Potential Netcat Reverse Shell Execution

Status: test
Severity: high
Log source: product linux, category process_creation
Author: @d4ns4n_, Nasreddine Bencherchali (Nextron Systems)
Source: github.com/SigmaHQ/sigma

Detects execution of netcat with the "-e" flag followed by common shells. This could be a sign of a potential reverse shell setup.

MITRE ATT&CK coverage

Tactic	Techniques
Execution	`T1059` Command and Scripting Interpreter

Event coverage

Provider	Event
Sysmon-for-Linux	Event ID 1

Rule body yaml

title: Potential Netcat Reverse Shell Execution
id: 7f734ed0-4f47-46c0-837f-6ee62505abd9
status: test
description: Detects execution of netcat with the "-e" flag followed by common shells. This could be a sign of a potential reverse shell setup.
references:
    - https://pentestmonkey.net/cheat-sheet/shells/reverse-shell-cheat-sheet
    - https://www.revshells.com/
    - https://www.hackingtutorials.org/networking/hacking-netcat-part-2-bind-reverse-shells/
    - https://www.infosecademy.com/netcat-reverse-shells/
    - https://man7.org/linux/man-pages/man1/ncat.1.html
author: '@d4ns4n_, Nasreddine Bencherchali (Nextron Systems)'
date: 2023-04-07
tags:
    - attack.execution
    - attack.t1059
logsource:
    category: process_creation
    product: linux
detection:
    selection_nc:
        Image|endswith:
            - '/nc'
            - '/ncat'
    selection_flags:
        CommandLine|contains:
            - ' -c '
            - ' -e '
    selection_shell:
        CommandLine|contains:
            - ' ash'
            - ' bash'
            - ' bsh'
            - ' csh'
            - ' ksh'
            - ' pdksh'
            - ' sh'
            - ' tcsh'
            - '/bin/ash'
            - '/bin/bash'
            - '/bin/bsh'
            - '/bin/csh'
            - '/bin/ksh'
            - '/bin/pdksh'
            - '/bin/sh'
            - '/bin/tcsh'
            - '/bin/zsh'
            - '$IFSash'
            - '$IFSbash'
            - '$IFSbsh'
            - '$IFScsh'
            - '$IFSksh'
            - '$IFSpdksh'
            - '$IFSsh'
            - '$IFStcsh'
            - '$IFSzsh'
    condition: all of selection_*
falsepositives:
    - Unlikely
level: high

Stages and Predicates

Stage 0: `condition`

all of selection_*

Stage 1: `selection_nc`

selection_nc:
    Image|endswith:
        - '/nc'
        - '/ncat'

Stage 2: `selection_flags`

selection_flags:
    CommandLine|contains:
        - ' -c '
        - ' -e '

Stage 3: `selection_shell`

selection_shell:
    CommandLine|contains:
        - ' ash'
        - ' bash'
        - ' bsh'
        - ' csh'
        - ' ksh'
        - ' pdksh'
        - ' sh'
        - ' tcsh'
        - '/bin/ash'
        - '/bin/bash'
        - '/bin/bsh'
        - '/bin/csh'
        - '/bin/ksh'
        - '/bin/pdksh'
        - '/bin/sh'
        - '/bin/tcsh'
        - '/bin/zsh'
        - '$IFSash'
        - '$IFSbash'
        - '$IFSbsh'
        - '$IFScsh'
        - '$IFSksh'
        - '$IFSpdksh'
        - '$IFSsh'
        - '$IFStcsh'
        - '$IFSzsh'

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
`CommandLine`	match	`-c` `-e` `ash` `bash` `bsh` `csh` `ksh` `pdksh` `sh` `tcsh` `$IFSash` `$IFSbash` `$IFSbsh` `$IFScsh` `$IFSksh` `$IFSpdksh` `$IFSsh` `$IFStcsh` `$IFSzsh` `/bin/ash` `/bin/bash` `/bin/bsh` `/bin/csh` `/bin/ksh` `/bin/pdksh` `/bin/sh` `/bin/tcsh` `/bin/zsh`
`Image`	ends_with	`/nc` `/ncat`

`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 Netcat Reverse Shell Execution

MITRE ATT&CK coverage

Event coverage

Rule body yaml

Stages and Predicates

Stage 0: `condition`

Stage 1: `selection_nc`

Stage 2: `selection_flags`

Stage 3: `selection_shell`

Indicators

Keyboard shortcuts

Search operators

Potential Netcat Reverse Shell Execution

MITRE ATT&CK coverage

Event coverage

Rule body yaml

Stages and Predicates

Stage 0: condition

Stage 1: selection_nc

Stage 2: selection_flags

Stage 3: selection_shell

Indicators

Stage 0: `condition`

Stage 1: `selection_nc`

Stage 2: `selection_flags`

Stage 3: `selection_shell`