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Agent skill

Test Server Side Fetching

by cyberful in cyberful/cyberful

Assess server-side URL retrieval and network egress during authorized penetration tests or code audits.

AGPL-3.0Auto-check passedBackend & APIs

Install Test Server Side Fetching

skills CLI
$ npx skills add cyberful/cyberful --skill test-server-side-fetching -a claude-code

Project install by default; add -g for ~/.claude/skills/.

GitHub CLI
$ gh skill install cyberful/cyberful test-server-side-fetching --agent claude-code

Project scope by default; add --scope user for a personal install. Needs GitHub CLI 2.90.0 or later (public preview).

Manual copy
$ git clone --depth 1 https://github.com/cyberful/cyberful.git skills-src && mkdir -p .claude/skills && cp -r skills-src/cyberful/builtin/skills/test-server-side-fetching .claude/skills/test-server-side-fetching && rm -rf skills-src

Use ~/.claude/skills/ instead of .claude/skills for a personal install. The folder must contain SKILL.md.

Claude Code skills documentation · loads skills from .claude/skills/

Facts

Skill name
test-server-side-fetching
GitHub stars
135
Token cost
~867 tokens
SKILL.md length
322 words
Files
4 (incl. references)
Skills in repo
85
Repo updated
First seen
Licence
AGPL-3.0

At a glance

Assess server-side URL retrieval and network egress during authorized penetration tests or code audits.

  • Works in 7 steps: parse and canonicalize the submitted URL; → validate scheme, authority, host, and… → resolve all address records; → …
  • URL parser confusion
  • SKILL.md covers Inventory Fetch Capabilities, Trace the Resolution Lifecycle, Confirm the Primitive and Audit Response and Credential…, plus 1 more section
  • Instructions only: no scripts, shell commands, URLs or credentials in SKILL.md

What it does

Test Server Side Fetching is an agent skill from cyberful/cyberful. Assess server-side URL retrieval and network egress during authorized penetration tests or code audits. Use for SSRF, blind SSRF, URL parser confusion, DNS rebinding, redirect validation, webhook callbacks, importers, previews, PDF or image fetchers, cloud metadata access, alternate protocols, internal service reachability, and egress-policy review.

Its SKILL.md is about 870 tokens, which your agent loads only when the skill is triggered. The skill folder holds 5 other files, including reference files (for example `agents/openai.yaml`, `references/egress-cloud.md` and `references/url-dns-redirects.md`).

It sits in Backend & APIs, covering Backend development, Web application vulnerabilities and Webhooks. The repository describes itself as: Cyberful is an open-source AI Red Team for discovering, exploiting, verifying, and remediating vulnerabilities. The licence is AGPL-3.0.

When your agent uses it

  • URL parser confusion
  • Redirect validation
  • Webhook callbacks
  • Cloud metadata access

Example prompts

  • “/test-server-side-fetching”

Workflow steps

7 steps, taken from the first numbered list in SKILL.md.

  1. parse and canonicalize the submitted URL;
  2. validate scheme, authority, host, and port;
  3. resolve all address records;
  4. classify every resolved address;
  5. connect to the validated address without unchecked re-resolution;
  6. validate every redirect target from scratch;
  7. constrain response size, time, type, and recursive fetches.

What it can do on your machine

Read from SKILL.md and the folder at commit ec598a6. It shows what the files ask for, not the result of running them.

  • Tool permissions

    Pre-approves nothing: there is no allowed-tools line, so your agent's usual permission prompts apply.

    From allowed-tools in the SKILL.md frontmatter.

  • Runs code

    No scripts in the folder and no shell commands in SKILL.md.

    From the folder's file list and the shell code blocks in SKILL.md.

  • Network

    No URLs in SKILL.md.

    From URLs in SKILL.md, links to its own repository left out.

  • Credentials

    Names no API keys, tokens, secrets or passwords.

    From names ending in _API_KEY, _TOKEN, _SECRET, _KEY or _PASSWORD in SKILL.md.

Context cost

Test Server Side Fetching loads about 867 tokens when it runs, and up to ~2k if it reads all its reference files. Until then it costs about 94 tokens; SKILL.md has 322 words of instructions outside code blocks.

