Potential CVE-2023-27997 Exploitation Indicators

Status: test
Severity: medium
Log source: category webserver
Author: Sergio Palacios Dominguez, Nasreddine Bencherchali (Nextron Systems)
Source: github.com/SigmaHQ/sigma

Detects indicators of potential exploitation of CVE-2023-27997 in Frotigate weblogs. To avoid false positives it is best to look for successive requests to the endpoints mentioned as well as weird values of the "enc" parameter

MITRE ATT&CK coverage

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

Rule body yaml

title: Potential CVE-2023-27997 Exploitation Indicators
id: 31e4e649-7394-4fd2-9ae7-dbc61eebb550
status: test
description: |
    Detects indicators of potential exploitation of CVE-2023-27997 in Frotigate weblogs.
    To avoid false positives it is best to look for successive requests to the endpoints mentioned as well as weird values of the "enc" parameter
references:
    - https://blog.lexfo.fr/Forensics-xortigate-notice.html
    - https://blog.lexfo.fr/xortigate-cve-2023-27997.html
    - https://research.kudelskisecurity.com/2023/06/12/cve-2023-27997-fortigate-ssl-vpn/
    - https://labs.watchtowr.com/xortigate-or-cve-2023-27997/
author: Sergio Palacios Dominguez, Nasreddine Bencherchali (Nextron Systems)
date: 2023-07-28
tags:
    - attack.initial-access
    - attack.t1190
    - cve.2023-27997
    - detection.emerging-threats
logsource:
    category: webserver
detection:
    selection_uri:
        cs-method:
            - 'GET'
            - 'POST'
        cs-uri-query|contains:
            - '/remote/hostcheck_validate'
            - '/remote/logincheck'
    selection_keywords:
        - 'enc='
    condition: all of selection_*
falsepositives:
    - Unknown
level: medium

Stages and Predicates

Stage 0: `condition`

all of selection_*

Stage 1: `selection_uri`

selection_uri:
    cs-method:
        - 'GET'
        - 'POST'
    cs-uri-query|contains:
        - '/remote/hostcheck_validate'
        - '/remote/logincheck'

Stage 2: `selection_keywords`

selection_keywords:
    - 'enc='

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-method`	eq	`GET` `POST`
`cs-uri-query`	match	`/remote/hostcheck_validate` `/remote/logincheck`

`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 CVE-2023-27997 Exploitation Indicators

MITRE ATT&CK coverage

Rule body yaml

Stages and Predicates

Stage 0: `condition`

Stage 1: `selection_uri`

Stage 2: `selection_keywords`

Indicators

Keyboard shortcuts

Search operators

Potential CVE-2023-27997 Exploitation Indicators

MITRE ATT&CK coverage

Rule body yaml

Stages and Predicates

Stage 0: condition

Stage 1: selection_uri

Stage 2: selection_keywords

Indicators

Stage 0: `condition`

Stage 1: `selection_uri`

Stage 2: `selection_keywords`