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MCP Supply Chain Verification for Claude Code — Beginner Guide (as of 29 Jul 2026)

Grading note. A dated snapshot — accurate as of 29 Jul 2026, frozen here as a permanent archive entry. This is a refresh of the 12 Jul 2026 beginner snapshot, re-leveled from the 29 Jul 2026 technical entry. Facts are unchanged from that technical entry. This version is written for readers new to AI, the command line, and Ubuntu.

⚠️ PROTOCOL UPDATE — 29 Jul 2026. MCP has a technical “rulebook” (called a specification) that describes exactly how Claude Code and MCP servers talk to each other behind the scenes. A new version of that rulebook, dated 2026-07-28, has now become the official, final version (it was only a draft at the time of the 12 Jul 2026 snapshot). It removes an old opening “handshake” step and a session-tracking detail called Mcp-Session-Id. On logins: a security standard called OAuth 2.1 remains the foundation for how servers prove who they are — an earlier draft had predicted it would be replaced by “OAuth 2.0/OIDC,” but that prediction turned out to be only partly right. A newer standard, RFC 9728 (“Protected Resource Metadata”), is now a hard requirement for both servers and the Claude Code client. A related standard, RFC 9207 (checking who issued a login token), works a bit differently on each side: the authorization server should include this check, but the client must verify it whenever it is present — it is not a strict requirement on both ends. None of this changes the safety practices below. They are about deciding which MCP server you choose to run, not about the technical handshake it uses to talk to Claude Code. If you are personally reviewing a server’s own login/security code (rather than just deciding whether to trust and run it), check it against the official 2026-07-28 rulebook, not older summaries. 🕒 verify live.

What you need to know before reading this guide

Claude Code is Anthropic’s AI coding assistant that runs on your computer as a command-line program. You give it tasks, and it can read files, write code, and run commands on your behalf.

MCP (Model Context Protocol) is the system that lets Claude Code connect to outside plug-ins called “MCP servers.” Each MCP server gives Claude Code a new set of tools — for example, the ability to search the web, send emails, or talk to GitHub. You install and run these servers yourself.

npm is a package manager — a tool that downloads and installs software written in JavaScript. Many MCP servers are distributed through npm. You use it on the command line with commands like npm install.

The core risk: An MCP server runs as a program on your machine with your own user account’s permissions. It can read your files, run commands, and connect to the internet. If you install a malicious MCP server, it can silently steal your passwords, SSH keys, and API credentials — before you even know anything is wrong.

This guide tells you exactly what to check before connecting any MCP server to Claude Code.

How to read the labels

Practice 1: Only connect MCP servers you have personally inspected, or that come from publishers you independently trust

Do: Before connecting any MCP server to Claude Code, find out who published it.

Why (beginner): A website you browse in a browser is sandboxed — it can only do so much. An MCP server is different: it is an executable program running under your own user account. Connecting one you have not inspected is the same as downloading a stranger’s program and running it on your computer. As of March 2026, roughly 15.4% of MCP servers had no public source code at all, making them impossible to audit before use.

What goes wrong if you skip this: You connect a server that looks useful. Behind the scenes it reads your SSH private keys, your AWS credentials, or your password manager’s vault and sends them to an attacker. You see no error; you lose access to your cloud accounts, your repositories, or your bank.

Caveat / contested: Anthropic maintains a connector directory that lists MCP servers. Anthropic itself states it does not security-audit those listings — the criteria are about format and metadata, not code safety. A server being “in the directory” does not mean it is safe.

Sources: code.claude.com/docs/en/security (Anthropic, fetched 2026-07-12) · code.claude.com/docs/en/mcp (Anthropic, fetched 2026-07-12) · nimblebrain.ai/blog/state-of-mcp-security-2026/ (NimbleBrain, March 2026) · toolradar.com/blog/mcp-server-security-best-practices (Toolradar, 2026)

Confidence: ✅ independently-corroborated

Practice 2: Always pin MCP server versions — never let npm fetch “whatever is newest”

What “pinning a version” means: When you install a package with npm, you can tell it to get a specific numbered release instead of automatically grabbing the newest one. Locking to a specific number is called “pinning.”