Always · name and description, kept in context so the agent knows when to use it
~94
When it runs · the whole SKILL.md, loaded when a task matches
~867
With references · SKILL.md plus every file in references/, read only if the agent opens them
~2k

Estimates: characters ÷ 4, the usual rule of thumb; real counts depend on the model's tokenizer. Scripts and assets cost tokens only if the agent reads them.

Safety

Auto-check passed

The automated check found no risky patterns in SKILL.md.

Automated static check — not a guarantee. Review scripts before installing. It scans the text of SKILL.md for risky patterns (piping downloads into a shell, reading credential files, hidden Unicode, destructive commands); files beside SKILL.md are not scanned.

SKILL.md

The full file from cyberful/cyberful at commit ec598a6, republished under its AGPL-3.0 licence (© cyberful). 322 words, ~867 tokens.

Download SKILL.mdSave it as .claude/skills/test-server-side-fetching/SKILL.md (or your agent's skills folder). This skill also uses 3 other files; get the full folder from GitHub.
name
test-server-side-fetching
description
Assess server-side URL retrieval and network egress during authorized penetration tests or code audits. Use for SSRF, blind SSRF, URL parser confusion, DNS rebinding, redirect validation, webhook callbacks, importers, previews, PDF or image fetchers, cloud metadata access, alternate protocols, internal service reachability, and egress-policy review.
metadata.domain
application-security
metadata.subdomain
server-side-fetching
metadata.triggers
ssrf testing, blind ssrf, server-side url fetch, dns rebinding, cloud metadata access, egress policy review
metadata.tags
ssrf, url-parser, dns, redirects, cloud-metadata, egress

Test Server-Side Fetching

Inventory Fetch Capabilities

Find every feature that causes a trusted component to resolve or connect to an influenced destination: URL previews, webhooks, imports, feed readers, image proxies, document renderers, SSO metadata, repository integrations, health checks, headless browsers, and indirect downstream fetchers.

For each fetcher, record input source, parser, normalization, DNS resolver, proxy, redirect policy, supported schemes, credential attachment, network namespace, response handling, retry behavior, and caller-visible output.

Read url-dns-redirects.md for validation differentials. Read egress-cloud.md for network and cloud containment.

Trace the Resolution Lifecycle

Test controls at every stage:

  1. parse and canonicalize the submitted URL;
  2. validate scheme, authority, host, and port;
  3. resolve all address records;
  4. classify every resolved address;
  5. connect to the validated address without unchecked re-resolution;
  6. validate every redirect target from scratch;
  7. constrain response size, time, type, and recursive fetches.

An allowlist checked only before redirects or against a string representation is incomplete.

Confirm the Primitive

Use engagement-controlled endpoints and unique correlation tokens. Begin with outbound DNS or HTTP and record source address, method, headers, timing, redirect behavior, and retry cadence. For blind paths, correlate asynchronous callbacks and separate resolver traffic from application fetch traffic.

Timing is supporting evidence only when matched controls exclude application variance. A benign internal status or controlled listener can establish reach without collecting sensitive internal data.

Audit Response and Credential Handling

Determine whether the fetcher:

  • returns bodies, headers, screenshots, parsed metadata, or timing;
  • forwards cookies, service credentials, proxy credentials, client certificates, or authorization headers;
  • renders active content or parses fetched files;
  • follows nested references;
  • stores content where another security boundary consumes it.

Chain into file-parser or browser analysis when the fetched object enters those interpreters.

Classify Reach Precisely

State whether the issue provides DNS interaction, arbitrary outbound connection, internal HTTP write, internal read, authenticated service access, response exfiltration, non-HTTP protocol access, or code-adjacent parser reach. Include network context, redirect and DNS behavior, and the narrowest architectural correction.

© cyberful, AGPL-3.0. Rendered from Markdown: HTML in the file is shown as text, images as links, and headings moved down two levels. Raw file

Files

SKILL.md and 3 other files (references) in cyberful/builtin/skills/test-server-side-fetching of cyberful/cyberful.

  • SKILL.md
  • agents/openai.yaml
  • references/egress-cloud.md
  • references/url-dns-redirects.md

Open the folder on GitHubat commit ec598a6

Compare with similar skills

Test Server Side Fetching next to the 5 skills that share the most tags, products or categories with it. Stars are the repository's; “used in” counts other GitHub owners with a copy.

