Potentially Suspicious Azure Front Door Connection

Status: test
Severity: medium
Log source: product windows, category network_connection
Author: Isaac Dunham
Source: github.com/SigmaHQ/sigma

Detects connections with Azure Front Door (known legitimate service that can be leveraged for C2) that fall outside of known benign behavioral baseline (not using common apps or common azurefd.net endpoints)

MITRE ATT&CK coverage

Tactic	Techniques
Command & Control	`T1090.004` Proxy: Domain Fronting, `T1102.002` Web Service: Bidirectional Communication

Event coverage

Provider	Event	Title
Sysmon	Event ID 3	Network connection

Rule body yaml

title: Potentially Suspicious Azure Front Door Connection
id: 8cb4d14e-776e-43c2-8fb9-91e7fcea32b4
status: test
description: |
    Detects connections with Azure Front Door (known legitimate service that can be leveraged for C2)
    that fall outside of known benign behavioral baseline (not using common apps or common azurefd.net endpoints)
references:
    - https://lots-project.com/site/2a2e617a75726566642e6e6574
    - https://medium.com/r3d-buck3t/red-teaming-in-cloud-leverage-azure-frontdoor-cdn-for-c2-redirectors-79dd9ca98178
    - https://www.fortalicesolutions.com/posts/hiding-behind-the-front-door-with-azure-domain-fronting
author: Isaac Dunham
date: 2024-11-07
tags:
    - attack.command-and-control
    - attack.t1102.002
    - attack.t1090.004
    - detection.threat-hunting
logsource:
    category: network_connection
    product: windows
detection:
    selection:
        DestinationHostname|contains: 'azurefd.net'
    filter_main_web_browsers:
        Image|endswith:
            - 'brave.exe'
            - 'chrome.exe'
            - 'chromium.exe'
            - 'firefox.exe'
            - 'msedge.exe'
            - 'msedgewebview2.exe'
            - 'opera.exe'
            - 'vivaldi.exe'
    filter_main_common_talkers:
        Image|endswith: 'searchapp.exe' # Windows search service uses signifcant amount of Azure FD
    filter_main_known_benign_domains:
        DestinationHostname|contains:
            - 'afdxtest.z01.azurefd.net' # used by Cortana; Cisco Umbrella top 1m
            - 'fp-afd.azurefd.net' # used by Cortana; Cisco Umbrella top 1m
            - 'fp-afdx-bpdee4gtg6frejfd.z01.azurefd.net' # used by Cortana; Cisco Umbrella top 1m
            - 'roxy.azurefd.net' # used by Cortana; Cisco Umbrella top 1m
            - 'powershellinfraartifacts-gkhedzdeaghdezhr.z01.azurefd.net' # Used by VS Code; Cisco Umbrella top 1m
            - 'storage-explorer-publishing-feapcgfgbzc2cjek.b01.azurefd.net' # Used by Azure Storage Explorer; Cisco Umbrella top 1m
            - 'graph.azurefd.net' # MS Graph; Cisco Umbrella top 1m
    condition: selection and not 1 of filter_main_*
falsepositives:
    - Results are not inherently suspicious, but should be investigated during threat hunting for potential cloud C2.
    - Organization-specific Azure Front Door endpoints
level: medium

Stages and Predicates

Stage 0: `condition`

selection and not 1 of filter_main_*

Stage 1: `selection`

selection:
    DestinationHostname|contains: 'azurefd.net'

Stage 2: `not filter_main_*`

filter_main_web_browsers:
    Image|endswith:
        - 'brave.exe'
        - 'chrome.exe'
        - 'chromium.exe'
        - 'firefox.exe'
        - 'msedge.exe'
        - 'msedgewebview2.exe'
        - 'opera.exe'
        - 'vivaldi.exe'
filter_main_common_talkers:
    Image|endswith: 'searchapp.exe'
filter_main_known_benign_domains:
    DestinationHostname|contains:
        - 'afdxtest.z01.azurefd.net'
        - 'fp-afd.azurefd.net'
        - 'fp-afdx-bpdee4gtg6frejfd.z01.azurefd.net'
        - 'roxy.azurefd.net'
        - 'powershellinfraartifacts-gkhedzdeaghdezhr.z01.azurefd.net'
        - 'storage-explorer-publishing-feapcgfgbzc2cjek.b01.azurefd.net'
        - 'graph.azurefd.net'

Exclusions

Top-level NOT(...) conjuncts: predicates this rule actively suppresses.

