> 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/unknown-dll-was-added-to-the-ad-fs-global-assembly-cache-path.md).

# Unknown DLL was added to the AD FS Global Assembly Cache path

### Synopsis

| Field                | Value                                          |
| -------------------- | ---------------------------------------------- |
| Activation Period    | 14 Days                                        |
| Training Period      | 30 Days                                        |
| Test Period          | N/A (single event)                             |
| Deduplication Period | 1 Day                                          |
| Required Data        | XDR Agent with eXtended Threat Hunting (XTH)   |
| Detection Modules    | Identity Analytics                             |
| Detector Tags        | Active Directory Federation Services Analytics |
| ATT\&CK Tactic       | Defense Impairment (TA0112)                    |
| ATT\&CK Technique    | Modify Authentication Process (T1556)          |
| Severity             | Informational                                  |

### Description

A new and unknown DLL was created within the Active Directory Federation Services (AD FS) Global Assembly Cache (GAC). Attackers may manipulate IdentityServer adapters to achieve persistence or execute malicious code within the AD FS environment.

### Attacker's Goals

Attackers may inject malicious code into the AD FS server and manipulate the IdentityServer adapters to gain persistence.

### Investigative actions

* Check if the AD FS service was stopped or restarted around the time of modification.
* Identify the user or process responsible for the file creation.
* Verify if the DLL is digitally signed by Microsoft.
* Compare the modification timestamp of this DLL against others in the same directory.

### Variations

<details>

<summary>Suspicious DLL was added to the AD FS Global Assembly Cache path</summary>

**Synopsis**

| Field             | Value                                 |
| ----------------- | ------------------------------------- |
| ATT\&CK Tactic    | Defense Impairment (TA0112)           |
| ATT\&CK Technique | Modify Authentication Process (T1556) |
| Severity          | Low                                   |

**Description**

A new and unknown DLL was created within the Active Directory Federation Services (AD FS) Global Assembly Cache (GAC). Attackers may manipulate IdentityServer adapters to achieve persistence or execute malicious code within the AD FS environment.

**Attacker's Goals**

Attackers may inject malicious code into the AD FS server and manipulate the IdentityServer adapters to gain persistence.

**Investigative actions**

* Check if the AD FS service was stopped or restarted around the time of modification.
* Identify the user or process responsible for the file creation.
* Verify if the DLL is digitally signed by Microsoft.
* Compare the modification timestamp of this DLL against others in the same directory.

</details>


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