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Gemini CLI Security for Beginners: Extensions & MCP Integration (as of 01 Aug 2026)

A note on this guide. This is a snapshot of what was true as of 01 Aug 2026 — a permanent, frozen archive page that will never be silently edited later (a future update would be a brand-new dated page). It’s a plain-language rewrite of a technical entry that was already fact-checked by a 3-lens review panel (a skeptic, a beginner reader, and someone checking dates and version numbers) plus a senior reviewer. This version doesn’t change any fact — only the words, so someone brand new to AI tools and to MCP can follow along.

Before you start: what is an MCP server?

Keep that idea in mind for the rest of this guide: every MCP server and every extension is code that can act for you. Everything below is about controlling how much you trust that code, and how much power you hand it.

How to read the labels

Practice: Never turn on trust: true or --yolo unless you personally control that server

Do: Leave MCP servers set to trust: false — that’s the default — so Gemini CLI always asks you to confirm before it runs a tool. Only switch a server to trust: true if you wrote it yourself and it runs on your own machine. Here’s what that looks like as a complete, working command:

gemini mcp add my-internal-server --trust --url http://localhost:8080

Never use --yolo (Gemini CLI’s “auto-approve everything, don’t ask” mode) on any machine that ever touches untrusted input — including a build/CI server.

Why (beginner): trust: true tells Gemini CLI “don’t ask me, just run whatever this server’s tools want to do” — including shell commands, file writes, or sending data over the network — with no chance for you to say no. Google’s own documentation says to use it “cautiously and only for servers you completely control.” Here’s what goes wrong when that’s ignored: Google shipped a version of --yolo that silently ignored the fine-grained list of tools a user had approved, and combined with a second bug — automated (CI) mode auto-trusting whatever folder it was pointed at — this became a maximum-severity bug. It let an attacker who could simply open a public GitHub issue reach full code execution on a company’s CI servers. Google fixed it in @google/gemini-cli version 0.39.1 and the run-gemini-cli GitHub Action version 0.1.22 (24 Apr 2026). It’s now tracked as CVE-2026-12537 (officially published 24 Jun 2026 — two vulnerability databases, NVD and CIRCL, confirm this is the same bug referenced elsewhere as GHSA-wpqr-6v78-jr5g, even though the original write-ups from April predate the CVE number being assigned).

Caveat / contested: 🕒 verify live — check you’re on a version at or after the 24 Apr 2026 fix (0.39.1 / 0.1.22). Security researchers disagree on exactly how severe this bug was: Google’s own team scores it 10.0 out of 10, the maximum (using the CVSS 4.0 scoring system); the independent NVD database scores it lower, 7.8 out of 10 — “HIGH” but not maximum (using the older CVSS 3.1 scoring system). Separately, this wasn’t the only trust-related bug found recently — Pillar Security’s “Week of Sandbox Escapes” research (20–23 Jul 2026) found a new Docker-related escape in Gemini CLI (since fixed) and a still-unpatched bypass in Google’s separate Antigravity tool. One more detail worth knowing: an extension’s own config file cannot set trust: true for itself — only your personal settings.json can do that — so a malicious extension can’t quietly grant itself no-confirmation mode on its own. But if you set trust: true yourself for that extension’s server, that protection disappears. Sources: Gemini CLI docs — MCP servers (fetched 01 Aug 2026) · GBHackers — Critical Gemini CLI Flaw Raises Supply Chain Security Concerns (27 Apr 2026) · Pillar Security — My Agentic Trust Issues (5 May 2026) · NVD — CVE-2026-12537 (CVSS scoring dispute + CVE↔GHSA link) · GitHub Advisory Database — GHSA-wpqr-6v78-jr5g · Pillar Security — The Week of Sandbox Escapes (Jul 2026) Confidence: independently-corroborated

Practice: Don’t assume “Trusted Folders” is protecting you — check it yourself, and read the popup