Field	Kind	Excluded values
`DestinationHostname`	match	`afdxtest.z01.azurefd.net`
`DestinationHostname`	match	`fp-afd.azurefd.net`
`DestinationHostname`	match	`fp-afdx-bpdee4gtg6frejfd.z01.azurefd.net`
`DestinationHostname`	match	`graph.azurefd.net`
`DestinationHostname`	match	`powershellinfraartifacts-gkhedzdeaghdezhr.z01.azurefd.net`
`DestinationHostname`	match	`roxy.azurefd.net`
`DestinationHostname`	match	`storage-explorer-publishing-feapcgfgbzc2cjek.b01.azurefd.net`
`Image`	ends_with	`brave.exe`
`Image`	ends_with	`chrome.exe`
`Image`	ends_with	`chromium.exe`
`Image`	ends_with	`firefox.exe`
`Image`	ends_with	`msedge.exe`
`Image`	ends_with	`msedgewebview2.exe`
`Image`	ends_with	`opera.exe`
`Image`	ends_with	`searchapp.exe`
`Image`	ends_with	`vivaldi.exe`

Indicators

Each row is a field, operator, and value that the rule matches. The corpus column counts how many other rules in the catalog look for the same combination: high numbers point to widely-used, community-vetted indicators. Blank or 1 shows that the indicator is specific to this rule.

Field	Kind	Values
`DestinationHostname`	match	`azurefd.net`

`j` / `k`	Scroll down / up
`d` / `u`	Half-page down / up
`gg` / `G`	Top / bottom
`h` / `l`	History back / forward
`f`	Follow link (`Shift` = new tab)
`/`	Focus search
`?`	Toggle this help
`↑` / `↓`	Navigate search results
`Enter`	Open highlighted result
`Esc`	Close results / dialog

`type:`	`events` / `rules` / `providers`
`vendor:`	`sigma` / `elastic` / `splunk` / `kusto` / `chronicle` (vendor name alone also works: `sigma:`, `kql:`, `secops:`…)
`tactic:`	TA-id, slug, or name: `credential_access`, `TA0006`
`technique:`	technique or sub-technique ID: `T1003`, `T1003.001` (alias `tech:`)
`severity:`	`critical` / `high` / `medium` / `low` / `informational` (alias `sev:`)
`risk_score`	Numeric comparison on the Elastic risk score (0 to 100): `risk_score>50`, `risk_score<=20`, `risk_score=99` (alias `risk`; Elastic rules only)
`stages:`	Rules with exactly N pipeline stages
`correlation:`	`single_event` / `sequence` / `alternatives` / `alternatives_cross_log` / `all_required` / `correlated`
`with:`	Co-occurrence event-id; stacks (`with:4624 with:4769`) to require all, while a comma list in one occurrence (`with:4624,4769`) is an either-or group. Implies multi-event
`like:`	Structural neighbors of a rule slug (equivalents + subsumption stricter / broader): `like:comsvcs_lsass_memory_dump-splunk-sysmon`
`groupby:`	Entity-grouping substring match against `group_by_keys`: `groupby:user`, `groupby:host`
`uses:`	Rules whose predicate tree touches the field (any kind, any value): `uses:CommandLine`
`excludes:`	Rules with top-level `not()` clauses on the field (FP whitelists): `excludes:ParentImage`
`field:` / `value:`	Predicate search; narrows rule cards to those with a matching leaf and drives the indicator tier. Unquoted = substring, wildcards allowed (`value:mimikatz`)
`indicator:`	Shorthand for `field:F value:V`: `indicator:Image=*\powershell.exe`
`kind:`	Filter by predicate kind. Narrows rule cards to those carrying a matching predicate leaf (`vendor:elastic kind:cidr_match`) and drives the indicator tier: `contains` / `starts_with` / `ends_with` / `regex` / `cidr` / `eq` / `in` … (operator aliases `op:`/`match:`)
`has:` / `no:`	`sample`, `field`, `notes`, `refs`, `trace`, `thirdparty`, `rule`, `pattern`, `timewindow`, `threshold`, `newterms`, `sigma`/`elastic`/`splunk`/`kusto`/`chronicle`
`-op:val`	Exclude matches; works on most operators but not `type:`/`like:`/`has:`/`no:` (use `no:<flag>` to exclude a rule flag): `tactic:execution -vendor:splunk`. Standalone `-kind:`/`-field:`/`-value:` drop every rule carrying a matching predicate leaf (`type:rules -kind:is_null`)
`field:"…"` / `value:"…"`	Quoted value = anchored exact match (also allows spaces): `value:"net user"`
`a,b`	Comma = OR inside one operator (`vendor:sigma,elastic`, `severity:high,critical`); repeating a facet merges the same way. `field:`/`value:` never split (literal commas)
`vendors:` / `stage:`	Singular and plural spellings fold to the canonical operator and value: `tactics:` = `tactic:`, `type:event` = `type:events`, `correlation:sequences` = `correlation:sequence`, `has:thresholds` = `has:threshold`
`"quoted phrase"`	Exact-match a multi-word phrase (free text)

Potentially Suspicious Azure Front Door Connection

MITRE ATT&CK coverage

Event coverage

Rule body yaml

Stages and Predicates

Stage 0: `condition`

Stage 1: `selection`

Stage 2: `not filter_main_*`

Exclusions

Indicators

Keyboard shortcuts

Search operators

Potentially Suspicious Azure Front Door Connection

MITRE ATT&CK coverage

Event coverage

Rule body yaml

Stages and Predicates

Stage 0: condition

Stage 1: selection

Stage 2: not filter_main_*

Exclusions

Indicators

Stage 0: `condition`

Stage 1: `selection`

Stage 2: `not filter_main_*`