Do: Instead of running:

npx -y @modelcontextprotocol/server-filesystem

run this instead, with an explicit version number you have already reviewed:

npx -y @modelcontextprotocol/server-filesystem@x.y.z

(Replace x.y.z with the real version number — see below for how to find it.)

In your .mcp.json configuration file (the file that tells Claude Code which MCP servers to use), write the version number in the args line:

"args": ["-y", "@modelcontextprotocol/server-filesystem@x.y.z"]

Also commit your package-lock.json file to version control (git). This file records the exact checksum of every package downloaded, so future installs match what you inspected.

🕒 verify live: Don’t copy a “current” version number from any guide, including this one — it goes stale within weeks no matter what is written here (the version cited in this entry’s 12 Jul 2026 predecessor was already two releases behind just 17 days later). Instead, ask npm directly what the current version is:

npm view @modelcontextprotocol/server-filesystem version

Read and review that release yourself, and only then pin that exact version number in your commands and config files.

⚠️ WARNING — upgrading can break older setups without warning. If you pin or upgrade an MCP server to a version that only understands the new 2026-07-28 rulebook (see the PROTOCOL UPDATE box above), any older (“legacy”) client that still expects the old handshake steps and the old session-tracking header will simply stop working — there is no automatic bridge back to the old way built into the spec. If you look after MCP servers for more than one machine or more than one person, check which protocol era a new pinned version speaks before rolling it out everywhere. For the full technical compatibility details, see the “mcp-server-security” and “mcp-security-fundamentals” best-practice entries.

Why (beginner): Attackers exploit version ranges by publishing a tiny “patch” release that adds malicious code. npm’s default behaviour silently upgrades to the newest version every time you run a command. The Mastra npm supply chain attack (June 2026, attributed to an attacker group called Sapphire Sleet) infected over 140 packages this way by pushing a malicious dependency update through a compromised maintainer account; every version of Mastra after 1.13.0 was affected. Pinning means you only get the code you reviewed.

What goes wrong if you skip this: You install an MCP server, review it, and decide it is safe. Weeks later npm silently upgrades it to a new version that contains a hidden payload. Your credentials are stolen from a version you never read.

Caveat / contested: Pinning stops silent upgrades, but if the version you pinned was already malicious at publish time, the pinned hash still matches — pinning alone is not a complete guarantee. Combine it with code inspection and run npm audit every time you bump to a new version.

Sources: stacklok.com/blog/examining-the-impact-of-npm-supply-chain-attacks-on-mcp/ (Stacklok, 2025) · microsoft.com/en-us/security/blog/2026/06/17/postinstall-payload-inside-mastra-npm-supply-chain-compromise/ (Microsoft Security, June 2026) · toolradar.com/blog/mcp-server-security-best-practices (Toolradar, 2026)

Confidence: ✅ independently-corroborated

Practice 3: Use --ignore-scripts when installing any new MCP server package for the first time

What “install scripts” means: npm packages can include small shell scripts that run automatically the moment you install the package — before you have read any of the code. These are called lifecycle hooks, with names like postinstall. They are meant for legitimate purposes like compiling native code, but a malicious package uses them to steal your secrets the instant you run npm install.

Do: When evaluating a new MCP server package for the first time, install it like this:

npm install --ignore-scripts <package-name>@<version>

The --ignore-scripts flag tells npm to download the files but not run any of the package’s shell scripts. Then read the package contents — especially the scripts section of package.json and any file it references in a postinstall entry — before deciding whether to trust and run the server.

⚠️ WARNING: If you run a plain npm install without --ignore-scripts, the package’s postinstall code runs the moment installation begins — before you have seen what it does. Both the Sapphire Sleet / Mastra attack (June 2026) and the “postmark-mcp” backdoor (see Practice 8 below) delivered their malicious payloads through postinstall hooks. The attack executes silently the moment you run npm install — installation alone is enough; you do not need to import or use the package in your code.

Why (beginner): Imagine opening an envelope and having it spray poison gas before you can read the letter inside. That is what a malicious postinstall hook does. The --ignore-scripts flag lets you open the envelope safely and read what is inside before deciding whether it is safe.

