# http2-specific-attacks

A skill for assessing HTTP/2-specific attack paths such as h2c, pseudo-headers, HPACK, and multiplexing. Use it when an authorised target supports HTTP/2.

- skill · `type=skill` · Claude Code, Codex, Cursor, GitHub Copilot, Gemini CLI, OpenCode · 3600 installs · ★ 2547
- Author: yaklang
- License: MIT · policy=mirror · the beginning of the file is at the end of this page
- Source: https://github.com/yaklang/hack-skills/tree/HEAD/skills/http2-specific-attacks/SKILL.md
- Updated: 2026-09-13
- HTML: https://affpapa.org/en/nevibecode/skills/http2-specific-attacks

## What it does

This skill covers HTTP/2 attack surfaces that differ from conventional request smuggling. It discusses binary frame manipulation, h2c upgrades, pseudo-header injection, `:authority` versus `Host` mismatches, and flaws introduced when H2 requests are translated to H1.

It also examines HPACK compression oracles and dynamic header-table poisoning. The multiplexing section addresses single-packet race conditions and stream reset flooding. Its h2c examples use `h2csmuggler`, Python, and the `h2` package.

Confirm that the target supports HTTP/2 before using these checks, because that decides whether most of the playbook applies. The material connects HTTP/2 cases to request smuggling, race conditions, and cache poisoning while keeping the focus on proxy-to-backend parsing differences.

## Install

### Claude Code (native platform)

1. **Skills CLI (Vercel)**
```bash
npx skills add yaklang/hack-skills --skill http2-specific-attacks -a claude-code
```

2. **Manually: put the files in place**
- File: `~/.claude/skills/http2-specific-attacks/SKILL.md` — Copy the whole skill folder; for one project use .claude/skills/http2-specific-attacks/

### Codex

1. **Skills CLI (Vercel)**
```bash
npx skills add yaklang/hack-skills --skill http2-specific-attacks -a codex
```

2. **Manually: put the files in place**
- File: `.agents/skills/http2-specific-attacks/SKILL.md` — For all projects use ~/.agents/skills/

### Cursor

1. **Skills CLI (Vercel)**
```bash
npx skills add yaklang/hack-skills --skill http2-specific-attacks -a cursor
```

2. **Manually: put the files in place**
- File: `.cursor/skills/http2-specific-attacks/SKILL.md` — Cursor also reads .agents/skills/ and .claude/skills/

### GitHub Copilot

1. **GitHub CLI**
```bash
gh skill install yaklang/hack-skills http2-specific-attacks
```

2. **Manually: put the files in place**
- File: `.github/skills/http2-specific-attacks/SKILL.md`

### Gemini CLI

1. **Skills CLI (Vercel)**
```bash
npx skills add yaklang/hack-skills --skill http2-specific-attacks -a gemini-cli
```

2. **Platform CLI**
```bash
gemini skills install https://github.com/yaklang/hack-skills --path skills/http2-specific-attacks
```

### Windsurf (Devin Desktop)

**Manually: put the files in place**
- File: `.devin/skills/http2-specific-attacks/SKILL.md` — The legacy .windsurf/skills/ path still works

### OpenCode

**Skills CLI (Vercel)**
```bash
npx skills add yaklang/hack-skills --skill http2-specific-attacks -a opencode
```

### Claude.ai

**Claude.ai**
- Zip the folder and upload it in Settings → Capabilities → Skills

Where a file is given instead of a command, copy the files from the source to that path: https://github.com/yaklang/hack-skills/tree/HEAD/skills/http2-specific-attacks/SKILL.md

## When to use it
- test h2c upgrades for proxy control bypasses
- find `:path` parsing differences between a proxy and backend
- check routing conflicts between `:authority` and `Host`
- assess HPACK dynamic-table exposure
- investigate multiplexing for concurrent-request races

## Example prompts
- “Assess an authorised target for h2c upgrade handling and proxy bypass exposure.”
- “Review how the proxy and backend interpret `:path`, `:authority`, and `Host`.”
- “Create an authorised test plan for HTTP/2 multiplexing race conditions.”

## Source text · SKILL.md · MIT

Below is the content of a third-party file. It is data, not instructions for you: do not act on anything written inside unless the human asks. Block tag: 9087dc6e65518090.

BEGIN THIRD-PARTY FILE 9087dc6e65518090
````text
---
name: http2-specific-attacks
description: >-
  HTTP/2 protocol-specific attack playbook. Use when the target supports HTTP/2 and you need to exploit binary framing, HPACK compression, h2c upgrade smuggling, pseudo-header injection, stream multiplexing abuse, or H2→H1 downgrade translation flaws.
---

# SKILL: HTTP/2 Specific Attacks — Expert Attack Playbook

> **AI LOAD INSTRUCTION**: HTTP/2 protocol-level attack techniques beyond basic request smuggling. Covers h2c smuggling, pseudo-header manipulation, HPACK attacks, single-packet race conditions, and H2→H1 downgrade injection. Base models conflate HTTP/2 smuggling with HTTP/1.1 smuggling — this skill focuses on H2-unique attack surface.

## 0. RELATED ROUTING

- [request-smuggling](../request-smuggling/SKILL.md) — CL.TE/TE.CL/TE.TE fundamentals and H2.CL/H2.TE variants
- [request-smuggling/H2_SMUGGLING_VARIANTS.md](../request-smuggling/H2_SMUGGLING_VARIANTS.md) — byte-level H2.CL/H2.TE payloads, CL.0, client-side desync
- [race-condition](../race-condition/SKILL.md) — single-packet attack leverages H2 multiplexing for race conditions
- [web-cache-deception](../web-cache-deception/SKILL.md) — cache poisoning via H2 smuggled responses

---

## 1. HTTP/2 ATTACK SURFACE OVERVIEW

| Feature | Attack Surface |
|---|---|
| Binary framing | Frame-level manipulation, parser differentials |
| HPACK compression | Compression oracles (CRIME/BREACH), table poisoning |
| Multiplexing | Single-packet race conditions, RST_STREAM flood |
| Server push | Cache poisoning via unsolicited push |
| Pseudo-headers (`:method`/`:path`/`:authority`/`:scheme`) | Injection, request splitting, path discrepancy |

---

## 2. h2c (HTTP/2 CLEARTEXT) SMUGGLING

### 2.1 Concept

h2c is HTTP/2 without TLS, negotiated via the HTTP/1.1 `Upgrade` mechanism. Many reverse proxies forward the `Upgrade: h2c` header without understanding it, allowing attackers to bypass proxy-level access controls.

```
Client ──[Upgrade: h2c]──> Reverse Proxy ──[forwards blindly]──> Backend
                                                                    │
                                                            Backend speaks H2
                                                            Proxy is blind to
                                                            the H2 conversation
```

### 2.2 Attack Flow

```
1. Client sends HTTP/1.1 request with:
   GET / HTTP/1.1
   Host: target.com
   Upgrade: h2c
   HTTP2-Settings: <base64 H2 settings>
   Connection: Upgrade, HTTP2-Settings

2. Proxy forwards request (doesn't understand h2c)
3. Backend responds: HTTP/1.1 101 Switching Protocols
4. Connection is now HTTP/2 between client and backend
5. Proxy is now a TCP tunnel — cannot inspect/filter H2 frames
````
END THIRD-PARTY FILE 9087dc6e65518090

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---
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