Suspicious Curl.EXE Download

Status: test
Severity: high
Log source: product windows, category process_creation
Author: Florian Roth (Nextron Systems), Nasreddine Bencherchali (Nextron Systems)
Source: github.com/SigmaHQ/sigma

Detects a suspicious curl process start on Windows and outputs the requested document to a local file

MITRE ATT&CK coverage

Tactic	Techniques
Command & Control	`T1105` Ingress Tool Transfer

Event coverage

Provider	Event	Title
Sysmon	Event ID 1	Process creation

Rule body yaml

title: Suspicious Curl.EXE Download
id: e218595b-bbe7-4ee5-8a96-f32a24ad3468
related:
    - id: bbeaed61-1990-4773-bf57-b81dbad7db2d # Basic curl execution
      type: derived
    - id: 9a517fca-4ba3-4629-9278-a68694697b81 # Curl download
      type: similar
status: test
description: Detects a suspicious curl process start on Windows and outputs the requested document to a local file
references:
    - https://twitter.com/max_mal_/status/1542461200797163522
    - https://web.archive.org/web/20200128160046/https://twitter.com/reegun21/status/1222093798009790464
    - https://github.com/pr0xylife/Qakbot/blob/4f0795d79dabee5bc9dd69f17a626b48852e7869/Qakbot_AA_23.06.2022.txt
    - https://www.volexity.com/blog/2022/07/28/sharptongue-deploys-clever-mail-stealing-browser-extension-sharpext/
    - https://github.com/redcanaryco/atomic-red-team/blob/0f229c0e42bfe7ca736a14023836d65baa941ed2/atomics/T1105/T1105.md#atomic-test-18---curl-download-file
author: Florian Roth (Nextron Systems), Nasreddine Bencherchali (Nextron Systems)
date: 2020-07-03
modified: 2023-02-21
tags:
    - attack.command-and-control
    - attack.t1105
logsource:
    category: process_creation
    product: windows
detection:
    selection_curl:
        - Image|endswith: '\curl.exe'
        - Product: 'The curl executable'
    selection_susp_locations:
        CommandLine|contains:
            - '%AppData%'
            - '%Public%'
            - '%Temp%'
            - '%tmp%'
            - '\AppData\'
            - '\Desktop\'
            - '\Temp\'
            - '\Users\Public\'
            - 'C:\PerfLogs\'
            - 'C:\ProgramData\'
            - 'C:\Windows\Temp\'
    selection_susp_extensions:
        CommandLine|endswith:
            - '.dll'
            - '.gif'
            - '.jpeg'
            - '.jpg'
            - '.png'
            - '.temp'
            - '.tmp'
            - '.txt'
            - '.vbe'
            - '.vbs'
    filter_optional_git_windows:
        # Example FP
        #   CommandLine: "C:\Program Files\Git\mingw64\bin\curl.exe" --silent --show-error --output C:/Users/test/AppData/Local/Temp/gfw-httpget-jVOEoxbS.txt --write-out %{http_code} https://gitforwindows.org/latest-tag.txt
        ParentImage: 'C:\Program Files\Git\usr\bin\sh.exe'
        Image: 'C:\Program Files\Git\mingw64\bin\curl.exe'
        CommandLine|contains|all:
            - '--silent --show-error --output '
            - 'gfw-httpget-'
            - 'AppData'
    condition: selection_curl and 1 of selection_susp_* and not 1 of filter_optional_*
falsepositives:
    - Unknown
level: high
regression_tests_path: regression_data/rules/windows/process_creation/proc_creation_win_curl_susp_download/info.yml
simulation:
    - type: atomic-red-team
      name: Curl Download File
      technique: T1105
      atomic_guid: 2b080b99-0deb-4d51-af0f-833d37c4ca6a

Stages and Predicates

Stage 0: `condition`

selection_curl and 1 of selection_susp_* and not 1 of filter_optional_*

Stage 1: `selection_curl`

selection_curl:
    - Image|endswith: '\curl.exe'
    - Product: 'The curl executable'

