MSSQL Server Failed Logon From External Network

Status: test
Severity: medium
Log source: product windows, service application
Author: j4son
Source: github.com/SigmaHQ/sigma

Detects failed logon attempts from clients with external network IP to an MSSQL server. This can be a sign of a bruteforce attack.

MITRE ATT&CK coverage

Tactic	Techniques
Credential Access	`T1110` Brute Force

Event coverage

Provider	Event
MSSQLSERVER	Event ID 18456

Rule body yaml

title: MSSQL Server Failed Logon From External Network
id: ebfe73c2-5bc9-4ed9-aaa8-8b54b2b4777d
related:
    - id: 218d2855-2bba-4f61-9c85-81d0ea63ac71
      type: similar
status: test
description: Detects failed logon attempts from clients with external network IP to an MSSQL server. This can be a sign of a bruteforce attack.
references:
    - https://cybersecthreat.com/2020/07/08/enable-mssql-authentication-log-to-eventlog/
    - https://www.experts-exchange.com/questions/27800944/EventID-18456-Failed-to-open-the-explicitly-specified-database.html
author: j4son
date: 2023-10-11
modified: 2025-05-28
tags:
    - attack.credential-access
    - attack.t1110
logsource:
    product: windows
    service: application
    definition: 'Requirements: Must enable MSSQL authentication.'
detection:
    selection:
        Provider_Name|contains: 'MSSQL' # Note: We use contains to account for other third party providers - See https://github.com/SigmaHQ/sigma/issues/4876
        EventID: 18456
    filter_main_local_ips:
        Data|contains:
            - 'CLIENT: 10.' # filter_range_IP: 10.0.0.0/8
            - 'CLIENT: 172.16.' # filter_range_IP: 172.16.0.0/12
            - 'CLIENT: 172.17.'
            - 'CLIENT: 172.18.'
            - 'CLIENT: 172.19.'
            - 'CLIENT: 172.20.'
            - 'CLIENT: 172.21.'
            - 'CLIENT: 172.22.'
            - 'CLIENT: 172.23.'
            - 'CLIENT: 172.24.'
            - 'CLIENT: 172.25.'
            - 'CLIENT: 172.26.'
            - 'CLIENT: 172.27.'
            - 'CLIENT: 172.28.'
            - 'CLIENT: 172.29.'
            - 'CLIENT: 172.30.'
            - 'CLIENT: 172.31.'
            - 'CLIENT: 192.168.' # filter_range_IP: 192.168.0.0/16
            - 'CLIENT: 127.' # filter_loop_back: 127.0.0.0/8
            - 'CLIENT: 169.254.' # fileter_link-local_addressing: 169.254.0.0/16
            - 'CLIENT: <local machine>'
    condition: selection and not 1 of filter_main_*
falsepositives:
    - Unknown
level: medium

Stages and Predicates

Stage 0: `condition`

selection and not 1 of filter_main_*

Stage 1: `selection`

selection:
    Provider_Name|contains: 'MSSQL'
    EventID: 18456

Stage 2: `not filter_main_local_ips`

filter_main_local_ips:
    Data|contains:
        - 'CLIENT: 10.'
        - 'CLIENT: 172.16.'
        - 'CLIENT: 172.17.'
        - 'CLIENT: 172.18.'
        - 'CLIENT: 172.19.'
        - 'CLIENT: 172.20.'
        - 'CLIENT: 172.21.'
        - 'CLIENT: 172.22.'
        - 'CLIENT: 172.23.'
        - 'CLIENT: 172.24.'
        - 'CLIENT: 172.25.'
        - 'CLIENT: 172.26.'
        - 'CLIENT: 172.27.'
        - 'CLIENT: 172.28.'
        - 'CLIENT: 172.29.'
        - 'CLIENT: 172.30.'
        - 'CLIENT: 172.31.'
        - 'CLIENT: 192.168.'
        - 'CLIENT: 127.'
        - 'CLIENT: 169.254.'
        - 'CLIENT: <local machine>'

Exclusions

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

Field	Kind	Excluded values
`Data`	match	`CLIENT: 10.`
`Data`	match	`CLIENT: 127.`
`Data`	match	`CLIENT: 169.254.`
`Data`	match	`CLIENT: 172.16.`
`Data`	match	`CLIENT: 172.17.`
`Data`	match	`CLIENT: 172.18.`
`Data`	match	`CLIENT: 172.19.`
`Data`	match	`CLIENT: 172.20.`
`Data`	match	`CLIENT: 172.21.`
`Data`	match	`CLIENT: 172.22.`
`Data`	match	`CLIENT: 172.23.`
`Data`	match	`CLIENT: 172.24.`
`Data`	match	`CLIENT: 172.25.`
`Data`	match	`CLIENT: 172.26.`
`Data`	match	`CLIENT: 172.27.`
`Data`	match	`CLIENT: 172.28.`
`Data`	match	`CLIENT: 172.29.`
`Data`	match	`CLIENT: 172.30.`
`Data`	match	`CLIENT: 172.31.`
`Data`	match	`CLIENT: 192.168.`
`Data`	match	`CLIENT: <local machine>`

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
`Provider_Name`	match	`MSSQL` corpus 7 (sigma 7)

`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)

MSSQL Server Failed Logon From External Network

MITRE ATT&CK coverage

Event coverage

Rule body yaml

Stages and Predicates

Stage 0: `condition`

Stage 1: `selection`

Stage 2: `not filter_main_local_ips`

Exclusions

Indicators

Keyboard shortcuts

Search operators

MSSQL Server Failed Logon From External Network

MITRE ATT&CK coverage

Event coverage

Rule body yaml

Stages and Predicates

Stage 0: condition

Stage 1: selection

Stage 2: not filter_main_local_ips

Exclusions

Indicators

Stage 0: `condition`

Stage 1: `selection`

Stage 2: `not filter_main_local_ips`