active on the database, explicitly narrowing down an attacker's objective solely to password cracking or credential stuffing loops.

When I re-tested the environment following their deployment patch, the original credential leak in the reset flow was resolved, but the login form continued to explicitly distinguish an unregistered username from a valid account with a mismatching password. They addressed only the isolated variables exposed in the direct report, rather than a broad structural audit of the identity access management stack.

Key Takeaways for Researchers:

• Audit Peripheral Flows: Never skip standard functional elements like account recovery mechanisms during scoping exercises. These areas frequently escape heavy engineering oversight compared to primary entry components.
• Client-Side Limits: Client-side cryptography operating on data the client already possesses offers zero protection.
• Anticipate Silent Fixes: Unacknowledged, silent patch deployments are common across independent disclosure paths. Maintain local copies of verification logs, transmission timelines, and replication proofs.