Potential Base64 Decoded From Images

Status: test
Severity: high
Log source: product macos, category process_creation
Author: Joseliyo Sanchez, @Joseliyo_Jstnk
Source: github.com/SigmaHQ/sigma

Detects the use of tail to extract bytes at an offset from an image and then decode the base64 value to create a new file with the decoded content. The detected execution is a bash one-liner.

MITRE ATT&CK coverage

Tactic	Techniques
Stealth	`T1140` Deobfuscate/Decode Files or Information

Event coverage

Provider	Event	Title
ESF	exec	Process Execution (Notify)

Rule body yaml

title: Potential Base64 Decoded From Images
id: 09a910bf-f71f-4737-9c40-88880ba5913d
status: test
description: |
    Detects the use of tail to extract bytes at an offset from an image and then decode the base64 value to create a new file with the decoded content. The detected execution is a bash one-liner.
references:
    - https://www.virustotal.com/gui/file/16bafdf741e7a13137c489f3c8db1334f171c7cb13b62617d691b0a64783cc48/behavior
    - https://www.virustotal.com/gui/file/483fafc64a2b84197e1ef6a3f51e443f84dc5742602e08b9e8ec6ad690b34ed0/behavior
author: Joseliyo Sanchez, @Joseliyo_Jstnk
date: 2023-12-20
tags:
    - attack.stealth
    - attack.t1140
logsource:
    product: macos
    category: process_creation
detection:
    # Example:  /bin/bash sh -c tail -c +21453 '/Volumes/Installer/Installer.app/Contents/Resources/workout-logo.jpeg' | base64 --decode > /tmp/54A0A2CD-FAD1-4D4D-AAF5-5266F6344ABE.zip
    # VT Query: 'behavior_processes:"tail" (behavior_processes:"jpeg" or behavior_processes:"jpg" or behavior_processes:"png" or behavior_processes:"gif") behavior_processes:"base64" behavior_processes:"--decode >" and tag:dmg'
    selection_image:
        Image|endswith: '/bash'
    selection_view:
        CommandLine|contains|all:
            - 'tail'
            - '-c'
    selection_b64:
        CommandLine|contains|all:
            - 'base64'
            - '-d' # Also covers "--decode"
            - '>'
    selection_files:
        CommandLine|contains:
            - '.avif'
            - '.gif'
            - '.jfif'
            - '.jpeg'
            - '.jpg'
            - '.pjp'
            - '.pjpeg'
            - '.png'
            - '.svg'
            - '.webp'
    condition: all of selection_*
falsepositives:
    - Unknown
level: high

Stages and Predicates

Stage 0: `condition`

all of selection_*

Stage 1: `selection_image`

selection_image:
    Image|endswith: '/bash'

Stage 2: `selection_view`

selection_view:
    CommandLine|contains|all:
        - 'tail'
        - '-c'

Stage 3: `selection_b64`

selection_b64:
    CommandLine|contains|all:
        - 'base64'
        - '-d'
        - '>'

Stage 4: `selection_files`

selection_files:
    CommandLine|contains:
        - '.avif'
        - '.gif'
        - '.jfif'
        - '.jpeg'
        - '.jpg'
        - '.pjp'
        - '.pjpeg'
        - '.png'
        - '.svg'
        - '.webp'

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` `-d` `.avif` `.gif` `.jfif` `.jpeg` `.jpg` `.pjp` `.pjpeg` `.png` `.svg` `.webp` `>` `base64` `tail`
`Image`	ends_with	`/bash`

`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 Base64 Decoded From Images

MITRE ATT&CK coverage

Event coverage

Rule body yaml

Stages and Predicates

Stage 0: `condition`

Stage 1: `selection_image`

Stage 2: `selection_view`

Stage 3: `selection_b64`

Stage 4: `selection_files`

Indicators

Keyboard shortcuts

Search operators

Potential Base64 Decoded From Images

MITRE ATT&CK coverage

Event coverage

Rule body yaml

Stages and Predicates

Stage 0: condition

Stage 1: selection_image

Stage 2: selection_view

Stage 3: selection_b64

Stage 4: selection_files

Indicators

Stage 0: `condition`

Stage 1: `selection_image`

Stage 2: `selection_view`

Stage 3: `selection_b64`

Stage 4: `selection_files`