Test Server Side Fetching compared with similar skills
SkillStarsUsed inTokensAuto-checkLicenceRepo updated
Test Server Side Fetching this skillcyberful/cyberful135—~867Automated safety check: PassAGPL-3.0
Web Ssrfs0ld13rr/pentestcode828—~660Automated safety check: WarnMIT
Cashier Stripe Developmentluadotsh/lua3431 repos~1.2kAutomated safety check: PassMIT
Rails Security Multitenancymarckohlbrugge/37signals-skills724—~1.7kAutomated safety check: PassNone
Django Access Reviewgetsentry/skills1k3 repos~2.6kAutomated safety check: NotesApache-2.0
Django Securityaffaan-m/ECC276k5 repos~4kAutomated safety check: NotesMIT

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Categories

Questions about Test Server Side Fetching

What does Test Server Side Fetching do?

Assess server-side URL retrieval and network egress during authorized penetration tests or code audits. Test Server Side Fetching is an agent skill from cyberful/cyberful. Assess server-side URL retrieval and network egress during authorized penetration tests or code audits.

When should I use Test Server Side Fetching?

Test Server Side Fetching fits situations like: URL parser confusion; redirect validation; webhook callbacks; cloud metadata access.

How do I install Test Server Side Fetching in Claude Code?

Run `npx skills add cyberful/cyberful --skill test-server-side-fetching -a claude-code`. Or copy the skill folder (cyberful/builtin/skills/test-server-side-fetching in cyberful/cyberful) into .claude/skills/test-server-side-fetching in your project. Claude Code loads it when a task matches its description.

How do I install Test Server Side Fetching in Codex?

Run `npx skills add cyberful/cyberful --skill test-server-side-fetching -a codex`. Or copy the skill folder (cyberful/builtin/skills/test-server-side-fetching in cyberful/cyberful) into .agents/skills/test-server-side-fetching in your project. Codex loads it when a task matches its description.

Can I use Test Server Side Fetching in Cursor, Gemini CLI or GitHub Copilot?

Cursor, Gemini CLI, GitHub Copilot and OpenCode also load SKILL.md folders. With the skills CLI, run `npx skills add cyberful/cyberful --skill test-server-side-fetching -a cursor` (or -a gemini-cli, github-copilot or opencode for the others). To copy it by hand, put the folder in .cursor/skills/test-server-side-fetching, .gemini/skills/test-server-side-fetching, .github/skills/test-server-side-fetching and .opencode/skills/test-server-side-fetching in your project.

What does Test Server Side Fetching need to run?

SKILL.md names no scripts, command-line tools or credentials: Test Server Side Fetching is instructions for the agent only.

Does Test Server Side Fetching access the network?

SKILL.md contains no URLs. Any network use would come from the scripts or tools the agent runs. This is read from the text; nothing was executed.

Is Test Server Side Fetching safe to install?

Our automated static check of SKILL.md found no risky patterns, such as piping downloads into a shell, reading credential files or hidden Unicode. It is not a guarantee. Review the folder before installing.

What licence does Test Server Side Fetching use?

Test Server Side Fetching is published under the AGPL-3.0 licence (the repository's licence). It allows redistribution, so the full SKILL.md is shown on this page.

How many tokens does Test Server Side Fetching use?

About 867 tokens (SKILL.md is roughly 3.5k characters). Agents keep only the skill's name and description in context until a task matches; then they load SKILL.md in full. Its references folder adds about 1.1k tokens, read only when the agent opens those files.

What are the alternatives to Test Server Side Fetching?

Skills that share tags, products or a category with Test Server Side Fetching: Web Ssrf (s0ld13rr/pentestcode, 828 stars), Cashier Stripe Development (luadotsh/lua, 343 stars), Rails Security Multitenancy (marckohlbrugge/37signals-skills, 724 stars) and Django Access Review (getsentry/skills, 1k stars). The comparison table on this page puts their stars, adoption, token cost, safety result and licence side by side.

Who maintains Test Server Side Fetching?

cyberful (a GitHub organization) maintains it in cyberful/cyberful, which has 135 GitHub stars. The repository holds 85 skills in this directory. The repository was last updated on August 24, 2026.

Source: cyberful/cyberful on GitHub. Facts on this page come from the repository at the commit we read; the author's words are quoted as theirs.