Use it when deciding which environment variables, tokens, authorization rules, API calls, or server actions may safely cross from a web server to the client.

It reduces the risk of leaking API keys or allowing users to influence sensitive business rules through code they can inspect and change in the browser.

security-client-server-boundary-and-secret-exp

Q: What is security-client-server-boundary-and-secret-exposure?

Security guidance for keeping private information and privileged operations on the server instead of exposing them to browser code. It covers public settings, private credentials, scoped tokens, and server-side checks.

Install

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.

Any agent

npx skills add andrewsrigom/agent-skills --skill security-client-server-boundary-and-secret-exposure

Clone the repo

git clone --depth 1 https://github.com/andrewsrigom/agent-skills

Made for: Claude Code, Codex.

Wrote 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.

README.md

[![agentmods](https://agentmods.dev/badge/skills/andrewsrigom/agent-skills/security-client-server-boundary-and-secret-exposure/github.svg)](https://agentmods.dev/skills/andrewsrigom/agent-skills/security-client-server-boundary-and-secret-exposure)

Your own site

<a href="https://agentmods.dev/skills/andrewsrigom/agent-skills/security-client-server-boundary-and-secret-exposure"><img src="https://agentmods.dev/badge/skills/andrewsrigom/agent-skills/security-client-server-boundary-and-secret-exposure/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.

Your own site · 80×15

<a href="https://agentmods.dev/skills/andrewsrigom/agent-skills/security-client-server-boundary-and-secret-exposure"><img src="https://agentmods.dev/badge/skills/andrewsrigom/agent-skills/security-client-server-boundary-and-secret-exposure.svg" alt="Reviewed on agentmods" width="80" height="20"></a>

Per session 52 Skills are progressive disclosure: only the name and description are preloaded; the body loads when the skill is used.

When invoked 497 The whole file, excluding the scripts and references it only reads on demand.

Security scan A 0 findings. A grade says what 26 rules found in the file — not that it is safe.

Origin original No closer match found in the catalogue.

Token cost

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	$0.00052	$0.00497
Opus 5.5	$0.00021	$0.00199
Sonnet 5.5	$0.00010	$0.00099
Haiku 4.5	$0.00005	$0.00050

Measured 2mo ago against content hash 4e9255ed5deb, method: parsed. Prices are Anthropic first-party input rates as of 2026-10-07, from the pricing page.

Security

Grade A, and why

security-client-server-boundary-and-secret-exposure 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 2mo 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.

security/security-client-server-boundary-and-secret-exposure/SKILL.md · 66 lines

How it starts

The opening of the file, as written. The whole thing — 66 lines — stays where its author put it; the contents beside it link to each section on GitHub.

Security Client Server Boundary And Secret Exposure

Use this skill when the risky move would be letting the browser see or influence something that should stay server-only.

Scope

secret exposure
public vs server-only environment variables
leaking privileged tokens to the browser
pushing business rules or authorization into client code
internal API and server action boundary mistakes

Default path

Assume the browser is untrusted.
Keep secrets, privileged credentials, and vendor private keys on the server only.
Expose only the minimum public identifiers or scoped tokens truly needed by the client.
Run privileged reads, writes, and policy checks on the server.
Treat hidden fields, local storage, and client props as attacker-controlled input.

When to deviate

Use short-lived scoped client tokens only when the vendor flow truly requires direct browser access.
Keep harmless publishable keys public only when the downstream provider explicitly designs them that way.
Allow client-side hints for UX, but re-check the privileged decision on the server.

Guardrails

NEXT_PUBLIC_* and equivalent client-exposed config are public by definition.
UI gating is not authorization.
Never rely on “users cannot see this value in the UI” as protection.
Keep third-party admin credentials, signing keys, and private API keys off the client entirely.

Avoid

putting secrets in client bundles or public env vars
trusting client-calculated price, role, or ownership values
exposing internal admin APIs because the route is “hard to find”
treating obfuscation as a security boundary

Verification checklist

all secrets are server-only
any client token is intentionally scoped and limited
privileged decisions execute on the server
the browser is treated as attacker-controlled
no UI-only guard is mistaken for authorization

Output Shape

When answering with this skill, prefer:

what must stay server-only
what may be exposed safely
which decisions must move server-side
the main secret-exposure risk

Read the full file on GitHub · 66 lines

Files

What ships with it

1 file beside SKILL.md in the same directory: the scripts, references and assets a skill reads on demand. Not counted in the per-session cost; read them before you install if any of them is executable.

references/secret-and-boundary-defaults.md 431 B

Changes

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.

2mo ago First seen · 66 lines · 52 tokens per session scan A 4e9255ed5deb

Subscribe to this mod's changes

security-client-server-boundary-and-secret-exposure is a skill published in the GitHub repository andrewsrigom/agent-skills (2 stars, last pushed 7mo ago), licensed MIT. It adds 52 tokens to every session and 497 once invoked, about $0.0002 per session on Opus 5.5. 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 2026-08-31.