---
title: "T-Mobile security incident"
description: "What the retained public evidence establishes about the T-Mobile incident, its attack path, impact, and response."
incident_type: "Cybersecurity incident"
status: "active"
last_modified: "2026-09-19"
canonical_url: "https://www.ally.security/incidents/t-mobile-security-incident-2021"
markdown_url: "https://www.ally.security/incidents/t-mobile-security-incident-2021.md"
stix_url: "https://www.ally.security/incidents/t-mobile-security-incident-2021/stix.json"
---

# T-Mobile security incident

API abuse requiring no login. 37M customer accounts' basic data exposed.

Last modified Sep 19, 2026 · 1 source

## Summary

- **Environment:** Telecom
- **Operational impact:** 37M customer accounts' basic data exposed
- **Financial impact:** No financial figure is established by the reviewed source evidence

## What happened

In 2023, T-Mobile experienced an incident in its telecom environment. The retained source describes the attack path as follows: API abuse requiring no login. [1](#source-1)

The documented consequence was: 37M customer accounts' basic data exposed. [1](#source-1)

## Impact

- Documented impact: 37M customer accounts' basic data exposed. [1](#source-1)
- No financial loss, ransom amount, recovery cost, or regulatory penalty is established by the reviewed source evidence.

## Threat Group & Attack Vector

The retained source describes the attack path as follows: API abuse requiring no login. The canonical record does not add intrusion steps beyond those supported by the source. [1](#source-1)

### Actors

- No threat actor group has been identified in the reviewed public evidence.

### TTPs

- No specific MITRE ATT\&CK technique is currently mapped for this case.

## Response

The retained source describes a forensic or specialist investigation of the incident. [1](#source-1)

This account is bounded to [T-Mobile US Form 10-K](https://www.sec.gov/Archives/edgar/data/1283699/000128369922000018/tmus-20211231.htm). Details absent from that evidence are left unresolved rather than inferred. [1](#source-1)

## Assets

[Download the case-scoped STIX 2.1 bundle](<https://www.ally.security/incidents/t-mobile-security-incident-2021/stix.json>)

## Sources

Primary source records used to research this incident.

<a id="source-1"></a>

### T-Mobile US Form 10-K

regulatory · U.S. Securities and Exchange Commission

<https://www.sec.gov/Archives/edgar/data/1283699/000128369922000018/tmus-20211231.htm>

<details>
<summary>Evidence ledger</summary>

Review the supporting structured claims.

1. **Resulted In · 90% confidence · current**  
   T-Mobile security incident: The retained source describes a forensic or specialist investigation of the incident.
2. **Resulted In · 83% confidence · current**  
   T-Mobile security incident: 37M customer accounts' basic data exposed.
3. **Resulted In · 100% confidence · current**  
   T-Mobile security incident: The reviewed source documents the t-mobile security incident involving T-Mobile.
4. **Resulted In · 84% confidence · current**  
   T-Mobile security incident: API abuse requiring no login.
5. **Affected Organization · 100% confidence · current**  
   T-Mobile security incident: T-Mobile

</details>