Microsoft IIS Connection Strings Decryption

Status: test
Severity: high
Log source: category process_creation, product windows
Author: Tim Rauch, Elastic (idea)
Source: github.com/SigmaHQ/sigma

Detects use of aspnet_regiis to decrypt Microsoft IIS connection strings. An attacker with Microsoft IIS web server access via a webshell or alike can decrypt and dump any hardcoded connection strings, such as the MSSQL service account password using aspnet_regiis command.

MITRE ATT&CK coverage

Tactic	Techniques
Credential Access	`T1003` OS Credential Dumping

Telemetry coverage

Provider	Record / event type
Sysmon	Event ID 1: Process creation

Rule body

title: Microsoft IIS Connection Strings Decryption
id: 97dbf6e2-e436-44d8-abee-4261b24d3e41
status: test
description: Detects use of aspnet_regiis to decrypt Microsoft IIS connection strings. An attacker with Microsoft IIS web server access via a webshell or alike can decrypt and dump any hardcoded connection strings, such as the MSSQL service account password using aspnet_regiis command.
references:
    - https://www.elastic.co/guide/en/security/current/microsoft-iis-connection-strings-decryption.html
author: Tim Rauch, Elastic (idea)
date: 2022-09-28
modified: 2022-12-30
tags:
    - attack.credential-access
    - attack.t1003
logsource:
    category: process_creation
    product: windows
detection:
    selection_name:
        - Image|endswith: '\aspnet_regiis.exe'
        - OriginalFileName: 'aspnet_regiis.exe'
    selection_args:
        CommandLine|contains|all:
            - 'connectionStrings'
            - ' -pdf'
    condition: all of selection*
falsepositives:
    - Unknown
level: high

Stages and Predicates

Stage 0: `condition`

all of selection*

Stage 1: `selection_name`

selection_name:
    - Image|endswith: '\aspnet_regiis.exe'
    - OriginalFileName: 'aspnet_regiis.exe'

Stage 2: `selection_args`

selection_args:
    CommandLine|contains|all:
        - 'connectionStrings'
        - ' -pdf'

Indicators

These rows show field, operator, and value matches.

Field	Kind	Values	Search
`CommandLine`	match	`-pdf` `connectionStrings`	`field:"CommandLine" kind:match`
`Image`	ends_with	`\aspnet_regiis.exe`	`field:"Image" kind:ends_with value:"\aspnet_regiis.exe"`
`OriginalFileName`	eq	`aspnet_regiis.exe` corpus 3 (elastic 2, sigma 1)	`field:"OriginalFileName" kind:eq value:"aspnet_regiis.exe"`