What goes wrong if you skip this: Your SSH private key (~/.ssh/id_rsa), API keys stored in environment variables, and credential files are silently sent to an attacker the moment you run npm install. You see no error message. The theft happens in milliseconds.

Caveat / contested: --ignore-scripts prevents the hooks from running, but it does not prevent the malicious source code from being placed on your disk. You still need to read the code. Also, some legitimate packages genuinely need postinstall to compile native components. Once you have read the code and decided you trust it, you can run the hooks deliberately with npm rebuild.

Sources: microsoft.com/en-us/security/blog/2026/05/28/typosquatted-npm-packages-used-steal-cloud-ci-cd-secrets/ (Microsoft Security, May 2026) · microsoft.com/en-us/security/blog/2026/06/17/postinstall-payload-inside-mastra-npm-supply-chain-compromise/ (Microsoft Security, June 2026) · checkmarx.com/learn/mcp-security-risks-real-world-incidents-and-security-controls/ (Checkmarx, 2025–2026)

Confidence: ✅ independently-corroborated

Practice 4: Read the tool descriptions an MCP server gives Claude Code before trusting them — look for hidden instructions

What “tool poisoning” means: When an MCP server connects to Claude Code, it sends a list of tools along with text descriptions of what each tool does. Claude Code reads those descriptions the same way it reads your own instructions. A malicious server can hide extra commands inside those descriptions — for example, an instruction to also send a copy of your SSH key to an attacker’s website. This attack is called “tool poisoning.”

Do: When you connect a new MCP server, type /mcp directly in Claude Code’s chat input (that is a forward-slash followed by mcp — it is a slash command, not a terminal command). This shows you the list of tools the server has sent. Read the descriptions yourself before Claude Code acts on them. Watch for:

Why (beginner): Think of tool descriptions as messages in a bottle coming from an outside server. If the server is malicious, those messages can contain hidden orders to Claude Code — orders you never wrote and never approved. A November 2025 attack on a WhatsApp MCP integration used exactly this technique to exfiltrate entire message histories. Academic research (arXiv 2603.22489, March 2026) rates this kind of attack as Critical severity.

What goes wrong if you skip this: Claude Code receives a hidden instruction from a malicious server telling it to read your ~/.ssh/id_rsa file and send its contents to attacker.example.com. Claude Code follows the instruction as though you asked for it. You lose your SSH access.

Caveat / contested: Claude Code has some built-in analysis to detect harmful instructions, but Anthropic does not claim this is a complete defence. Research notes that “client implementations remain largely undefended against sophisticated metadata manipulation attacks.” Reading the descriptions yourself is an important extra layer. The core safety rule behind this practice — that tool descriptions from an untrusted server should not be treated as trustworthy — is confirmed to carry over unchanged into the new 2026-07-28 protocol rulebook; that update does not add any new specific defence against tool poisoning, but it also does not weaken or remove this guidance.

Sources: owasp.org/www-project-mcp-top-10/2025/MCP03-2025–Tool-Poisoning (OWASP MCP Top 10 beta v0.1, 2025) · arxiv.org/html/2603.22489v1 (academic paper, March 2026) · truefoundry.com/blog/blog-mcp-tool-poisoning-gateway-defense (TrueFoundry, 2025–2026) · code.claude.com/docs/en/security (Anthropic, fetched 2026-07-12)

Confidence: ✅ independently-corroborated

Practice 5: Limit what each MCP server is allowed to do using Claude Code’s permission rules

What permissions mean here: Claude Code has a settings file called .claude/settings.json (inside your home directory). In it you can write rules that control which MCP tools Claude Code is allowed to call automatically, and which ones it must ask you about first. This is called “least-privilege” — giving each server only the minimum access it actually needs.

Do: Open .claude/settings.json and use these rules:

Never give an MCP server access to filesystem folders wider than the single project it needs to work with. When a server needs a password or token (such as a GitHub token or Jira API key), request the minimum permission level that server actually needs — not administrator access.

⚠️ WARNING: There is a mode called bypassPermissions (and a related flag --dangerously-skip-permissions sometimes mentioned for automated use). Turning this on skips all permission prompts — including MCP tool calls. This means Claude Code will read, write, delete, and execute shell commands across your entire home directory with no confirmation prompts at all. Anthropic’s own documentation states: “Only use this mode in isolated environments like containers or VMs where Claude Code can’t cause damage.” Do not use bypassPermissions on your normal working computer. If you enable it and a malicious MCP server is connected, it can delete everything in your home directory or send all your files to an attacker, and Claude Code will not stop to ask you.

