Potential CVE-2021-27905 Exploitation Attempt

Status: test
Severity: medium
Log source: category webserver
Author: @gott_cyber
Source: github.com/SigmaHQ/sigma

Detects exploitation attempt of the CVE-2021-27905 which affects all Apache Solr versions prior to and including 8.8.1.

MITRE ATT&CK coverage

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

Rule body yaml

title: Potential CVE-2021-27905 Exploitation Attempt
id: 0bbcd74b-0596-41a4-94a0-4e88a76ffdb3
status: test
description: Detects exploitation attempt of the CVE-2021-27905 which affects all Apache Solr versions prior to and including 8.8.1.
references:
    - https://twitter.com/Al1ex4/status/1382981479727128580
    - https://twitter.com/sec715/status/1373472323538362371
    - https://nsfocusglobal.com/apache-solr-arbitrary-file-read-and-ssrf-vulnerability-threat-alert/
    - https://mp.weixin.qq.com/s?__biz=Mzg3NDU2MTg0Ng==&mid=2247484117&idx=1&sn=2fdab8cbe4b873f8dd8abb35d935d186
    - https://github.com/murataydemir/CVE-2021-27905
author: '@gott_cyber'
date: 2022-12-11
modified: 2023-03-24
tags:
    - attack.initial-access
    - attack.t1190
    - cve.2021-27905
    - detection.emerging-threats
logsource:
    category: webserver
detection:
    selection_request1:
        cs-uri-query|contains|all:
            - '/solr/'
            - '/debug/dump?'
            - 'param=ContentStream'
        sc-status: 200
    selection_request2:
        cs-method: 'GET'
        cs-uri-query|contains|all:
            - '/solr/'
            - 'command=fetchindex'
            - 'masterUrl='
        sc-status: 200
    condition: 1 of selection_*
falsepositives:
    - Vulnerability Scanners
level: medium

Stages and Predicates

Stage 0: `condition`

1 of selection_*

Stage 1: `selection_request1`

selection_request1:
    cs-uri-query|contains|all:
        - '/solr/'
        - '/debug/dump?'
        - 'param=ContentStream'
    sc-status: 200

Stage 2: `selection_request2`

selection_request2:
    cs-method: 'GET'
    cs-uri-query|contains|all:
        - '/solr/'
        - 'command=fetchindex'
        - 'masterUrl='
    sc-status: 200

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`
`cs-uri-query`	match	`/debug/dump?` `/solr/` `command=fetchindex` `masterUrl=` `param=ContentStream`
`sc-status`	eq	`200`

`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-2021-27905 Exploitation Attempt

MITRE ATT&CK coverage

Rule body yaml

Stages and Predicates

Stage 0: `condition`

Stage 1: `selection_request1`

Stage 2: `selection_request2`

Indicators

Keyboard shortcuts

Search operators

Potential CVE-2021-27905 Exploitation Attempt

MITRE ATT&CK coverage

Rule body yaml

Stages and Predicates

Stage 0: condition

Stage 1: selection_request1

Stage 2: selection_request2

Indicators

Stage 0: `condition`

Stage 1: `selection_request1`

Stage 2: `selection_request2`