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/header-injectionnpx skills add thejefflarson/soundcheck --skill header-injectiongit 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 | $0.00060 | $0.00651 |
| Opus 5 | $0.00030 | $0.00326 |
| Sonnet 5 | $0.00012 | $0.00130 |
| Haiku 4.5 | $0.00006 | $0.00065 |
Grade A, and why
header-injection 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 — 61 lines — stays where its author put it; the contents beside it link to each section on GitHub.
HTTP Header Injection Security Check (CWE-113)
What this checks
Protects against HTTP response header injection where user input containing CR/LF characters is included in response headers, allowing attackers to inject arbitrary headers or split the HTTP response. Exploitation leads to cache poisoning, session fixation, XSS via injected headers, and response splitting.
Vulnerable patterns
- Response header value set directly from a request parameter with no CR/LF stripping
Content-Dispositionfilename interpolated from user input without normalizationLocationredirect header built from a caller-supplied URL with no validation- Forwarded request header (correlation ID, user agent, custom header) echoed into an outgoing response without sanitization
- Email or SMTP header value built from form input without rejecting newline characters
Fix immediately
Flag the vulnerable code and explain the risk. Translate the principles below to the audited file's language and HTTP framework — use that stack's documented header API and string-sanitization helpers.
For each finding, establish these properties:
- Every header value derived from user input is stripped of CR/LF before it reaches the header-set call. A single newline sequence in a value ends the header block and starts a new header (or a new response body). The strip happens at or before the call site — relying on the framework to reject it is fragile across versions.
- Forwarded headers are sanitized too. Values read from incoming requests (correlation IDs, user agents, custom headers) are attacker-controlled just like form inputs. Echoing them into outgoing responses or logs without stripping is the same vulnerability.
- Content-Disposition filenames are normalized — CR/LF stripped, quotes
escaped, and for international characters use the RFC 5987
filename*form rather than raw UTF-8 in the header value. - Redirect Location headers are validated too. A newline sequence in the
target URL splits the response; see the
open-redirectskill for target-host validation.
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 · 61 lines · 60 tokens per session scan A 390abe969f31
header-injection is a skill published in the GitHub repository thejefflarson/soundcheck (20 stars, last pushed 1mo ago), licensed MIT. It adds 60 tokens to every session and 651 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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