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.
npx agentmods add skills/thejefflarson/soundcheck/file-uploadnpx skills add thejefflarson/soundcheck --skill file-uploadgit clone --depth 1 https://github.com/thejefflarson/soundcheckWhat 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 | $0.00062 | $0.00787 |
| Opus 5 | $0.00031 | $0.00394 |
| Sonnet 5 | $0.00012 | $0.00157 |
| Haiku 4.5 | $0.00006 | $0.00079 |
Grade A, and why
file-upload 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 2d 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.
How it starts
The opening of the file, as written. The whole thing — 68 lines — stays where its author put it; the contents beside it link to each section on GitHub.
File Upload Security Check (A04:2025)
What this checks
Protects against unrestricted file upload attacks where an attacker uploads executable files (web shells, scripts, HTML with embedded JS) that the server later serves or executes. Exploitation leads to remote code execution, stored XSS, or full server compromise.
Vulnerable patterns
- Upload handler that writes the user-supplied filename directly to disk, enabling path traversal and extension bypass
- Upload destination inside the web-accessible document root, so uploaded content is served directly by the web server
- No extension allowlist, or an extension denylist that misses novel or case-varied extensions
- No upload-size cap enforced at the framework or reverse-proxy level
- MIME type taken from the client-supplied
Content-Typeheader rather than the file's magic bytes
Fix immediately
Flag the vulnerable code and explain the risk. Then suggest a fix that establishes these properties:
- Extension is validated against an allowlist, never a denylist. Denylists
miss novel extensions (
.phtml,.phar,.svg) and case variations. An allowlist of expected types (.png,.jpg,.pdf) fails closed. - The user-supplied filename is never used as the storage path. Rename the
file to a CSPRNG identifier, preserving only the validated extension. This
defeats path traversal (
../../etc/passwd), overwrites of existing files, and filename-based XSS on download. - Uploads are written outside the webroot. The server must not serve them
directly — files are handed out through an application route that sets a safe
Content-TypeandContent-Disposition: attachment, never inferred from the filename. - Size is capped at the framework or proxy level, not just inside the handler. A handler-only check lets a multi-gigabyte upload exhaust memory before the check runs.
- MIME type is validated against the file's magic bytes, not the
Content-Typeheader the client sent — the header is attacker-controlled.
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.
- 2d ago First seen · 68 lines · 62 tokens per session scan A 82f86b040cf8
file-upload is a skill published in the GitHub repository thejefflarson/soundcheck (20 stars, last pushed 1mo ago), licensed MIT. It adds 62 tokens to every session and 787 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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