CVE-2021-41773 Exploitation Attempt

Status: test
Severity: high
Log source: category webserver
Author: daffainfo, Florian Roth
Source: github.com/SigmaHQ/sigma

Detects exploitation of flaw in path normalization in Apache HTTP server 2.4.49. An attacker could use a path traversal attack to map URLs to files outside the expected document root. If files outside of the document root are not protected by "require all denied" these requests can succeed. Additionally this flaw could leak the source of interpreted files like CGI scripts. This issue is known to be exploited in the wild. This issue only affects Apache 2.4.49 and not earlier versions.

MITRE ATT&CK coverage

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

Rule body yaml

title: CVE-2021-41773 Exploitation Attempt
id: 3007fec6-e761-4319-91af-e32e20ac43f5
status: test
description: |
  Detects exploitation of flaw in path normalization in Apache HTTP server 2.4.49.
  An attacker could use a path traversal attack to map URLs to files outside the expected document root.
  If files outside of the document root are not protected by "require all denied" these requests can succeed.
  Additionally this flaw could leak the source of interpreted files like CGI scripts.
  This issue is known to be exploited in the wild. This issue only affects Apache 2.4.49 and not earlier versions.
references:
    - https://nvd.nist.gov/vuln/detail/CVE-2021-41773
    - https://github.com/apache/httpd/commit/e150697086e70c552b2588f369f2d17815cb1782
    - https://twitter.com/ptswarm/status/1445376079548624899
    - https://twitter.com/h4x0r_dz/status/1445401960371429381
    - https://github.com/projectdiscovery/nuclei-templates/blob/9d2889356eebba661c8407038e430759dfd4ec31/cves/2021/CVE-2021-41773.yaml
    - https://twitter.com/bl4sty/status/1445462677824761878
author: daffainfo, Florian Roth
date: 2021-10-05
modified: 2023-01-02
tags:
    - attack.initial-access
    - attack.t1190
    - cve.2021-41773
    - detection.emerging-threats
logsource:
    category: webserver
detection:
    selection:
        cs-uri-query|contains:
            - '/cgi-bin/.%2e/'
            - '/icons/.%2e/'
            - '/cgi-bin/.%%32%65/'
            - '/icons/.%%32%65/'
            - '/cgi-bin/.%%%25%33'
            - '/icons/.%%%25%33'
        sc-status:
            - 200
            - 301
    condition: selection
falsepositives:
    - Unknown
level: high

Stages and Predicates

Stage 0: `condition`

selection

Stage 1: `selection`

selection:
    cs-uri-query|contains:
        - '/cgi-bin/.%2e/'
        - '/icons/.%2e/'
        - '/cgi-bin/.%%32%65/'
        - '/icons/.%%32%65/'
        - '/cgi-bin/.%%%25%33'
        - '/icons/.%%%25%33'
    sc-status:
        - 200
        - 301

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	`/cgi-bin/.%%%25%33` `/cgi-bin/.%%32%65/` `/cgi-bin/.%2e/` `/icons/.%%%25%33` `/icons/.%%32%65/` `/icons/.%2e/`
`sc-status`	eq	`200` `301`

`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)

CVE-2021-41773 Exploitation Attempt

MITRE ATT&CK coverage

Rule body yaml

Stages and Predicates

Stage 0: `condition`

Stage 1: `selection`

Indicators

Keyboard shortcuts

Search operators

CVE-2021-41773 Exploitation Attempt

MITRE ATT&CK coverage

Rule body yaml

Stages and Predicates

Stage 0: condition

Stage 1: selection

Indicators

Stage 0: `condition`

Stage 1: `selection`