Why (beginner): Without permission rules, every MCP server you connect can ask Claude Code to do anything your user account can do. If one server is compromised or malicious, it can abuse those permissions to touch files and services that have nothing to do with its stated purpose. Permission rules act like a bouncer — the server can only do what you specifically approved.

What goes wrong if you skip this: A server you connected for one purpose (say, sending emails) uses its inherited permissions to also read files in your project folder, your home directory, and your credential files. You never notice because no permission prompt appears.

Caveat / contested: These permission rules are enforced by Claude Code itself, not by the operating system. A malicious MCP server running as a separate process can still make direct system calls regardless of what Claude Code’s rules say. The next practice (sandboxing) adds the OS-level enforcement layer that catches this.

Sources: code.claude.com/docs/en/permissions (Anthropic, fetched 2026-07-12) · checkmarx.com/learn/mcp-security-risks-real-world-incidents-and-security-controls/ (Checkmarx, 2025–2026) · stacklok.com/blog/the-enterprise-it-security-guide-to-claude-and-mcp/ (Stacklok, 2025–2026)

Confidence: ✅ independently-corroborated

Practice 6: Run MCP servers inside a container or sandbox to cage them at the operating system level

What a sandbox / container means: A sandbox is a cage at the operating system level — the kernel itself (the core of Linux) enforces what the caged program can and cannot do. A container (such as Docker) is a similar idea: the program runs in an isolated box where it cannot see your real home directory or make arbitrary network connections. Both make it much harder for a malicious server to do damage even if it tries.

Do: Pick at least one of these layers and enable it:

Option A — Claude Code’s built-in sandbox (easiest starting point). In your Claude Code settings, set "sandbox": {"enabled": true}. On Ubuntu this uses a tool called bubblewrap (which must be installed: sudo apt install bubblewrap socat) to restrict Bash commands and child processes to your project’s working directory and approved network domains. Also set "failIfUnavailable": true so Claude Code refuses to run at all if the sandbox cannot start, and "allowUnsandboxedCommands": false so nothing escapes the cage.

Option B — Dev containers (stronger isolation). Run Claude Code inside a Docker container using a .devcontainer/devcontainer.json file with the Claude Code Dev Container Feature. This isolates Claude Code and all MCP servers from your real host filesystem. Add a startup script (init-firewall.sh, included in the reference container) to block unexpected outbound network connections.

Option C — Stacklok ToolHive (open-source, per-server cages). An external tool that wraps each MCP server in its own container with configurable network and filesystem permissions. 🕒 verify live — confirm current availability and version before using.

⚠️ WARNING: Claude Code’s built-in sandbox does NOT isolate the built-in Read, Edit, and Write file tools — only Bash commands and child processes. Also, --dangerously-skip-permissions (sometimes recommended for CI pipelines) bypasses the permission system entirely. Only use it inside a container where an escape causes no damage to your real machine.

Why (beginner): The MCP protocol itself does not sandbox servers. Without OS-level containment, a compromised server subprocess can read ~/.aws/credentials, ~/.ssh/id_rsa, and any other file your user account can access, then send it over a network connection. Sandboxing means the server is caged at the kernel level regardless of what Claude Code decides to do.

What goes wrong if you skip this: A malicious MCP server bypasses Claude Code’s permission rules by making direct operating-system calls. Without a sandbox, the kernel has no fence to enforce. The server reads and exfiltrates your credentials directly.

Caveat / contested: The Linux bubblewrap sandbox requires the bubblewrap and socat packages and does not work on native Windows (you need WSL2 on Windows). The sandbox documentation also notes that allowing broad domains such as github.com can create paths for data exfiltration via domain fronting; a custom TLS-terminating proxy is needed for strict network inspection.

