2597 Foundations 9e0eb90b

Shared Foundations: Attacker-Controlled Input Model

This reference documents cross-cutting concepts that all 9 attack vector detection heuristics depend on. Read this before analyzing individual vectors.

Attacker-Controlled GitHub Context Expressions

These github.event.* expressions resolve to content an external attacker can influence. Dangerous contexts typically end with: body, default_branch, email, head_ref, label, message, name, page_name, ref, title.

High-frequency (seen across PoC workflows):

• github.event.issue.body -- issue body text
• github.event.issue.title -- issue title
• github.event.comment.body -- comment text on issues or PRs
• github.event.pull_request.body -- PR description
• github.event.pull_request.title -- PR title
• github.event.pull_request.head.ref -- PR source branch name
• github.event.pull_request.head.sha -- PR commit SHA (used in checkout)

Lower-frequency but still dangerous:

• github.event.review.body -- review comment text
• github.event.discussion.body, github.event.discussion.title
• github.event.pages.*.page_name -- wiki page name
• github.event.commits.*.message, github.event.commits.*.author.email, github.event.commits.*.author.name
• github.event.head_commit.message, github.event.head_commit.author.email, github.event.head_commit.author.name
• github.head_ref -- branch name (attacker-controlled in fork PRs)

Any ${{ }} expression referencing these contexts carries attacker-controlled content into whatever consumes the resolved value.

How env: Blocks Work in GitHub Actions

Environment variables can be set at three scopes:

1. Workflow-level env: (top of file) -- inherited by all jobs and steps
2. Job-level env: (under jobs.<id>:) -- inherited by all steps in that job
3. Step-level env: (under a step) -- available only to that step

Narrower scopes override broader ones. Critically, ${{ }} expressions in env: values are evaluated BEFORE the step runs. The step only sees the resolved string value, never the expression. This is the mechanism behind Vector A: the AI agent receives attacker content through an env var without any ${{ }} expression appearing in the prompt field itself.

env:
  ISSUE_BODY: ${{ github.event.issue.body }}   # evaluated at workflow parse time
# By the time the step runs, ISSUE_BODY contains the raw attacker text

Security-Relevant Trigger Events

These on: events expose workflows to external attacker-controlled input:

Note: push events from the default branch and pull_request events that do not grant secrets to forks are generally lower risk for prompt injection because the attacker cannot influence the content that reaches the AI agent without already having write access.

Data Flow Model

Attacker input reaches AI agents through three distinct paths:

Path 1 -- Direct expression interpolation:

github.event.*.body  ->  ${{ }} in prompt field  ->  AI processes attacker text

Path 2 -- Env var intermediary:

github.event.*.body  ->  env: VAR: ${{ }}  ->  prompt reads $VAR  ->  AI processes attacker text

Path 3 -- Runtime fetch:

github.event.*.number  ->  gh issue view N  ->  API returns attacker body  ->  AI processes attacker text

Path 2 requires extra attention because the prompt field contains zero ${{ }} expressions, making the injection invisible in the prompt itself. Path 3 is missed because the attacker content is not present in the workflow YAML at all -- it is fetched at runtime.

AI Action Prompt Field Names

Where each supported action receives prompt content that could carry attacker input:

When checking for attacker-controlled content in prompts, examine ALL fields listed for the relevant action, not just the primary prompt field.