Suspicious User Agent

Status: test
Severity: high
Log source: category proxy
Author: Florian Roth (Nextron Systems)
Source: github.com/SigmaHQ/sigma

Detects suspicious malformed user agent strings in proxy logs

MITRE ATT&CK coverage

Tactic	Techniques
Command & Control	`T1071.001` Application Layer Protocol: Web Protocols

Rule body yaml

title: Suspicious User Agent
id: 7195a772-4b3f-43a4-a210-6a003d65caa1
status: test
description: Detects suspicious malformed user agent strings in proxy logs
references:
    - https://github.com/fastly/waf_testbed/blob/8bfc406551f3045e418cbaad7596cff8da331dfc/templates/default/scanners-user-agents.data.erb
author: Florian Roth (Nextron Systems)
date: 2017-07-08
modified: 2022-10-31
tags:
    - attack.command-and-control
    - attack.t1071.001
logsource:
    category: proxy
detection:
    selection1:
        c-useragent|startswith:
            - 'user-agent'  # User-Agent: User-Agent:
            - 'Mozilla/3.0 '
            - 'Mozilla/2.0 '
            - 'Mozilla/1.0 '
            - 'Mozilla '  # missing slash
            - ' Mozilla/'  # leading space
            - 'Mozila/'  # single 'l'
            - 'Mozilla/4.0 (compatible; MSIE 6.0; MS Web Services Client Protocol'  # https://twitter.com/NtSetDefault/status/1303643299509567488
    selection2:
        c-useragent|contains:
            - ' (compatible;MSIE '  # typical typo - missing space
            - '.0;Windows NT '  # typical typo - missing space
            - 'loader'  # https://twitter.com/securityonion/status/1522614635152744453?s=20&t=gHyPTSq5A27EqKwrCd9ohg
    selection3:
        c-useragent:
            - '_'
            - 'CertUtil URL Agent'  # https://twitter.com/stvemillertime/status/985150675527974912
            - 'Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:60.0)'  # CobaltStrike Beacon https://unit42.paloaltonetworks.com/tracking-oceanlotus-new-downloader-kerrdown/
            - 'Mozilla/5.0 (Windows NT 6.3; WOW64; rv:28.0) Gecko/20100101 Firefox/28.0'  # used by APT28 malware https://threatvector.cylance.com/en_us/home/inside-the-apt28-dll-backdoor-blitz.html
            - 'HTTPS'  # https://twitter.com/stvemillertime/status/1204437531632250880
            - 'Erbium-UA-4ce7c27cb4be9d32e333bf032c88235a'  # https://www.cyfirma.com/outofband/erbium-stealer-malware-report
            - 'x'  # Use by Racoon Stealer but could be something else
            - 'xxx'  # Use by Racoon Stealer but could be something else
    falsepositives:
        - c-useragent: 'Mozilla/3.0 * Acrobat *'  # Acrobat with linked content
        - cs-host|endswith: # Adobe product traffic, example: Mozilla/3.0 (compatible; Adobe Synchronizer 10.12.20000)
              - '.acrobat.com'
              - '.adobe.com'
              - '.adobe.io'
    condition: 1 of selection* and not falsepositives
falsepositives:
    - Unknown
level: high

Stages and Predicates

Stage 0: `condition`

1 of selection* and not falsepositives

Stage 1: `selection1`

selection1:
    c-useragent|startswith:
        - 'user-agent'
        - 'Mozilla/3.0 '
        - 'Mozilla/2.0 '
        - 'Mozilla/1.0 '
        - 'Mozilla '
        - ' Mozilla/'
        - 'Mozila/'
        - 'Mozilla/4.0 (compatible; MSIE 6.0; MS Web Services Client Protocol'

Stage 2: `selection2`

selection2:
    c-useragent|contains:
        - ' (compatible;MSIE '
        - '.0;Windows NT '
        - 'loader'

Stage 3: `selection3`

selection3:
    c-useragent:
        - '_'
        - 'CertUtil URL Agent'
        - 'Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:60.0)'
        - 'Mozilla/5.0 (Windows NT 6.3; WOW64; rv:28.0) Gecko/20100101 Firefox/28.0'
        - 'HTTPS'
        - 'Erbium-UA-4ce7c27cb4be9d32e333bf032c88235a'
        - 'x'
        - 'xxx'

Stage 4: `not falsepositives`

falsepositives:
    - c-useragent: 'Mozilla/3.0 * Acrobat *'
    - cs-host|endswith:
          - '.acrobat.com'
          - '.adobe.com'
          - '.adobe.io'

Exclusions

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

Field	Kind	Excluded values
`c-useragent`	wildcard	`Mozilla/3.0 * Acrobat *`
`cs-host`	ends_with	`.acrobat.com`
`cs-host`	ends_with	`.adobe.com`
`cs-host`	ends_with	`.adobe.io`

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
`c-useragent`	eq	`CertUtil URL Agent` `Erbium-UA-4ce7c27cb4be9d32e333bf032c88235a` `HTTPS` `Mozilla/5.0 (Windows NT 10.0; Win64; x64; rv:60.0)` `Mozilla/5.0 (Windows NT 6.3; WOW64; rv:28.0) Gecko/20100101 Firefox/28.0` `_` `x` `xxx`
`c-useragent`	match	`(compatible;MSIE` `.0;Windows NT` `loader`
`c-useragent`	starts_with	`Mozilla/` `Mozila/` `Mozilla` `Mozilla/1.0` `Mozilla/2.0` `Mozilla/3.0` `Mozilla/4.0 (compatible; MSIE 6.0; MS Web Services Client Protocol` `user-agent`

`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 User Agent

MITRE ATT&CK coverage

Rule body yaml

Stages and Predicates

Stage 0: `condition`

Stage 1: `selection1`

Stage 2: `selection2`

Stage 3: `selection3`

Stage 4: `not falsepositives`

Exclusions

Indicators

Keyboard shortcuts

Search operators

Suspicious User Agent

MITRE ATT&CK coverage

Rule body yaml

Stages and Predicates

Stage 0: condition

Stage 1: selection1

Stage 2: selection2

Stage 3: selection3

Stage 4: not falsepositives

Exclusions

Indicators

Stage 0: `condition`

Stage 1: `selection1`

Stage 2: `selection2`

Stage 3: `selection3`

Stage 4: `not falsepositives`