Stage 2: `selection_susp_locations`

selection_susp_locations:
    CommandLine|contains:
        - '%AppData%'
        - '%Public%'
        - '%Temp%'
        - '%tmp%'
        - '\AppData\'
        - '\Desktop\'
        - '\Temp\'
        - '\Users\Public\'
        - 'C:\PerfLogs\'
        - 'C:\ProgramData\'
        - 'C:\Windows\Temp\'

Stage 3: `selection_susp_extensions`

selection_susp_extensions:
    CommandLine|endswith:
        - '.dll'
        - '.gif'
        - '.jpeg'
        - '.jpg'
        - '.png'
        - '.temp'
        - '.tmp'
        - '.txt'
        - '.vbe'
        - '.vbs'

Stage 4: `not filter_optional_git_windows`

filter_optional_git_windows:
    ParentImage: 'C:\Program Files\Git\usr\bin\sh.exe'
    Image: 'C:\Program Files\Git\mingw64\bin\curl.exe'
    CommandLine|contains|all:
        - '--silent --show-error --output '
        - 'gfw-httpget-'
        - 'AppData'

Exclusions

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

Field	Kind	Excluded values
`CommandLine`	match	`--silent --show-error --output`
`CommandLine`	match	`AppData`
`CommandLine`	match	`gfw-httpget-`
`Image`	eq	`C:\Program Files\Git\mingw64\bin\curl.exe`
`ParentImage`	eq	`C:\Program Files\Git\usr\bin\sh.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
`CommandLine`	ends_with	`.dll` corpus 7 (sigma 7) `.gif` corpus 3 (sigma 3) `.jpeg` corpus 3 (sigma 3) `.jpg` corpus 2 (sigma 2) `.png` corpus 3 (sigma 3) `.temp` corpus 2 (sigma 2) `.tmp` corpus 2 (sigma 2) `.txt` corpus 4 (sigma 4) `.vbe` corpus 7 (sigma 7) `.vbs` corpus 7 (sigma 7)
`CommandLine`	match	`%AppData%` corpus 13 (sigma 11, chronicle 2) `%Public%` corpus 7 (sigma 5, chronicle 2) `%Temp%` corpus 16 (sigma 13, chronicle 2, kusto 1) `%tmp%` corpus 15 (sigma 13, chronicle 2) `C:\PerfLogs\` corpus 2 (sigma 2) `C:\ProgramData\` corpus 6 (sigma 6) `C:\Windows\Temp\` corpus 7 (sigma 7) `\AppData\` corpus 8 (sigma 8) `\Desktop\` corpus 13 (sigma 13) `\Temp\` corpus 5 (sigma 5) `\Users\Public\` corpus 17 (sigma 17)
`Image`	ends_with	`\curl.exe` corpus 30 (sigma 30)
`Product`	eq	`The curl executable` corpus 5 (sigma 5)

`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)

Suspicious Curl.EXE Download

MITRE ATT&CK coverage

Event coverage

Rule body yaml

Stages and Predicates

Stage 0: `condition`

Stage 1: `selection_curl`

Stage 2: `selection_susp_locations`

Stage 3: `selection_susp_extensions`

Stage 4: `not filter_optional_git_windows`

Exclusions

Indicators

Keyboard shortcuts

Search operators

Suspicious Curl.EXE Download

MITRE ATT&CK coverage

Event coverage

Rule body yaml

Stages and Predicates

Stage 0: condition

Stage 1: selection_curl

Stage 2: selection_susp_locations

Stage 3: selection_susp_extensions

Stage 4: not filter_optional_git_windows

Exclusions

Indicators

Stage 0: `condition`

Stage 1: `selection_curl`

Stage 2: `selection_susp_locations`

Stage 3: `selection_susp_extensions`

Stage 4: `not filter_optional_git_windows`