Do: Google’s own documentation contradicts itself about whether the “Trusted Folders” safety feature is on or off by default — don’t just assume either way. When Gemini CLI shows you a first-time popup asking whether to trust a new folder, actually read what it lists (any MCP servers, custom commands, and other things that folder would activate) before clicking “Trust folder.” Don’t click “Trust parent folder” (which trusts everything above it too), and don’t trust a folder just because the popup is annoying — especially not on a computer that also runs automated jobs against code you didn’t write yourself. If you never see this popup at all, that’s a sign the feature might already be on for you by default — check the /permissions command, and if you want to be sure the popup is active, explicitly turn it on by setting "security": {"folderTrust": {"enabled": true}} in your settings. Why (beginner): In a folder Gemini CLI doesn’t trust, it refuses to auto-connect to MCP servers, blocks installing extensions, and makes you approve every tool call by hand. This is your safety net against a folder — or someone else’s code you’re reviewing — that secretly ships a poisoned config file or a malicious MCP server definition. The CVE from the practice above happened specifically because automated (“headless”) mode skipped this exact safety check and auto-trusted whatever folder it was pointed at, loading a malicious hidden config file before any protection could kick in. Caveat / contested: This mainly protects you when you’re using Gemini CLI directly, sitting at the keyboard. The bug above was in automated/CI mode, which skipped this check entirely — so an automated pipeline needs the fixed software version, not just correct trust settings. You need both: the right settings AND an up-to-date version. Sources: Gemini CLI docs — Trusted Folders (fetched 01 Aug 2026) · github.com/google-gemini/gemini-cli — docs/cli/trusted-folders.md (“disabled by default” — contradicts the settings-reference table’s stated true default; fetched 01 Aug 2026) · The Hacker News — Google Fixes CVSS 10 Gemini CLI CI RCE and Cursor Flaws Enable Code Execution (30 Apr 2026) · Cloud Security Alliance Labs — Gemini CLI CVSS 10.0: RCE in AI Developer Tools (2 May 2026) Confidence: independently-corroborated

Practice: Treat every extension as unchecked code — even Google’s own — and look before you install

Do: Before you run an install command like this complete example:

gemini extensions install https://github.com/gemini-cli-extensions/security

open that GitHub repository first. Skim its gemini-extension.json file (what tools or servers does it add?) and any bundled program source, especially if it will process things you don’t control — GitHub issues, pull requests, web pages. Don’t assume something is safe just because it looks official or was written by Google. Why (beginner): Google’s own extension gallery warns that it does not vet, endorse, or guarantee the security of anything listed there, and tells users to “carefully inspect any extension and its source code before installing.” This isn’t just cautious wording — it’s happened. Google’s own official “security” extension (which is supposed to scan pull requests for vulnerabilities) shipped with a bug called “prototype pollution” — a type of JavaScript bug where crafted input corrupts a shared template that other code relies on, letting an attacker’s data quietly override how unrelated code behaves. A pull request containing just the single word “constructor” in the right spot could trigger it (researchers scored this bug 7.1 out of 10 for severity, catalogued as CWE-1321, a standard reference number for this class of bug). Researchers escalated it into running arbitrary commands, and even used it to hide their findings from the very tool meant to catch them. Google fixed it by changing how the code stores data (merged 15 Oct 2025), but the bug wasn’t publicly written up until five months later, in March 2026. The lesson: extension code runs with full power on your machine the moment you install it — an official label doesn’t change that. Caveat / contested: We could only find one independent write-up of this specific bug (plus Google’s own fix commit, which confirms the bug existed but isn’t a second independent security review). So we’re labeling this “thin” evidence rather than “well-corroborated,” to be honest about how solid the sourcing actually is. Sources: Gemini CLI Extensions Gallery (fetched 01 Aug 2026) · Symbiotic Security — Who Scans the Scanners? Uncovering a Critical Bug in Gemini’s Security CLI (13 Mar 2026) · gemini-cli-extensions/security PR #91 (merged 15 Oct 2025) Confidence: thin

Practice: Add an explicit block-list for dangerous actions — don’t rely on the popup alone

Do: In your settings file, you can list specific dangerous actions you never want Gemini CLI to even offer, for example:

"excludeTools": ["run_shell_command(rm -rf *)"]

