Understanding the Verizon Call Filter API Vulnerability

Understanding the Verizon Call Filter API Vulnerability

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The recent discovery of a vulnerability in the Verizon Call Filter API has raised significant concerns about data security and privacy. This flaw, uncovered by security researcher Evan Connelly, allowed unauthorized access to Verizon Wireless customers’ incoming call logs through an unsecured API request. The vulnerability was reported to Verizon on February 22, 2025, and acknowledged by the company two days later (BleepingComputer). The flaw was rooted in the backend API used by the Verizon Call Filter app, which failed to verify that the phone number requested for call history matched the authenticated user’s number. This oversight enabled attackers with a valid JSON Web Token (JWT) to manipulate request headers and retrieve call logs for any Verizon customer (CyberInsider).

Discovery of the Vulnerability

The Verizon Call Filter API vulnerability was discovered by independent security researcher Evan Connelly on February 22, 2025. This flaw allowed unauthorized access to Verizon Wireless customers’ incoming call logs through an unsecured API request. The vulnerability was reported to Verizon on the same day, and the company acknowledged the issue by February 24, 2025 (BleepingComputer). The flaw was subsequently fixed by March 25, 2025, although the exact date of patching remains unclear (CyberInsider).

Technical Details of the API Flaw

The vulnerability was rooted in the backend API used by the Verizon Call Filter app, which failed to verify that the phone number requested for call history matched the authenticated user’s number. This oversight allowed any attacker with a valid JSON Web Token (JWT) to manipulate the request header and retrieve call logs for any Verizon customer (CyberInsider).

To understand this better, think of a JWT as a digital ID card. Just like a security guard checks your ID to ensure you belong in a building, the API should check the JWT to ensure the request is legitimate. However, in this case, the API was like a guard who didn’t check if the ID matched the person entering.

The vulnerable endpoint, https://clr-aqx.cequintvzwecid.com/clr/callLogRetrieval, accepted a JWT in the Authorization header and a phone number in the X-Ceq-MDN header. Although the JWT’s payload included the authenticated user’s number as the sub (subject) field, the server did not enforce a check to ensure this matched the phone number being requested (BleepingComputer).

JWT and API Request Manipulation

A JWT typically consists of three parts: a header, a payload, and a signature. It is often used for authentication and authorization in web applications. In this case, the JWT payload included various data, including the phone number of the logged-in user making the request to the API. However, the phone number in the JWT payload was not verified against the phone number whose incoming call logs were being requested (BleepingComputer).

As a result, any user could send requests using their own valid JWT token but replace the X-Ceq-MDN header value with another Verizon phone number to retrieve their incoming call history. This flaw was particularly sensitive for high-value targets like politicians, journalists, and law enforcement agents, as their sources, contacts, and daily routines could be mapped out (BleepingComputer).

Role of Third-Party Vendor Cequint

The vulnerable API endpoint used by the Call Filter app was hosted on a server owned by a separate telecommunications technology firm called Cequint. Cequint specializes in caller identification services and was responsible for the infrastructure powering the Verizon Call Filter app (CyberInsider). The involvement of a third-party vendor like Cequint raises broader concerns about the role and accountability of such vendors in managing sensitive telecom infrastructure.

Cequint’s website is currently offline, and little public information is available about their data handling policies or security posture. This situation raises questions about the oversight of firms entrusted with access to high-value telecom data (BleepingComputer).

Security Implications and Recommendations

The Verizon Call Filter API flaw highlights the importance of implementing robust security measures to protect sensitive customer data. Call metadata, while seemingly harmless, can become a powerful surveillance tool in the wrong hands. With unrestricted access to another user’s call history, an attacker could reconstruct daily routines, identify frequent contacts, and infer personal relationships (BleepingComputer).

To prevent similar vulnerabilities in the future, it is crucial for companies to enforce strict access control measures and ensure that API requests are properly authenticated and authorized. Here are some recommendations:

  • Enforce Access Control: Ensure that API requests are authenticated and authorized.
  • Rate Limiting: Implement rate limiting to prevent mass data scraping.
  • Periodic Reviews: Regularly review app permissions and account settings to minimize data exposure.

In conclusion, the discovery and technical breakdown of the Verizon Call Filter API vulnerability underscore the need for continuous vigilance and improvement in cybersecurity practices to protect customer data and maintain trust in digital services.

Final Thoughts

The Verizon Call Filter API vulnerability serves as a stark reminder of the critical importance of robust security measures in protecting sensitive customer data. While Verizon has addressed the issue, the incident underscores the need for continuous vigilance and improvement in cybersecurity practices. Companies must enforce strict access control measures and ensure that API requests are properly authenticated and authorized to prevent similar vulnerabilities in the future (CyberInsider). Additionally, the role of third-party vendors like Cequint in managing sensitive telecom infrastructure highlights the broader concerns about accountability and oversight in the digital age (BleepingComputer).

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