header-injection

header-injection is a skill for Claude Code from PurpleAILAB/Decepticon. It costs 45 tokens per session (2,958 once invoked), scanned A, original, Apache-2.0.

A security testing guide for HTTP header injection, where untrusted input is copied into response headers or used to build links. It covers issues such as response splitting, cache poisoning, and forged password-reset links.

In plain words
What is it for?
Testing web applications and proxy setups for unsafe header handling, cache poisoning, reset-link poisoning, rate-limit bypass, and injected filenames or cookies.
Why use it?
It helps find cases where attackers can alter how browsers, caches, or users interpret a server response. It also highlights risks in proxy headers and account-recovery flows.

Skill for Claude Code

Written for Claude Code: allowed-tools in frontmatter.

Good fit Testing web applications and proxy setups for unsafe header handling, cache poisoning, reset-link poisoning, rate-limit bypass, and injected filenames or cookies.

Compare 6 skills from other repositories ↓
Install with agentmods
npx agentmods add skills/purpleailab/decepticon/header-injection
About the project

Decepticon is an autonomous red-team agent that coordinates AI agents, security tools, sandboxes, and supporting services for authorized cybersecurity assessments. Security researchers and red teams can run it through its Docker stack, cloud service, command-line interface, or Python SDK, with the catalogue entries representing its available skills.

PurpleAILAB/Decepticon · 5,497 stars · on GitHub · decepticon.red

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.

Any agent
npx skills add PurpleAILAB/Decepticon --skill header-injection
Clone the repo
git clone --depth 1 https://github.com/PurpleAILAB/Decepticon

Made for: Claude Code.

Wrote 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.

agentmods badge for header-injection

README.md
[![agentmods](https://agentmods.dev/badge/skills/purpleailab/decepticon/header-injection/github.svg)](https://agentmods.dev/skills/purpleailab/decepticon/header-injection)
Your own site
<a href="https://agentmods.dev/skills/purpleailab/decepticon/header-injection"><img src="https://agentmods.dev/badge/skills/purpleailab/decepticon/header-injection/github.svg" alt="Measured on agentmods" height="20"></a>

Or the 80×15 button, for a site that already has a row of RSS and ATOM ones. Only the verdict fits; the numbers stay here.

agentmods 80×15 button for header-injection

Your own site · 80×15
<a href="https://agentmods.dev/skills/purpleailab/decepticon/header-injection"><img src="https://agentmods.dev/badge/skills/purpleailab/decepticon/header-injection.svg" alt="Reviewed on agentmods" width="80" height="20"></a>
Per session 45 Skills are progressive disclosure: only the name and description are preloaded; the body loads when the skill is used.
When invoked 2,958 The whole file, excluding the scripts and references it only reads on demand.
Security scan A 1 finding. A grade says what 26 rules found in the file — not that it is safe. Third-party audits
  • NVIDIA SkillSpector warn 7 Sept 2026
SkillSpector: 5 findings, up to high

These are SkillSpector’s own severities. On a checked sample its high-severity flags on skills were ~96% false positives — a documented command, a public API, a “never do X” rule — so we show them as a caution to read, not a verdict. Why →

  • high Supply Chain · line 127
    Remote code is downloaded and executed. This bypasses code review and could introduce malicious code.
    Fix: Avoid downloading and executing remote scripts. Use trusted packages from PyPI/npm. If remote fetch is required, verify checksums and use HTTPS.
  • medium Data Exfiltration · line 91
    Data is being sent to an external URL. This could be legitimate telemetry or data exfiltration. Manual review is recommended.
    Fix: Verify the destination URL is trusted and necessary. Remove or replace with documented APIs. Ensure no secrets, tokens, or PII are transmitted.
  • medium Data Exfiltration · line 97
    Data is being sent to an external URL. This could be legitimate telemetry or data exfiltration. Manual review is recommended.
    Fix: Verify the destination URL is trusted and necessary. Remove or replace with documented APIs. Ensure no secrets, tokens, or PII are transmitted.
  • medium Data Exfiltration · line 127
    Data is being sent to an external URL. This could be legitimate telemetry or data exfiltration. Manual review is recommended.
    Fix: Verify the destination URL is trusted and necessary. Remove or replace with documented APIs. Ensure no secrets, tokens, or PII are transmitted.
  • medium Data Exfiltration · line 161
    Data is being sent to an external URL. This could be legitimate telemetry or data exfiltration. Manual review is recommended.
    Fix: Verify the destination URL is trusted and necessary. Remove or replace with documented APIs. Ensure no secrets, tokens, or PII are transmitted.
How audits are shown
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 5.1 $0.00045 $0.02958
Opus 5 $0.00023 $0.01479
Sonnet 5 $0.00009 $0.00592
Haiku 4.5 $0.00005 $0.00296

