Log4j RCE CVE-2021-44228 in Fields

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

Detects exploitation attempt against log4j RCE vulnerability reported as CVE-2021-44228 in different header fields found in web server logs (Log4Shell)

MITRE ATT&CK coverage

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

Rule body yaml

title: Log4j RCE CVE-2021-44228 in Fields
id: 9be472ed-893c-4ec0-94da-312d2765f654
status: test
description: Detects exploitation attempt against log4j RCE vulnerability reported as CVE-2021-44228 in different header fields found in web server logs (Log4Shell)
references:
    - https://web.archive.org/web/20231230220738/https://www.lunasec.io/docs/blog/log4j-zero-day/
    - https://news.ycombinator.com/item?id=29504755
    - https://github.com/tangxiaofeng7/apache-log4j-poc
    - https://gist.github.com/Neo23x0/e4c8b03ff8cdf1fa63b7d15db6e3860b
    - https://github.com/YfryTchsGD/Log4jAttackSurface
    - https://twitter.com/shutingrz/status/1469255861394866177?s=21
author: Florian Roth (Nextron Systems)
date: 2021-12-10
modified: 2023-01-02
tags:
    - attack.initial-access
    - attack.t1190
    - cve.2021-44228
    - detection.emerging-threats
logsource:
    category: webserver
detection:
    selection1:
        cs-user-agent|contains:
            - '${jndi:ldap:/'
            - '${jndi:rmi:/'
            - '${jndi:ldaps:/'
            - '${jndi:dns:/'
            - '/$%7bjndi:'
            - '%24%7bjndi:'
            - '$%7Bjndi:'
            - '%2524%257Bjndi'
            - '%2F%252524%25257Bjndi%3A'
            - '${jndi:${lower:'
            - '${::-j}${'
            - '${jndi:nis'
            - '${jndi:nds'
            - '${jndi:corba'
            - '${jndi:iiop'
            - 'Reference Class Name: foo'
            - '${${env:BARFOO:-j}'
            - '${::-l}${::-d}${::-a}${::-p}'
            - '${base64:JHtqbmRp'
            - '${${env:ENV_NAME:-j}ndi${env:ENV_NAME:-:}$'
            - '${${lower:j}ndi:'
            - '${${upper:j}ndi:'
            - '${${::-j}${::-n}${::-d}${::-i}:'
    # selection2:
        # user-agent|contains:
            # - '${jndi:ldap:/'
            # - '${jndi:rmi:/'
            # - '${jndi:ldaps:/'
            # - '${jndi:dns:/'
            # - '/$%7bjndi:'
            # - '%24%7bjndi:'
            # - '$%7Bjndi:'
            # - '%2524%257Bjndi'
            # - '%2F%252524%25257Bjndi%3A'
            # - '${jndi:${lower:'
            # - '${::-j}${'
            # - '${jndi:nis'
            # - '${jndi:nds'
            # - '${jndi:corba'
            # - '${jndi:iiop'
            # - 'Reference Class Name: foo'
            # - '${${env:BARFOO:-j}'
            # - '${::-l}${::-d}${::-a}${::-p}'
            # - '${base64:JHtqbmRp'
            # - '${${env:ENV_NAME:-j}ndi${env:ENV_NAME:-:}$'
            # - '${${lower:j}ndi:'
            # - '${${upper:j}ndi:'
            # - '${${::-j}${::-n}${::-d}${::-i}:'
    selection3:
        cs-uri-query|contains:
            - '${jndi:ldap:/'
            - '${jndi:rmi:/'
            - '${jndi:ldaps:/'
            - '${jndi:dns:/'
            - '/$%7bjndi:'
            - '%24%7bjndi:'
            - '$%7Bjndi:'
            - '%2524%257Bjndi'
            - '%2F%252524%25257Bjndi%3A'
            - '${jndi:${lower:'
            - '${::-j}${'
            - '${jndi:nis'
            - '${jndi:nds'
            - '${jndi:corba'
            - '${jndi:iiop'
            - 'Reference Class Name: foo'
            - '${${env:BARFOO:-j}'
            - '${::-l}${::-d}${::-a}${::-p}'
            - '${base64:JHtqbmRp'
            - '${${env:ENV_NAME:-j}ndi${env:ENV_NAME:-:}$'
            - '${${lower:j}ndi:'
            - '${${upper:j}ndi:'
            - '${${::-j}${::-n}${::-d}${::-i}:'
    selection4:
        cs-referer|contains:
            - '${jndi:ldap:/'
            - '${jndi:rmi:/'
            - '${jndi:ldaps:/'
            - '${jndi:dns:/'
            - '/$%7bjndi:'
            - '%24%7bjndi:'
            - '$%7Bjndi:'
            - '%2524%257Bjndi'
            - '%2F%252524%25257Bjndi%3A'
            - '${jndi:${lower:'
            - '${::-j}${'
            - '${jndi:nis'
            - '${jndi:nds'
            - '${jndi:corba'
            - '${jndi:iiop'
            - 'Reference Class Name: foo'
            - '${${env:BARFOO:-j}'
            - '${::-l}${::-d}${::-a}${::-p}'
            - '${base64:JHtqbmRp'
            - '${${env:ENV_NAME:-j}ndi${env:ENV_NAME:-:}$'
            - '${${lower:j}ndi:'
            - '${${upper:j}ndi:'
            - '${${::-j}${::-n}${::-d}${::-i}:'
    condition: 1 of selection*
falsepositives:
    - Vulnerability scanning
level: high

