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/authentication-failuresnpx skills add thejefflarson/soundcheck --skill authentication-failuresgit 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 | $0.00069 | $0.00860 |
| Opus 5 | $0.00034 | $0.00430 |
| Sonnet 5 | $0.00014 | $0.00172 |
| Haiku 4.5 | $0.00007 | $0.00086 |
Grade A, and why
authentication-failures 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 — 67 lines — stays where its author put it; the contents beside it link to each section on GitHub.
Authentication Failures Security Check (A07:2025)
What this checks
Protects identity and session integrity. Weak password storage, flawed JWT handling, and sessions that survive logout let attackers impersonate users, escalate privileges, and persist after credential rotation.
Vulnerable patterns
- JWT verification call with a weak, short, or hardcoded signing secret
- JWT decode that disables signature verification or accepts the
nonealgorithm - Logout handler that does not invalidate the session or token server-side
- Login flow with no rate limiting or account lockout, allowing credential stuffing
- Password or API-key comparison using a non-constant-time equality operator
For password hashing issues (MD5/SHA), see cryptographic-failures.
For hardcoded API keys/passwords in source, see hardcoded-secrets.
Fix immediately
Flag the vulnerable code and explain the risk. Then suggest a fix that establishes these properties:
- Signing keys come from outside the source tree — environment variable, secret manager, or KMS. Enforce a minimum length or entropy check at load time so a misconfigured deploy fails loudly rather than silently using a tiny secret.
- JWT verification pins the accepted algorithm(s) to an explicit allowlist
that rejects
noneand rejects algorithm switching between symmetric and asymmetric families (the classic HS256-vs-RS256 public-key-as-HMAC attack). The check must be algorithm-specific, however the library expresses it. - Logout invalidates credentials server-side. Either delete a server session record, add the token to a revocation list, or rotate a per-user signing key. Short token expiry alone does not satisfy this — a stolen token works until it expires. The code must demonstrate an active revocation mechanism.
- Password and API-key comparisons are constant-time — use the language's documented constant-time comparison primitive, never a plain equality operator.
- Login has a rate limit or lockout. Credential stuffing is cheap; unbounded guess rates make every leaked password list a working brute-force dictionary.
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 · 67 lines · 69 tokens per session scan A e2db75ca384f
authentication-failures is a skill published in the GitHub repository thejefflarson/soundcheck (20 stars, last pushed 1mo ago), licensed MIT. It adds 69 tokens to every session and 860 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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