Sources: code.claude.com/docs/en/sandboxing (Anthropic, fetched 2026-07-12) · code.claude.com/docs/en/devcontainer (Anthropic, fetched 2026-07-12) · stacklok.com/blog/examining-the-impact-of-npm-supply-chain-attacks-on-mcp/ (Stacklok, 2025) · nimblebrain.ai/blog/state-of-mcp-security-2026/ (NimbleBrain, March 2026)

Confidence: ✅ independently-corroborated

Practice 7: Treat .claude/settings.json and .mcp.json as executable code — review them carefully when you clone any project

What these files do: .claude/settings.json is Claude Code’s configuration file. .mcp.json lists which MCP servers a project uses. Claude Code reads these files and acts on them. “Hooks” in settings.json are shell commands that Claude Code runs automatically. A malicious version of these files is effectively a program that runs on your computer.

Do:

⚠️ WARNING: A real vulnerability, CVE-2025-59536 (CVSS score 8.7 out of 10 — rated “High”), allowed remote code execution through a malicious .claude/settings.json committed to a repository. Claude Code executed hooks defined in that file before the user saw any trust dialog. This was patched in Claude Code version v1.0.111 and later (patch released October 2025). This fixed version number is the important fact here and does not change no matter how many newer versions come out afterward.

🕒 verify live: Don’t rely on any hardcoded “current version” number from a guide — Claude Code moves fast, and by the time you read this, a version number written today will already be many releases behind (the version this entry’s 12 Jul 2026 predecessor listed as “current” was already 12+ patch releases behind just 17 days later). Instead, check code.claude.com/docs/en/changelog directly for whatever the current version actually is when you read this.

Why (beginner): Most people think of JSON files as harmless data. Claude Code treats .claude/settings.json as active logic: hooks run shell commands, MCP entries launch server processes, and permission allow-rules expand what the AI can do without asking you. A single pull request from a compromised contributor can install a persistent backdoor that runs every time you open the project.

What goes wrong if you skip this: You clone a repository from a colleague or from the internet. It contains a .mcp.json that points to a malicious MCP server. The next time you open the project in Claude Code, that server connects automatically and begins stealing your credentials. You never added it yourself; it was already there.

Caveat / contested: The workspace trust dialog that Anthropic added after CVE-2025-59536 provides an extra checkpoint, but the CVE itself demonstrated it can be bypassed in certain flows. Insider threats and compromised CI/CD pipelines remain attack vectors regardless of the dialog.

Sources: mintmcp.com/blog/claude-code-supply-chain-attacks (MintMCP, 2026) · mintmcp.com/blog/claude-code-cve (MintMCP, 2026) · csoonline.com/article/4181230/claude-code-has-an-mcp-security-problem-and-your-developers-are-already-using-it.html (CSO Online / Mitiga Labs research, 2026)

Confidence: ✅ independently-corroborated

Practice 8: Check the exact npm package name and publisher before installing — attackers use look-alike names

What typosquatting means: Attackers publish packages with names that look almost identical to legitimate ones, hoping you misread or mistype. For example, a legitimate package might be @modelcontextprotocol/server-filesystem (with the @ scope prefix) and an attacker publishes modelcontextprotocol-server-filesystem (without the @, so it looks similar but is a completely different, unreviewed package).

Do: Before installing any npm-distributed MCP server, check all four of these things:

  1. The exact namespace. Official MCP reference servers are under @modelcontextprotocol/. Vendor servers should be under the vendor’s own verified npm organisation (for example @sentry/mcp-server-sentry). A package without an @ scope that claims to be an official server is suspicious.

  2. The publish date and version history. A package with very few releases, or dozens of releases squeezed into a short time window, deserves extra scrutiny.

  3. The download count. A package claiming to be a popular service’s official MCP server but with almost no downloads is a red flag.

  4. The vendor’s own documentation. Find the vendor’s official website and check what package name they list there. Do not rely on the npm page alone.

Why (beginner): The “postmark-mcp” attack is a real example of how patient and clever these attacks can be. An attacker published a package matching the legitimate Postmark MCP server name. For 15 releases the package was completely clean — nothing suspicious. Then in version 1.0.16 only, the package silently BCCed (blind carbon copied) every outgoing email to an attacker-controlled domain. The clean history of the first 15 releases made the malicious version much harder to detect. A separate report from Antiy CERT found 1,184 malicious packages in one marketplace at peak.