Stages and Predicates

Stage 0: `condition`

1 of selection*

Stage 1: `selection1`

selection1:
    cs-user-agent|contains:
        - '${jndi:ldap:/'
        - '${jndi:rmi:/'
        - '${jndi:ldaps:/'
        - '${jndi:dns:/'
        - '/$%7bjndi:'
        - '%24%7bjndi:'
        - '$%7Bjndi:'
        - '%2524%257Bjndi'
        - '%2F%252524%25257Bjndi%3A'
        - '${jndi:${lower:'
        - '${::-j}${'
        - '${jndi:nis'
        - '${jndi:nds'
        - '${jndi:corba'
        - '${jndi:iiop'
        - 'Reference Class Name: foo'
        - '${${env:BARFOO:-j}'
        - '${::-l}${::-d}${::-a}${::-p}'
        - '${base64:JHtqbmRp'
        - '${${env:ENV_NAME:-j}ndi${env:ENV_NAME:-:}$'
        - '${${lower:j}ndi:'
        - '${${upper:j}ndi:'
        - '${${::-j}${::-n}${::-d}${::-i}:'

Stage 2: `selection3`

selection3:
    cs-uri-query|contains:
        - '${jndi:ldap:/'
        - '${jndi:rmi:/'
        - '${jndi:ldaps:/'
        - '${jndi:dns:/'
        - '/$%7bjndi:'
        - '%24%7bjndi:'
        - '$%7Bjndi:'
        - '%2524%257Bjndi'
        - '%2F%252524%25257Bjndi%3A'
        - '${jndi:${lower:'
        - '${::-j}${'
        - '${jndi:nis'
        - '${jndi:nds'
        - '${jndi:corba'
        - '${jndi:iiop'
        - 'Reference Class Name: foo'
        - '${${env:BARFOO:-j}'
        - '${::-l}${::-d}${::-a}${::-p}'
        - '${base64:JHtqbmRp'
        - '${${env:ENV_NAME:-j}ndi${env:ENV_NAME:-:}$'
        - '${${lower:j}ndi:'
        - '${${upper:j}ndi:'
        - '${${::-j}${::-n}${::-d}${::-i}:'