Add this in addition to — not instead of — leaving trust: false on. You can also limit what each individual MCP server is allowed to do, using includeTools / excludeTools per server. Why (beginner): A confirmation popup only protects you if you read it carefully every single time. When you’re in a hurry, it’s easy to click “yes” on something destructive — like a command that deletes files — without really reading it. excludeTools removes the option entirely, so there’s no popup left to accidentally click through. Caveat / contested: This pattern is described only on Google’s own documentation pages — we didn’t find independent security research testing how well it actually holds up against a determined malicious server (in theory, a compromised server could rename its own tool to dodge your block-list pattern). Sources: Gemini CLI docs — Extensions best practices (fetched 01 Aug 2026) · Gemini CLI docs — MCP servers (fetched 01 Aug 2026) · Gemini CLI docs — Extension reference (fetched 01 Aug 2026) Confidence: vendor-documented

Practice: Don’t paste real API keys straight into an MCP server’s config file

Do: Don’t write a real API key or password as literal text inside an MCP server’s env section in your settings file, unless you genuinely mean to hand that exact value to that exact server. Instead, reference your own terminal’s environment, like "MY_KEY": "$MY_KEY", so the real secret lives only in your terminal — not in a config file that could get shared, backed up, or committed to a repository. Remember: the moment you explicitly name a variable this way, Gemini CLI treats that as your informed permission and skips the automatic protection described below. Why (beginner): Gemini CLI automatically hides (“redacts”) variables with names matching patterns like *TOKEN*, *SECRET*, *PASSWORD*, *KEY*, *AUTH*, *CREDENTIAL*, GEMINI_API_KEY, and GOOGLE_API_KEY, before passing your environment to MCP servers and extensions. That’s a good safety net — but it switches off the instant you explicitly list one of those variables for a specific server. Extensions also don’t automatically see your whole terminal environment; only variables you’ve allowed through reach them, which is a second safety net — but again, one you can accidentally switch off yourself without realizing it. What goes wrong if you ignore this: a real secret sitting in plain text in a config file can leak the moment that file gets shared, backed up, or pushed to a public repository. Caveat / contested: This is described only in Google’s own documentation; we couldn’t find independent security research specifically testing this protection for ways around it. Sources: Gemini CLI docs — MCP servers (fetched 01 Aug 2026) · Gemini CLI docs — Extension reference (fetched 01 Aug 2026) Confidence: vendor-documented

Practice: ⚠️ Don’t use underscores in MCP server names — it can silently break your security rules

Do: When you name an MCP server in your settings file, use dashes, not underscores — write my-server, not my_server. Why (beginner): Gemini CLI’s own documentation warns that an underscore in a server name confuses the part of the software that matches names against rules, and this “can cause wildcard rules and security policies to fail silently.” In plain words: a security rule you think is protecting you might quietly not be applied at all — and nothing tells you it failed. That’s what makes this dangerous: it doesn’t throw an error message. It just quietly doesn’t work. Caveat / contested: This comes from one source only (Google’s own documentation) and hasn’t been independently confirmed elsewhere. But it costs nothing to follow — it’s just a naming choice — so it’s worth doing even though it’s unverified. Sources: Gemini CLI docs — MCP servers (fetched 01 Aug 2026) Confidence: thin

Practice: Treat anything an MCP tool tells you (or its own description) as untrusted

Do: Assume that any text an MCP tool sends back — or even text baked into how the tool describes itself — could secretly contain instructions aimed at the AI model, not at you. This is called “tool poisoning” or indirect prompt injection: a malicious or hacked MCP server hides instructions inside what looks like ordinary data, and the model — which can’t reliably tell the difference between data and instructions — may just follow them. Be extra careful with any MCP server that reads content you don’t control, like GitHub issues, pull requests, web pages, or emails, and then feeds the results back to the model. Why (beginner): This has actually happened to Gemini CLI. Independent researchers (a study called “Comment and Control,” published 15 Apr 2026) showed they could hijack Gemini CLI — plus Claude Code and GitHub Copilot — just by hiding malicious instructions inside GitHub issue text, pull-request titles, and comments that the AI agent read as part of its normal work. In vulnerable setups, this led to CI secrets being stolen. Separately, a different reported flaw let instructions hidden inside a repository’s README file get carried out as if they were commands the user had typed themselves (fixed in Gemini CLI version 0.1.14, 25 Jul 2025). This general category of attack — “tool poisoning” — is a known risk across the whole MCP ecosystem, not something unique to Gemini CLI. Caveat / contested: We couldn’t find documentation or independent research saying whether Gemini CLI’s interface actually warns you when a tool description looks suspicious, the way some other AI tools do. Treat that specific question as unresolved — we found no evidence either way. Sources: Aptible — Prompt Injection in MCP: Tool Poisoning and Blast Radius (fetched 01 Aug 2026) · CSO Online — Google patches Gemini CLI tool after prompt injection flaw uncovered (29 Jul 2025) · SecurityWeek — Claude Code, Gemini CLI, GitHub Copilot Agents Vulnerable to Prompt Injection via Comments (15 Apr 2026, “Comment and Control” research) · Cloud Security Alliance Labs research note (2 May 2026) Confidence: contested

