Potential Webshell Creation On Static Website

Status: test
Severity: medium
Log source: product windows, category file_event
Author: Beyu Denis, oscd.community, Tim Shelton, Thurein Oo
Source: github.com/SigmaHQ/sigma

Detects the creation of files with certain extensions on a static web site. This can be indicative of potential uploads of a web shell.

MITRE ATT&CK coverage

Tactic	Techniques
Persistence	`T1505.003` Server Software Component: Web Shell

Event coverage

Provider	Event	Title
Sysmon	Event ID 11	FileCreate

Rule body yaml

title: Potential Webshell Creation On Static Website
id: 39f1f9f2-9636-45de-98f6-a4046aa8e4b9
status: test
description: Detects the creation of files with certain extensions on a static web site. This can be indicative of potential uploads of a web shell.
references:
    - PT ESC rule and personal experience
    - https://github.com/swisskyrepo/PayloadsAllTheThings/blob/c95a0a1a2855dc0cd7f7327614545fe30482a636/Upload%20Insecure%20Files/README.md
author: Beyu Denis, oscd.community, Tim Shelton, Thurein Oo
date: 2019-10-22
modified: 2023-10-15
tags:
    - attack.persistence
    - attack.t1505.003
logsource:
    product: windows
    category: file_event
detection:
    selection_wwwroot_path:
        TargetFilename|contains: '\inetpub\wwwroot\'
    selection_wwwroot_ext:
        TargetFilename|contains:
            - '.ashx'
            - '.asp'
            - '.ph'
            - '.soap'
    selection_htdocs_path:
        TargetFilename|contains:
            - '\www\'
            - '\htdocs\'
            - '\html\'
    selection_htdocs_ext:
        TargetFilename|contains: '.ph'
    # selection_tomcat_path:
    #     TargetFilename|contains: '\webapps\ROOT'
    # selection_tomcat_ext:
    #     TargetFilename|contains:
    #         - '.jsp' # .jspx, .jspf
    #         - '.jsv'
    #         - '.jsw'
    filter_main_temp:  # FP when unpacking some executables in $TEMP
        TargetFilename|contains:
            - '\AppData\Local\Temp\'
            - '\Windows\Temp\'
    filter_main_system:
        Image: 'System' # FP when backup/restore from drivers
    filter_main_legitimate:
        TargetFilename|contains: '\xampp'
    condition: (all of selection_wwwroot_* or all of selection_htdocs_*) and not 1 of filter_main_*
falsepositives:
    - Legitimate administrator or developer creating legitimate executable files in a web application folder
level: medium

Stages and Predicates

Stage 0: `condition`

(all of selection_wwwroot_* or all of selection_htdocs_*) and not 1 of filter_main_*

Stage 1: `selection_wwwroot_path`

selection_wwwroot_path:
    TargetFilename|contains: '\inetpub\wwwroot\'

Stage 2: `selection_wwwroot_ext`

selection_wwwroot_ext:
    TargetFilename|contains:
        - '.ashx'
        - '.asp'
        - '.ph'
        - '.soap'

Stage 3: `selection_htdocs_path`

selection_htdocs_path:
    TargetFilename|contains:
        - '\www\'
        - '\htdocs\'
        - '\html\'

Stage 4: `selection_htdocs_ext`

selection_htdocs_ext:
    TargetFilename|contains: '.ph'

Stage 5: `not filter_main_*`

filter_main_temp:
    TargetFilename|contains:
        - '\AppData\Local\Temp\'
        - '\Windows\Temp\'
filter_main_system:
    Image: 'System'
filter_main_legitimate:
    TargetFilename|contains: '\xampp'

Exclusions

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

Field	Kind	Excluded values
`Image`	eq	`System`
`TargetFilename`	match	`\AppData\Local\Temp\`
`TargetFilename`	match	`\Windows\Temp\`
`TargetFilename`	match	`\xampp`

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
`TargetFilename`	match	`.ashx` `.asp` `.ph` `.soap` `\htdocs\` `\html\` `\inetpub\wwwroot\` corpus 2 (sigma 2) `\www\`

`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)

Potential Webshell Creation On Static Website

MITRE ATT&CK coverage

Event coverage

Rule body yaml

Stages and Predicates

Stage 0: `condition`

Stage 1: `selection_wwwroot_path`

Stage 2: `selection_wwwroot_ext`

Stage 3: `selection_htdocs_path`

Stage 4: `selection_htdocs_ext`

Stage 5: `not filter_main_*`

Exclusions

Indicators

Keyboard shortcuts

Search operators

Potential Webshell Creation On Static Website

MITRE ATT&CK coverage

Event coverage

Rule body yaml

Stages and Predicates

Stage 0: condition

Stage 1: selection_wwwroot_path

Stage 2: selection_wwwroot_ext

Stage 3: selection_htdocs_path

Stage 4: selection_htdocs_ext

Stage 5: not filter_main_*

Exclusions

Indicators

Stage 0: `condition`

Stage 1: `selection_wwwroot_path`

Stage 2: `selection_wwwroot_ext`

Stage 3: `selection_htdocs_path`

Stage 4: `selection_htdocs_ext`

Stage 5: `not filter_main_*`