What goes wrong if you skip this: You install what you believe is the official email-sending MCP server. Every email Claude Code sends on your behalf is also secretly copied to an attacker. Your business emails, password reset links, and confidential correspondence are exposed.

Caveat / contested: Typosquatting detection is inherently reactive — the attacker publishes first, and detection comes later. npm audit catches known CVEs but will not flag a newly published malicious package that has a clean history. Cross-referencing vendor documentation is a useful check but not a guarantee. Registry-level provenance attestations (technologies called SLSA and sigstore) are emerging but not yet widely adopted in the MCP ecosystem.

Sources: checkmarx.com/learn/mcp-security-risks-real-world-incidents-and-security-controls/ (Checkmarx, 2025–2026) · microsoft.com/en-us/security/blog/2026/05/28/typosquatted-npm-packages-used-steal-cloud-ci-cd-secrets/ (Microsoft Security, May 2026) · blog.cyberdesserts.com/ai-agent-security-risks/ (CyberDesserts, 2026) · toolradar.com/blog/mcp-server-security-best-practices (Toolradar, 2026)

Confidence: ✅ independently-corroborated

Advanced practice (summary only): Team-wide allowlists using managed MCP configuration

If you are setting up Claude Code for a team, Anthropic provides a managed-mcp.json file that can be deployed to every developer’s machine through device management software (Jamf, Intune, or similar fleet tools). When this file is present, Claude Code only connects to the servers it lists — individual developers cannot add their own. The file locations are:

Do not store API keys or credentials inside managed-mcp.json — any local user can read the file. Use environment variable expansion (${VAR}), OAuth flows, or a headersHelper to inject credentials at connection time.

This practice requires familiarity with device management tools and is best handled by someone with IT or DevOps experience. The sources below have full details.

Sources: code.claude.com/docs/en/managed-mcp (Anthropic, fetched 2026-07-12) · code.claude.com/docs/en/permissions (Anthropic, fetched 2026-07-12) · stacklok.com/blog/the-enterprise-it-security-guide-to-claude-and-mcp/ (Stacklok, 2025–2026)

Confidence: ✅ independently-corroborated (Anthropic docs + independent Stacklok analysis)

Held pending fixes

CHANGELOG

  1. Re-leveled from the 2026-07-29 technical entry; facts unchanged.
  2. Updated the top PROTOCOL UPDATE box to reflect that the MCP 2026-07-28 specification is now final (not a draft, as in the 12 Jul 2026 beginner snapshot), including the plain-language correction that OAuth 2.1 remains the base login framework rather than being replaced by “OAuth 2.0/OIDC,” and the SHOULD/MUST split on issuer-validation (RFC 9207).
  3. Practice 2 (“pin versions”) no longer shows a hardcoded “current version” example — instead it shows the npm view @modelcontextprotocol/server-filesystem version command to look up whatever is current, matching the corrected technical entry (the old hardcoded number was already stale within 17 days).
  4. Added the new ⚠️ WARNING to Practice 2 about the legacy-compatibility gap: pinning or upgrading to a version that only speaks the new 2026-07-28 rulebook will break older clients, with no automatic fallback.
  5. Practice 4’s caveat updated to note the 2026-07-28 rulebook carries the tool-poisoning-safety principle forward unchanged — no new specific defence was added, but the guidance is not out of date either.
  6. Practice 7’s version-check note updated: instead of a hardcoded “current Claude Code version” figure, it now points to code.claude.com/docs/en/changelog directly, since any hardcoded number goes stale within weeks. The load-bearing fixed-in version (v1.0.111+) is unchanged.
  7. Added a new “Held pending fixes” item: whether/when Claude Code’s MCP client itself added support for the 2026-07-28 spec could not be confirmed.
  8. “What you need to know,” “What goes wrong if you skip this,” and plain-language term definitions retained from the 12 Jul 2026 beginner snapshot’s structure.
  9. All ⚠️ WARNING blocks retained verbatim in substance, including the bypassPermissions WARNING with its specific consequence and the new legacy-compatibility-gap WARNING.
  10. All source URLs and Confidence labels kept verbatim from the technical entry. No new facts or URLs introduced.