Stage 3: `selection4`

selection4:
    cs-referer|contains:
        - '${jndi:ldap:/'
        - '${jndi:rmi:/'
        - '${jndi:ldaps:/'
        - '${jndi:dns:/'
        - '/$%7bjndi:'
        - '%24%7bjndi:'
        - '$%7Bjndi:'
        - '%2524%257Bjndi'
        - '%2F%252524%25257Bjndi%3A'
        - '${jndi:${lower:'
        - '${::-j}${'
        - '${jndi:nis'
        - '${jndi:nds'
        - '${jndi:corba'
        - '${jndi:iiop'
        - 'Reference Class Name: foo'
        - '${${env:BARFOO:-j}'
        - '${::-l}${::-d}${::-a}${::-p}'
        - '${base64:JHtqbmRp'
        - '${${env:ENV_NAME:-j}ndi${env:ENV_NAME:-:}$'
        - '${${lower:j}ndi:'
        - '${${upper:j}ndi:'
        - '${${::-j}${::-n}${::-d}${::-i}:'

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-referer`	match	`$%7Bjndi:` `${${::-j}${::-n}${::-d}${::-i}:` `${${env:BARFOO:-j}` `${${env:ENV_NAME:-j}ndi${env:ENV_NAME:-:}$` `${${lower:j}ndi:` `${${upper:j}ndi:` `${::-j}${` `${::-l}${::-d}${::-a}${::-p}` `${base64:JHtqbmRp` `${jndi:${lower:` `${jndi:corba` `${jndi:dns:/` `${jndi:iiop` `${jndi:ldap:/` `${jndi:ldaps:/` `${jndi:nds` `${jndi:nis` `${jndi:rmi:/` `%24%7bjndi:` `%2524%257Bjndi` `%2F%252524%25257Bjndi%3A` `/$%7bjndi:` `Reference Class Name: foo`
`cs-uri-query`	match	`$%7Bjndi:` `${${::-j}${::-n}${::-d}${::-i}:` `${${env:BARFOO:-j}` `${${env:ENV_NAME:-j}ndi${env:ENV_NAME:-:}$` `${${lower:j}ndi:` `${${upper:j}ndi:` `${::-j}${` `${::-l}${::-d}${::-a}${::-p}` `${base64:JHtqbmRp` `${jndi:${lower:` `${jndi:corba` `${jndi:dns:/` `${jndi:iiop` `${jndi:ldap:/` `${jndi:ldaps:/` `${jndi:nds` `${jndi:nis` `${jndi:rmi:/` `%24%7bjndi:` `%2524%257Bjndi` `%2F%252524%25257Bjndi%3A` `/$%7bjndi:` `Reference Class Name: foo`
`cs-user-agent`	match	`$%7Bjndi:` `${${::-j}${::-n}${::-d}${::-i}:` `${${env:BARFOO:-j}` `${${env:ENV_NAME:-j}ndi${env:ENV_NAME:-:}$` `${${lower:j}ndi:` `${${upper:j}ndi:` `${::-j}${` `${::-l}${::-d}${::-a}${::-p}` `${base64:JHtqbmRp` `${jndi:${lower:` `${jndi:corba` `${jndi:dns:/` `${jndi:iiop` `${jndi:ldap:/` `${jndi:ldaps:/` `${jndi:nds` `${jndi:nis` `${jndi:rmi:/` `%24%7bjndi:` `%2524%257Bjndi` `%2F%252524%25257Bjndi%3A` `/$%7bjndi:` `Reference Class Name: foo`

`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)

Log4j RCE CVE-2021-44228 in Fields

MITRE ATT&CK coverage

Rule body yaml

Stages and Predicates

Stage 0: `condition`

Stage 1: `selection1`

Stage 2: `selection3`

Stage 3: `selection4`

Indicators

Keyboard shortcuts

Search operators

Log4j RCE CVE-2021-44228 in Fields

MITRE ATT&CK coverage

Rule body yaml

Stages and Predicates

Stage 0: condition

Stage 1: selection1

Stage 2: selection3

Stage 3: selection4

Indicators

Stage 0: `condition`

Stage 1: `selection1`

Stage 2: `selection3`

Stage 3: `selection4`