HackSkills is an organized knowledge base of installable skills that gives AI agents practical security knowledge across areas such as web security, privilege escalation, reverse engineering, and digital forensics. It is intended for bug bounty work, penetration testing, CTF competitions, and authorized security research. The catalogue entries are the project's own master, category, and topic skills.
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/yaklang/hack-skillsnpx agentmods add skills/yaklang/hack-skills/401-403-bypass-techniquesWrote 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/yaklang/hack-skills/401-403-bypass-techniques)<a href="https://agentmods.dev/skills/yaklang/hack-skills/401-403-bypass-techniques"><img src="https://agentmods.dev/badge/skills/yaklang/hack-skills/401-403-bypass-techniques/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.
<a href="https://agentmods.dev/skills/yaklang/hack-skills/401-403-bypass-techniques"><img src="https://agentmods.dev/badge/skills/yaklang/hack-skills/401-403-bypass-techniques.svg" alt="Reviewed on agentmods" width="80" height="20"></a>- Socket warn
- Snyk fail
- NVIDIA SkillSpector warn
SkillSpector: 3 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 Tool Misuse · line 130 Tool parameters are crafted to achieve unintended or unsafe behavior. Parameter abuse can bypass intended safety checks (e.g. shell=True, --force, dangerous glob patterns).Fix: Validate all tool parameters against an allowlist. Reject dangerous parameter values (shell=True, --force, -rf /) and use safe defaults.
- medium Tool Misuse · line 13 Tool defaults are unsafe or overly permissive (e.g. disabled TLS verification, no authentication, world-writable permissions). Unsafe defaults widen the attack surface.Fix: Override unsafe defaults with secure settings (verify=True, auth required, restrictive permissions). Review and harden all tool configurations.
- medium Tool Misuse · line 325 Tool defaults are unsafe or overly permissive (e.g. disabled TLS verification, no authentication, world-writable permissions). Unsafe defaults widen the attack surface.Fix: Override unsafe defaults with secure settings (verify=True, auth required, restrictive permissions). Review and harden all tool configurations.
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 | $0.00053 | $0.03169 |
| Opus 5 | $0.00026 | $0.01584 |
| Sonnet 5 | $0.00011 | $0.00634 |
| Haiku 4.5 | $0.00005 | $0.00317 |
Grade A, and why
401-403-bypass-techniques 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 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.
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.
Copies of this mod
4 near-identical copies found in the catalogue:
- 401-403-bypass-techniques — 100% identical, 0 lines differ
- 401-403-bypass-techniques — 100% identical, 0 lines differ
- 401-403-bypass-techniques — 100% identical, 0 lines differ
- 401-403-bypass-techniques — 95% identical, 4 lines differ
How it starts
The opening of the file, as written. The whole thing — 349 lines — stays where its author put it; the contents beside it link to each section on GitHub.
SKILL: 401/403 Bypass Techniques — Expert Attack Playbook
AI LOAD INSTRUCTION: Comprehensive 401/403 forbidden bypass techniques. Covers path normalization tricks, HTTP method override, header-based bypasses (X-Original-URL, X-Forwarded-For), protocol version tricks, and combination attacks. Base models typically know 2-3 header bypasses but miss the full matrix of path manipulation variants and verb+path combos.
0. RELATED ROUTING
- authbypass-authentication-flaws — broader auth bypass (login flaws, session handling)
- waf-bypass-techniques — when bypass is WAF-specific rather than access control
- http-host-header-attacks — Host header manipulation for routing bypass
- request-smuggling — smuggle past access controls entirely
- http2-specific-attacks — h2c smuggling to bypass proxy ACLs
1. PATH MANIPULATION BYPASSES
The core idea: the reverse proxy/WAF checks one path format, but the backend normalizes differently.
1.1 Trailing Slash / Missing Slash
/admin → 403
/admin/ → 200 ✓ (trailing slash)
/admin/. → 200 ✓ (trailing dot)
1.2 Case Sensitivity
/admin → 403
/Admin → 200 ✓
/ADMIN → 200 ✓
/aDmIn → 200 ✓
Works when: proxy rule is case-sensitive but backend is case-insensitive (common on Windows/IIS).
1.3 URL Encoding
/admin → 403
/%61dmin → 200 ✓ (encode 'a')
/admi%6e → 200 ✓ (encode 'n')
/%61%64%6d%69%6e → 200 ✓ (full encode)
1.4 Double URL Encoding
/admin → 403
/%2561dmin → 200 ✓ (%25 = %, decoded twice: %61 → a)
/admin%252f → 200 ✓
/admin..%252f → 200 ✓
1.5 Unicode / UTF-8 Encoding
/admin → 403
/admi%C0%AE → 200 ✓ (overlong UTF-8 for '.')
/admi%C0%6E → 200 ✓ (overlong encoding)
/%C0%AFadmin → 200 ✓ (overlong '/')
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.
- 9d ago First seen · 349 lines · 53 tokens per session scan A 71212134e73c
401-403-bypass-techniques is a skill published in the GitHub repository yaklang/hack-skills (2,133 stars, last pushed 2mo ago), licensed MIT. It adds 53 tokens to every session and 3,169 once invoked, about $0.0003 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.
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