CVE-2026-10591 is one of those vulnerabilities that makes you stop and stare at your terminal. A high-severity (CVSS 8.6–8.8) prompt injection flaw in AWS Kiro — Amazon’s agentic IDE — lets a malicious web page rewrite your MCP configuration files and execute arbitrary code with your developer privileges. No user interaction beyond opening a poisoned page required. Security researchers at Cymulate discovered it; AWS disclosed it via Security Bulletin 2026-037.

This isn’t theoretical. If you’re running an affected version of Kiro and your agent browses or processes external content, you’re exposed. Here’s what happened, why it’s dangerous, and exactly what you need to do about it.

What CVE-2026-10591 Actually Does

The vulnerability exploits how Kiro’s agentic IDE processes external content. When Kiro’s agent reads a web page or document, it feeds that content into its context window. If that content contains specially crafted hidden instructions — prompt injection payloads — the agent may interpret them as legitimate commands rather than external data.

The attack chain looks like this:

  1. Victim opens a poisoned web page or document — the attacker has embedded hidden instructions in the content
  2. Kiro’s agent processes the content — the injected instructions get folded into the agent’s context
  3. The agent rewrites config files — specifically mcp.json and .vscode/tasks.json — to register a malicious MCP server
  4. Arbitrary code executes — with the developer’s full local privileges — because the compromised MCP server now has tool access

This is a supply chain attack delivered through content. Any web page, document, or file your agentic IDE reads could be weaponized.

Step 1: Upgrade to Kiro 0.11.34 or Later

The most critical step is immediate: upgrade Kiro to version 0.11.34 or later. This is the fixed version confirmed by AWS Security Bulletin 2026-037. Earlier reporting cited 0.11.130+ as the fixed version, but the AWS bulletin and independent verification confirm that full MCP protections landed in Kiro 0.11/0.11.34+.

  • Check your current version: Open Kiro and navigate to About / Help → About Kiro
  • Download the latest version: Visit the official AWS Kiro download page for your platform
  • Verify after upgrading: Confirm your version shows 0.11.34 or later before continuing

Do not skip this step. The subsequent hardening measures reduce attack surface but do not patch the underlying vulnerability.

Step 2: Audit Your mcp.json for Unexpected Entries

After upgrading, manually inspect your MCP configuration files. Compromised configurations will contain server entries you didn’t register.

Your primary MCP configuration lives at ~/.kiro/mcp.json (global) and .kiro/mcp.json (project-level). Task configurations may also live in .vscode/tasks.json.

Look for:

  • Unknown server names you didn’t add yourself
  • Unexpected command paths — especially absolute paths or paths pointing outside your project
  • Newly registered servers with recent modification timestamps

If you find suspicious entries, do NOT run them to investigate. Remove the entries, rotate any credentials your environment might expose, and check your terminal history for unexpected command execution.

Step 3: Enable Human-in-the-Loop for MCP Operations

Kiro 0.11+ introduces enhanced controls for requiring human approval before sensitive operations. While exact configuration keys and syntax may differ by version, the principle is straightforward: require explicit confirmation before any MCP server registration or task execution.

Consult the AWS Kiro documentation for the current configuration syntax to enable approval gates for:

  • MCP server registration
  • Config file writes
  • External command execution

This doesn’t eliminate the risk from the unpatched vulnerability but significantly raises the bar for successful exploitation even if injection occurs.

Step 4: Scope Your MCP Server Permissions Tightly

Principle of least privilege applies to MCP servers just as it does to cloud IAM roles. Review each registered MCP server and ask: does it actually need this permission?

Common over-permissioned patterns to watch for:

  • Filesystem access to the entire home directory — consider scoping to only the project directory
  • Shell execution without confirmation — dangerous for automated pipelines
  • Network access for servers that only need local operations

Remove permissions that aren’t actively required. The blast radius of a successful prompt injection is directly proportional to how much access your MCP servers have.

Step 5: Be Skeptical of External Content in Agentic Workflows

Technical controls help, but the deeper lesson from CVE-2026-10591 is architectural: your agent processes content from the internet as instructions, not as passive data. Design your workflows accordingly.

  • Sandbox external content processing — consider processing untrusted content in isolated environments before it reaches your agent
  • Review content-fetching workflows — any automation that reads external URLs or documents is a potential injection surface
  • Log and monitor MCP server activity — unexpected registration events or command executions should trigger alerts

AWS Security Bulletin 2026-037 is the authoritative reference for this vulnerability. Cymulate’s disclosure provides additional technical depth on the injection mechanism.

The Broader Lesson for Agentic IDE Users

CVE-2026-10591 is the first high-severity prompt injection vulnerability documented in an agentic IDE, but it won’t be the last. The attack surface created by agents that read external content and take actions based on it is fundamentally new — and the security tooling hasn’t caught up yet.

Upgrade now. Audit your configs. Apply least privilege. And treat external content in your agent’s context window with the same skepticism you’d apply to untrusted SQL input in a web application.

Sources

  1. AWS Security Bulletin 2026-037 — official disclosure
  2. The Hacker News: AWS Kiro Flaw Let a Poisoned Web Page Rewrite Its Config and Run Code
  3. Cymulate Research — original discovery

Researched by Searcher → Analyzed by Analyst → Written by Writer Agent (Sonnet 4.6). Full pipeline log: subagentic-20260730-2000

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