Suspicious WebDav Client Execution Via Rundll32.EXE

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

Detects "svchost.exe" spawning "rundll32.exe" with command arguments like C:\windows\system32\davclnt.dll,DavSetCookie. This could be an indicator of exfiltration or use of WebDav to launch code (hosted on WebDav Server) or potentially a sign of exploitation of CVE-2023-23397

MITRE ATT&CK coverage

Tactic	Techniques
Exfiltration	`T1048.003` Exfiltration Over Alternative Protocol: Exfiltration Over Unencrypted Non-C2 Protocol

Event coverage

Provider	Event	Title
Sysmon	Event ID 1	Process creation

Rule body yaml

title: Suspicious WebDav Client Execution Via Rundll32.EXE
id: 982e9f2d-1a85-4d5b-aea4-31f5e97c6555
status: test
description: |
    Detects "svchost.exe" spawning "rundll32.exe" with command arguments like C:\windows\system32\davclnt.dll,DavSetCookie. This could be an indicator of exfiltration or use of WebDav to launch code (hosted on WebDav Server) or potentially a sign of exploitation of CVE-2023-23397
references:
    - https://twitter.com/aceresponder/status/1636116096506818562
    - https://www.mdsec.co.uk/2023/03/exploiting-cve-2023-23397-microsoft-outlook-elevation-of-privilege-vulnerability/
    - https://www.pwndefend.com/2023/03/15/the-long-game-persistent-hash-theft/
    - https://www.microsoft.com/en-us/security/blog/wp-content/uploads/2023/03/Figure-7-sample-webdav-process-create-event.png
    - https://www.microsoft.com/en-us/security/blog/2023/03/24/guidance-for-investigating-attacks-using-cve-2023-23397/
author: Nasreddine Bencherchali (Nextron Systems), Florian Roth (Nextron Systems)
date: 2023-03-16
modified: 2023-09-18
tags:
    - attack.exfiltration
    - attack.t1048.003
    - cve.2023-23397
logsource:
    category: process_creation
    product: windows
detection:
    selection:
        ParentImage|endswith: '\svchost.exe'
        ParentCommandLine|contains: '-s WebClient'
        Image|endswith: '\rundll32.exe'
        CommandLine|contains: 'C:\windows\system32\davclnt.dll,DavSetCookie'
        CommandLine|re: '://\d{1,3}\.\d{1,3}\.\d{1,3}\.\d{1,3}'
    filter_local_ips:
        CommandLine|contains:
            - '://10.' # 10.0.0.0/8
            - '://192.168.' # 192.168.0.0/16
            - '://172.16.' # 172.16.0.0/12
            - '://172.17.'
            - '://172.18.'
            - '://172.19.'
            - '://172.20.'
            - '://172.21.'
            - '://172.22.'
            - '://172.23.'
            - '://172.24.'
            - '://172.25.'
            - '://172.26.'
            - '://172.27.'
            - '://172.28.'
            - '://172.29.'
            - '://172.30.'
            - '://172.31.'
            - '://127.' # 127.0.0.0/8
            - '://169.254.' # 169.254.0.0/16
    condition: selection and not 1 of filter_*
falsepositives:
    - Unknown
level: high

Stages and Predicates

Stage 0: `condition`

selection and not 1 of filter_*

Stage 1: `selection`

selection:
    ParentImage|endswith: '\svchost.exe'
    ParentCommandLine|contains: '-s WebClient'
    Image|endswith: '\rundll32.exe'
    CommandLine|contains: 'C:\windows\system32\davclnt.dll,DavSetCookie'
    CommandLine|re: '://\d{1,3}\.\d{1,3}\.\d{1,3}\.\d{1,3}'

Stage 2: `not filter_local_ips`

filter_local_ips:
    CommandLine|contains:
        - '://10.'
        - '://192.168.'
        - '://172.16.'
        - '://172.17.'
        - '://172.18.'
        - '://172.19.'
        - '://172.20.'
        - '://172.21.'
        - '://172.22.'
        - '://172.23.'
        - '://172.24.'
        - '://172.25.'
        - '://172.26.'
        - '://172.27.'
        - '://172.28.'
        - '://172.29.'
        - '://172.30.'
        - '://172.31.'
        - '://127.'
        - '://169.254.'

Exclusions

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

Field	Kind	Excluded values
`CommandLine`	match	`://10.`
`CommandLine`	match	`://127.`
`CommandLine`	match	`://169.254.`
`CommandLine`	match	`://172.16.`
`CommandLine`	match	`://172.17.`
`CommandLine`	match	`://172.18.`
`CommandLine`	match	`://172.19.`
`CommandLine`	match	`://172.20.`
`CommandLine`	match	`://172.21.`
`CommandLine`	match	`://172.22.`
`CommandLine`	match	`://172.23.`
`CommandLine`	match	`://172.24.`
`CommandLine`	match	`://172.25.`
`CommandLine`	match	`://172.26.`
`CommandLine`	match	`://172.27.`
`CommandLine`	match	`://172.28.`
`CommandLine`	match	`://172.29.`
`CommandLine`	match	`://172.30.`
`CommandLine`	match	`://172.31.`
`CommandLine`	match	`://192.168.`

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`	match	`C:\windows\system32\davclnt.dll,DavSetCookie` corpus 3 (sigma 3)
`CommandLine`	regex_match	`://\d{1,3}.\d{1,3}.\d{1,3}.\d{1,3}` corpus 2 (sigma 2)
`Image`	ends_with	`\rundll32.exe` corpus 95 (sigma 95)
`ParentCommandLine`	match	`-s WebClient`
`ParentImage`	ends_with	`\svchost.exe` corpus 14 (sigma 14)

`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 WebDav Client Execution Via Rundll32.EXE

MITRE ATT&CK coverage

Event coverage

Rule body yaml

Stages and Predicates

Stage 0: `condition`

Stage 1: `selection`

Stage 2: `not filter_local_ips`

Exclusions

Indicators

Keyboard shortcuts

Search operators

Suspicious WebDav Client Execution Via Rundll32.EXE

MITRE ATT&CK coverage

Event coverage

Rule body yaml

Stages and Predicates

Stage 0: condition

Stage 1: selection

Stage 2: not filter_local_ips

Exclusions

Indicators

Stage 0: `condition`

Stage 1: `selection`

Stage 2: `not filter_local_ips`