Arcadyan Router Exploitations

Status: test
Severity: critical
Log source: category webserver
Author: Bhabesh Raj
Source: github.com/SigmaHQ/sigma

Detects exploitation of vulnerabilities in Arcadyan routers as reported in CVE-2021-20090 and CVE-2021-20091.

MITRE ATT&CK coverage

Tactic	Techniques
Initial Access	`T1190` Exploit Public-Facing Application

Rule body yaml

title: Arcadyan Router Exploitations
id: f0500377-bc70-425d-ac8c-e956cd906871
status: test
description: Detects exploitation of vulnerabilities in Arcadyan routers as reported in CVE-2021-20090 and CVE-2021-20091.
references:
    - https://medium.com/tenable-techblog/bypassing-authentication-on-arcadyan-routers-with-cve-2021-20090-and-rooting-some-buffalo-ea1dd30980c2
    - https://www.tenable.com/security/research/tra-2021-13
    - https://blogs.juniper.net/en-us/security/freshly-disclosed-vulnerability-cve-2021-20090-exploited-in-the-wild
author: Bhabesh Raj
date: 2021-08-24
modified: 2023-01-02
tags:
    - attack.initial-access
    - attack.t1190
    - cve.2021-20090
    - cve.2021-20091
    - detection.emerging-threats
logsource:
    category: webserver
detection:
    path_traversal:
        # CVE-2021-20090 (Bypass Auth: Path Traversal)
        cs-uri-query|contains: '..%2f'
    config_file_inj:
        cs-uri-query|contains|all:
            # Chaining of CVE-2021-20090 (Bypass Auth) and CVE-2021-20091 (Config File Injection)
            - '..%2f'
            - 'apply_abstract.cgi'
    noauth_list:
        cs-uri-query|contains:
            - '/images/'
            - '/js/'
            - '/css/'
            - '/setup_top_login.htm'
            - '/login.html'
            - '/loginerror.html'
            - '/loginexclude.html'
            - '/loginlock.html'
    condition: (path_traversal or config_file_inj) and noauth_list
falsepositives:
    - Unknown
level: critical

Stages and Predicates

Stage 0: `condition`

(path_traversal or config_file_inj) and noauth_list

Stage 1: `path_traversal`

path_traversal:
    cs-uri-query|contains: '..%2f'

Stage 2: `config_file_inj`

config_file_inj:
    cs-uri-query|contains|all:
        - '..%2f'
        - 'apply_abstract.cgi'

Stage 3: `noauth_list`

noauth_list:
    cs-uri-query|contains:
        - '/images/'
        - '/js/'
        - '/css/'
        - '/setup_top_login.htm'
        - '/login.html'
        - '/loginerror.html'
        - '/loginexclude.html'
        - '/loginlock.html'

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
`cs-uri-query`	match	`..%2f` `/css/` `/images/` `/js/` `/login.html` `/loginerror.html` `/loginexclude.html` `/loginlock.html` `/setup_top_login.htm` `apply_abstract.cgi`

`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)

Arcadyan Router Exploitations

MITRE ATT&CK coverage

Rule body yaml

Stages and Predicates

Stage 0: `condition`

Stage 1: `path_traversal`

Stage 2: `config_file_inj`

Stage 3: `noauth_list`

Indicators

Keyboard shortcuts

Search operators

Arcadyan Router Exploitations

MITRE ATT&CK coverage

Rule body yaml

Stages and Predicates

Stage 0: condition

Stage 1: path_traversal

Stage 2: config_file_inj

Stage 3: noauth_list

Indicators

Stage 0: `condition`

Stage 1: `path_traversal`

Stage 2: `config_file_inj`

Stage 3: `noauth_list`