Measured 9d ago against content hash fd93e3dee4a7, method: parsed. Prices are Anthropic first-party input rates as of 2026-09-12, from the pricing page.

Security

Grade A, and why

header-injection scanned grade A with 1 finding 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 9d 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.

Makes network callslowCapability

Not a fault in itself. Listed so you know the mod talks to something, and to what.

curl -sv "${TARGET}/redirect?url=https://evil.com%0d%0aSet-Cookie:%20admin=1" 2>&1 \
packages/decepticon/decepticon/skills/standard/exploit/web/header-injection/SKILL.md · 230 lines

How it starts

The opening of the file, as written. The whole thing — 230 lines — stays where its author put it; the contents beside it link to each section on GitHub.

HTTP Header Injection

Header injection turns user-controlled input into HTTP protocol control. When server code copies a request value into a response header without CR/LF stripping, an attacker can terminate the current header and inject new headers or an entire second HTTP response. At the cache layer, headers that influence the response body but are excluded from the cache key enable persistent cache poisoning. The same family of bugs drives Host-header password-reset link hijacking, X-Forwarded-For rate-limit bypass, and Content-Disposition filename injection.

Authorized use only. Only test systems you are explicitly authorized to assess. Cache poisoning attacks affect all users of a shared cache resource.

Attack Surface

  • Query/body/path parameters echoed into Location, Set-Cookie, Content-Type, Content-Disposition, Link, or custom X-* headers
  • Request headers re-reflected into responses: Referer, User-Agent, X-Forwarded-Host, correlation IDs
  • Password-reset / account-recovery flows where the reset link is built from the Host header
  • OAuth/SSO redirect flows where Location is derived from user input
  • CDN/reverse-proxy stacks where X-Forwarded-Host or X-Forwarded-Proto is echoed into canonical URLs
  • Content-Disposition: attachment; filename=<user_input> file-download endpoints
  • Outbound email headers populated from user-supplied fields (To/From/Subject)

CR/LF Injection Payload Set

These are the characters and encodings to inject. Try each until one survives to the response header:

Encoding Value Notes
URL bare LF %0a Most permissive servers
URL bare CR %0d Rarely effective alone
URL CRLF %0d%0a Classic; many filters strip this
Double-encoded %250d%250a Bypasses single-decode WAFs
Tab %09 RFC 7230 permits tab in field values; some parsers fold into preceding header
Null byte %00 Truncates value in some C-based parsers
Unicode LS/PS %e2%80%a8 / %e2%80%a9 U+2028/U+2029; some intermediaries fold to LF
Overlong UTF-8 CR %c0%8d Invalid per spec but accepted by some older parsers

Read the full file on GitHub · 230 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. 9d ago First seen · 230 lines · 45 tokens per session scan A fd93e3dee4a7

Subscribe to this mod's changes

header-injection is a skill published in the GitHub repository PurpleAILAB/Decepticon (5,497 stars, last pushed 13d ago), licensed Apache-2.0. It adds 45 tokens to every session and 2,958 once invoked, about $0.0002 per session on Opus 5. A static security scan graded it A with 1 finding (makes network calls). No closer match exists in the catalogue, so it is treated as the original; first seen 2026-09-03.