A flaw in Verizon’s iOS Call Filter app exposed call records of millions, revealing a severe vulnerability in a widely used telecom application. Discovered by security researcher Evan Connelly and reported to Verizon on February 22, 2025, this flaw allowed unauthorized access to incoming call metadata via an insecure API endpoint. Patched in mid-March, the exposure window left millions of Verizon Wireless customers at risk of surveillance and privacy breaches. This post dives into the technical underpinnings of the vulnerability, detection methods, and mitigation strategies for SOC analysts, threat hunters, and security engineers.
Table of Contents
- Technical Analysis of the Verizon Call Filter Flaw
- Detection Strategies for Exposed Call Records
- Mitigation Techniques for API Vulnerabilities
Technical Analysis of the Verizon Call Filter Flaw
The vulnerability resided in the Verizon Call Filter app’s backend API, specifically the endpoint https://clr-aqx.cequintvzwecid.com/clr/callLogRetrieval. This endpoint, managed by third-party provider Cequint, failed to enforce proper authorization checks. By crafting a request with a valid JSON Web Token (JWT) and an arbitrary phone number in the X-Ceq-MDN header, attackers could retrieve incoming call logs—including phone numbers and timestamps—for any Verizon customer.
How the Flaw Worked
The API’s design flaw allowed attackers to bypass identity verification. A legitimate user’s JWT, obtained through standard app authentication, could be repurposed to query call logs for unrelated phone numbers. Below is an example of a malicious HTTP request:
GET /clr/callLogRetrieval HTTP/1.1
Host: clr-aqx.cequintvzwecid.com
Authorization: Bearer eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9...
X-Ceq-MDN: 555-123-4567
The response would return a JSON payload containing call metadata, exposing patterns that could be cross-referenced with OSINT to map victim behaviors. While no evidence of exploitation surfaced, the lack of rate limiting suggests mass scraping was feasible.
Attack Surface Implications
This flaw in Verizon’s iOS Call Filter app exposed call records to potential abuse by nation-state actors, cybercriminals, or even domestic adversaries. For high-value targets—journalists, law enforcement, or abuse survivors—this metadata could enable physical tracking or social engineering. The involvement of Cequint, a third-party vendor, underscores the risks of outsourced telecom infrastructure.
Detection Strategies for Exposed Call Records
Detecting exploitation of this vulnerability requires monitoring for anomalous API requests and correlating them with user activity. Below are actionable detection methods tailored for SOC teams and threat hunters.
Network Traffic Analysis with Wireshark
Filter for requests to the vulnerable endpoint using this Wireshark display filter:
http.request.method == "GET" && http.host == "clr-aqx.cequintvzwecid.com" && http.request.uri contains "/clr/callLogRetrieval"
Look for discrepancies between the JWT’s subject claim (sub) and the X-Ceq-MDN header value. A mismatch indicates potential abuse.
Sigma Rule for SIEM Detection
Deploy this Sigma rule in your SIEM to flag suspicious API calls:
title: Suspicious Verizon Call Filter API Request
id: 8f3b2c1a-4e5d-11ec-81d3-0242ac130003
description: Detects potential abuse of Verizon Call Filter API for unauthorized call log retrieval
logsource:
category: network
product: firewall
detection:
selection:
http_method: "GET"
http_host: "clr-aqx.cequintvzwecid.com"
http_uri: "/clr/callLogRetrieval"
http_header|X-Ceq-MDN: "*"
condition: selection
fields:
- src_ip
- http_header|Authorization
- http_header|X-Ceq-MDN
level: high
This rule triggers on any GET request to the endpoint, enabling analysts to investigate the source IP and header values.
PowerShell Script for Log Analysis
Use this PowerShell script to parse proxy logs for signs of exploitation:
$logs = Import-Csv -Path "proxy_logs.csv"
$flagged = $logs | Where-Object {
$_.Method -eq "GET" -and
$_.Host -eq "clr-aqx.cequintvzwecid.com" -and
$_.Uri -match "/clr/callLogRetrieval" -and
$_.Headers -match "X-Ceq-MDN"
}
$flagged | Export-Csv -Path "suspicious_verizon_api_calls.csv" -NoTypeInformation
Adjust the input file path and column names based on your log format.
Mitigation Techniques for API Vulnerabilities
Preventing similar flaws requires robust API security controls. Below are configuration recommendations and best practices for security engineers.
API Authentication Hardening
Enforce strict validation between the JWT and request parameters. Update the API to reject requests where the X-Ceq-MDN value doesn’t match the JWT’s authenticated phone number. Example pseudocode for server-side validation:
if (jwt.claims.phone_number != request.headers["X-Ceq-MDN"]) {
return HTTP 403 Forbidden
}
Firewall Configuration
Deploy a Web Application Firewall (WAF) rule to block mismatched requests:
SecRule REQUEST_HEADERS:X-Ceq-MDN "!@eq %{AUTHENTICATED_USER_PHONE}" \
"id:10001,phase:2,block,msg:'Unauthorized Verizon API Access Attempt'"
This ModSecurity rule assumes the authenticated phone number is available as a variable.
Third-Party Vendor Oversight
The flaw in Verizon’s iOS Call Filter app exposed call records partly due to Cequint’s infrastructure. Enterprises must audit third-party APIs for compliance with NIST SP 800-53 controls, particularly SA-9 (External System Services). Require vendors to implement rate limiting and logging per MITRE ATT&CK T1190.
Endpoint Monitoring
Enable verbose logging on API gateways to capture all X-Ceq-MDN and JWT pairs. Integrate with a SIEM for real-time alerting on anomalies, reducing the window of exposure.
For more on securing server-side applications, see our Server Hardening Tips and Threat Detection Guide.
Conclusion
The flaw in Verizon’s iOS Call Filter app exposed call records of millions, highlighting the fragility of telecom APIs and the risks of third-party dependencies. While patched, the incident serves as a wake-up call for organizations to harden API endpoints, monitor for abuse, and enforce strict access controls. Leverage the detection rules and mitigation strategies outlined here to safeguard your infrastructure. Stay vigilant—explore our Zero-Day Response Plan for proactive defense tactics.
Tools like Wireshark and frameworks like MITRE ATT&CK remain essential for dissecting and defending against such threats.