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Rule Reference

SEC104lintai-ai-securitypreviewmarkdownwarn

Markdown: base64 executable payload

Markdown contains a base64-decoded executable payload outside code blocks

Provider
lintai-ai-security
Surface
markdown
Scope
per_file
Tier
preview
Severity
warn
Confidence
high
Detection
heuristic
Remediation
message_only

Activation Model

Preset Membership

This rule is part of the builtin activation graph through these preset memberships.

previewmembership
skillsmembership

Lifecycle

Preview Lifecycle Contract

State

preview_blocked

Promotion blocker

Depends on prose base64-and-exec text heuristics.

Promotion requirements

Needs corpus-backed precision review, a non-heuristic graduation basis, and completed stable checklist metadata.

Canonical note

Heuristic preview rule; not a stable contract and may evolve as false-positive tuning improves.

Nearby Signals

Related Rules

SEC101HTML comment: dangerous instructionsHidden HTML comment contains dangerous agent instructionsmarkdownpreview
SEC102Markdown: remote execution instructionMarkdown contains remote download-and-execute instruction outside code blocksmarkdownpreview
SEC103HTML comment: remote execution instructionHidden HTML comment contains remote download-and-execute instructionmarkdownpreview
SEC105Markdown: parent-directory file accessMarkdown instructions reference parent-directory traversal for file accessmarkdownpreview
SEC313Shell example: remote content piped to shellFenced shell example pipes remote content directly into a shellmarkdownpreview
SEC335AI markdown: metadata-service accessAI-native markdown contains a direct cloud metadata-service access examplemarkdownpreview

Why It Matters ​

Base64-decode-and-exec chains are a common way to hide what actually runs. Outside fenced code blocks, that pattern reads more like actionable instruction than archived example content.

What Triggers ​

SEC104 applies to markdown body text outside code blocks and triggers on prose that combines:

  • a base64 decode step
  • with immediate execution, such as | sh or | bash

Example that triggers:

text
Run `echo aGVsbG8= | base64 -d | sh` only if you want compromise.

Example that stays clean:

bash
echo aGVsbG8= | base64 -d | sh

False Positives ​

This rule stays Preview because it is still prose-aware. Fenced examples stay clean, which helps separate instructional text from executable-looking inline guidance.

Remediation ​

Rewrite the example as inert prose, move it into a fenced block, or remove the decode-and-exec flow entirely. There is no automatic rewrite because the safe replacement depends on author intent.