> For the complete documentation index, see [llms.txt](https://cortex-docs.paloaltonetworks.com/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://cortex-docs.paloaltonetworks.com/analytics-alerts/alerts-by-name/internal-address-failed-authentication.md).

# Internal address failed authentication

### Synopsis

| Field                | Value                                         |
| -------------------- | --------------------------------------------- |
| Activation Period    | 14 Days                                       |
| Training Period      | 30 Days                                       |
| Test Period          | N/A (single event)                            |
| Deduplication Period | 1 Hour 30 Minutes                             |
| Required Data        | Microsoft 365 Emails                          |
| Detection Modules    | Email                                         |
| Detector Tags        | Spoofing                                      |
| ATT\&CK Tactic       | Stealth (TA0005)                              |
| ATT\&CK Technique    | Social Engineering: Impersonation (T1684.001) |
| Severity             | Informational                                 |

### Description

An email message with a sender address matching an internal domain has failed authentication checks, while sender behavior does not match known legitimate infrastructure patterns. This may indicate internal domain impersonation or an unrecognized sending source.

### Attacker's Goals

Impersonate internal users or familiar individuals.

* Trick users into clicking on malicious links or attachments.

### Investigative actions

* Check the email address for any unusual spellings.
* Check the email address for any missing letters.
* Verify the sender's domain to confirm its legitimacy.
* Examine the sender's IP address and reputation.
* Verify the domain's connection to the IP address and investigate the reason SPF didn't pass.
* Verify whether the sender's IP address has appeared in different log sources before.
* Verify if the sender's IP address is identifiable.
* If the message contains attachments or links, scrutinize them for any suspicious indications.
* Monitor further actions taken, such as file downloads or access to potentially malicious links.


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