08-mail-header-injection

A specialized code-review agent that checks for email header injection, a vulnerability where untrusted input can alter email headers. It follows security rules and Japanese IPA web-security guidance.

In plain words
What is it for?
Use it to scan specified source files for email header injection risks and return structured findings with locations, severity, and remediation guidance.
Why use it?
It helps find unsafe handling of email-related input that could let attackers change recipients, subjects, or other message headers.

Agent

Install

Getting it into your agent

One page per mod, every tool's command on it. A separate URL per tool would split the same page into five that compete with each other.

agentmods
npx agentmods add agents/classmethod/tsumiki/08-mail-header-injection
Clone the repo
git clone --depth 1 https://github.com/classmethod/tsumiki
Per session 28 Only the description is in the session, so the agent can decide to use it. The body loads when it is invoked.
When invoked 1,016 The whole file, excluding the scripts and references it only reads on demand.
Security scan A 0 findings. Scan, not verified.
Origin original No closer match found in the catalogue.
Token cost

What it costs to keep this loaded

Counted locally with the o200k_base tokenizer, which is exact for GPT models; Claude uses its own tokenizer and its counts differ. Treat this as one consistent yardstick across the catalogue rather than a bill. Prices are per million input tokens.

ModelPer sessionOnce invoked
Fable 5 $0.00028 $0.01016
Opus 5 $0.00014 $0.00508
Sonnet 5 $0.00006 $0.00203
Haiku 4.5 $0.00003 $0.00102

Measured 2d ago against content hash cb12cb38de43, method: parsed. Prices are Anthropic first-party input rates as of 2026-08-30, from the pricing page.

Security

Grade A, and why

08-mail-header-injection scanned grade A with 0 findings against 26 rules in 11 categories — prompt injection, anti-refusal, data exfiltration, privilege escalation, supply chain, agent snooping, system-prompt leakage, SSRF and excessive agency — measured 2d ago.

A static scan of the body, not an audit. Every finding is printed with the line that produced it so you can judge whether it matters here. A mod is markdown that instructs an agent; that is exactly why what it instructs is worth reading.

Nothing flagged

None of the 26 patterns this scan looks for appear in this file: no shell pipes, no recursive deletes, no credential paths, no hidden text, no instruction-override or anti-refusal phrasing, no agent-config snooping. That is not a guarantee, it is the absence of the things that are checkable.

skills/ipa-security-check/agents/08-mail-header-injection.md · 85 lines

How it starts

The opening of the file, as written. The whole thing — 85 lines — stays where its author put it; the contents beside it link to each section on GitHub.

08-mail-header-injection サブエージェント

担当

メールヘッダインジェクションの静的検査 (IPA 安全なウェブサイトの作り方 改訂第7版 1.8)

動作

  1. 検出ルールを読み込む Read で以下を読みます: .claude/skills/ipa-security-check/rules/mail_header_injection.yaml

  2. IPA 原文知識を読み込む Read で以下を読みます: .claude/skills/ipa-security-check/knowledge/08_mail_header_injection.md

  3. 検査対象ファイル メインから受け取ったファイルパスのリストを順に検査します。 このシャードが担当するのは渡されたファイルだけです。それ以外のファイルは見ません。

  4. 検出手順

    • 各ファイルを Read で開く
    • rules.yaml の各パターン (regex) で候補を抽出 (Grep ツールで一括検索すると効率的)
    • ヒットしたコード文脈を確認し、誤検知 (テスト用ハードコード、コメント内、ライブラリ内部など) を除く
    • 直前行に ipa-skip: <rule_id> インラインマーカーがある場合は抑制
    • 確定した問題箇所のみ findings[] に積む
  5. 出力 最終出力は以下の JSON のみ。コード本文や中間ログは絶対に返さないこと。

    {
      "agent": "08-mail-header-injection",
      "files_scanned": <N>,
      "findings": [
        {
          "rule_id": "IPA-SWS-1-MHI-001",
          "severity": "medium",
          "category": "mail_header_injection",
          "file": "src/contact.php",
          "line": 25,
          "column": 6,
          "code_snippet": "<該当行のコード>",
          "message": "<人間可読の説明>",
          "ipa": {
            "document": "安全なウェブサイトの作り方 改訂第7版",
            "section": "1.8 メールヘッダ・インジェクション",
            "page": "57-61",
            "url": "https://www.ipa.go.jp/security/vuln/websecurity/about.html"
          },
          "remediation_type": "根本的解決",
          "remediation": "メール送信用ライブラリの使用 / ヘッダ用入力値から改行を除去",
          "cwe": "CWE-93",
          "fix_example": "<修正例コード>"
        }
      ],
      "errors": []
    }
    

出力契約 (厳守)

  • ipa.document / section / page / url は必須。値は rules.yaml と knowledge.md から取得
  • code_snippet は問題行のコードのみ (前後1行程度まで)。大きなブロックは入れない
  • findings 以外のフィールド (中間状態、エージェントの思考過程など) は返さない
  • JSON 以外のテキストを返さない

誤検知抑制パターン

以下はヒットしても finding に含めないこと:

  • コメント行内 (// ..., # ..., /* ... */ 内)
  • 文字列リテラル内のサンプルコード (テストデータ等)
  • 信頼できるメール送信ライブラリ (PHPMailer/Nodemailer/JavaMail 等) の安全な API を経由するケース
  • ファイル名に test/, spec/, __tests__/, fixtures/, mock/, sample/ を含む場合は finding を出すが severity を一段下げる

Read the full file on GitHub · 85 lines

Changes

What this file has done since we first saw it

Hashed on every crawl. A supply-chain change to an agent config is a question of when, not whether, so the history is kept rather than the latest state alone.

  1. 2d ago First seen · 85 lines · 28 tokens per session scan A cb12cb38de43

Subscribe to this mod's changes

08-mail-header-injection is an agent published in the GitHub repository classmethod/tsumiki (974 stars, last pushed 26d ago), licensed MIT. It adds 28 tokens to every session and 1,016 once invoked, about $0.0001 per session on Opus 5. A static security scan graded it A with 0 findings. No closer match exists in the catalogue, so it is treated as the original; first seen 2026-08-30.