Practice: Only install Gemini CLI from the official npm package or Google’s own docs — never from a search result

Do: Install with this exact command, typed by hand rather than copy-pasted from a random blog or advertisement:

npm install -g @google/gemini-cli

Only use another install method if it’s linked directly from Google’s own documentation. Never paste a one-line “installer” command from a page you found by searching, without first checking that the web address is genuinely Google’s. Why (beginner): Between roughly March and May 2026, attackers ran a fake-website campaign that got malicious lookalike sites (for example, geminicli[.]co[.]com) to rank above the real Gemini CLI documentation in search results. Here’s the trick: the fake page’s copy-paste command really did install the genuine, uncompromised Gemini CLI from npm — so nothing looked wrong. But it also quietly ran a hidden PowerShell program in the background that stole browser cookies, login tokens, CI/CD credentials, and VPN keys. This particular attack targeted how you install the base tool, not the extension system — but it’s the exact same habit you need for vetting extensions: never blindly run code from a source you haven’t checked yourself. Caveat / contested: This specific campaign targeted the base installer, not the extension marketplace directly. Also, one of our two sources says openly that it’s reporting on someone else’s research rather than doing its own independent investigation — so even though it’s two different publishers, it’s really one finding reported twice, not two separate confirmations. This is why we’re labeling it “thin” rather than “well-corroborated.” Sources: EclecticIQ — SEO poisoning campaign leverages Gemini and Claude Code impersonation to deliver infostealer (21 May 2026) · GBHackers — Hackers Use SEO Poisoning to Fake Gemini CLI, Claude Installers (23 May 2026, derivative of EclecticIQ’s research per its own text) Confidence: thin

If you’re a team/org admin (optional — skip this if you’re an individual user)

The technical version of this entry includes one more practice about organization-wide MCP-server allowlists, managed through Google’s admin console. That practice is genuinely useful — but only if you administer Gemini CLI for a company or team. It requires enterprise/admin access most individual beginners won’t have, so it’s left out of this beginner version. If that applies to you, read the full technical entry for that practice, its worked example, and its sources.

Still being checked (not fully confirmed yet)

CHANGELOG (grading → this entry)

  1. Re-leveled from the graded 2026-08-01 technical entry (best-practices/gemini-cli-extensions-mcp-security/2026-08-01) for readers brand new to AI tools and to MCP. Facts, commands, dates, version numbers, and sources are unchanged — only the wording, structure, and reading level changed. See that entry’s own CHANGELOG for the original grading corrections this rewrite inherits.
  2. Dropped as too advanced: the enterprise/admin MCP-server-allowlist practice. Its own caveat in the technical entry says it’s “an enterprise-tier feature, not something most individual/beginner users will touch,” so it’s summarized in an “if you’re an admin” note instead of given as a full practice; its sources were not carried into this entry along with it, per Translator rules (no source is stripped from a practice that’s kept — this one wasn’t kept as a full practice).
  3. Kept all 7 remaining practices, every ⚠️ WARNING, the top-of-entry MCP definition (simplified further into a bulleted explainer), the gemini mcp add ... --trust worked example, the gemini extensions install <github-url> worked example, and the install prerequisite note — all carried over with the same facts as the technical entry.
  4. Simplified language throughout, defined jargon on first use, and added plain-language “what goes wrong” framing to each practice’s “Why” section. No new facts, commands, dates, or URLs were introduced anywhere in this rewrite.