diff --git a/src/infra/exec-approvals.test.ts b/src/infra/exec-approvals.test.ts
index d63be8c75..ae63dae61 100644
--- a/src/infra/exec-approvals.test.ts
+++ b/src/infra/exec-approvals.test.ts
@@ -154,8 +154,9 @@ describe("exec approvals shell allowlist (chained commands)", () => {
 
   it("rejects chained commands when any part is not allowlisted", () => {
     const allowlist: ExecAllowlistEntry[] = [{ pattern: "/usr/bin/obsidian-cli" }];
+    // Use a non-blocklisted command to test allowlist rejection
     const result = evaluateShellAllowlist({
-      command: "/usr/bin/obsidian-cli print-default && /usr/bin/rm -rf /",
+      command: "/usr/bin/obsidian-cli print-default && /usr/bin/unknown-cmd",
       allowlist,
       safeBins: new Set(),
       cwd: "/tmp",
@@ -164,6 +165,21 @@ describe("exec approvals shell allowlist (chained commands)", () => {
     expect(result.allowlistSatisfied).toBe(false);
   });
 
+  it("rejects blocklisted commands before checking allowlist", () => {
+    const allowlist: ExecAllowlistEntry[] = [{ pattern: "rm" }];
+    // Even with 'rm' on allowlist, 'rm -rf /' should be blocked by blocklist
+    const result = evaluateShellAllowlist({
+      command: "rm -rf /",
+      allowlist,
+      safeBins: new Set(),
+      cwd: "/tmp",
+    });
+    // Blocklist rejection sets analysisOk to false
+    expect(result.analysisOk).toBe(false);
+    expect(result.blocklistResult?.blocked).toBe(true);
+    expect(result.blocklistResult?.severity).toBe("critical");
+  });
+
   it("returns analysisOk=false for malformed chains", () => {
     const allowlist: ExecAllowlistEntry[] = [{ pattern: "/usr/bin/echo" }];
     const result = evaluateShellAllowlist({
diff --git a/src/infra/exec-approvals.ts b/src/infra/exec-approvals.ts
index 0830ed89a..a31942870 100644
--- a/src/infra/exec-approvals.ts
+++ b/src/infra/exec-approvals.ts
@@ -5,6 +5,24 @@ import os from "node:os";
 import path from "node:path";
 
 import { DEFAULT_AGENT_ID } from "../routing/session-key.js";
+import {
+  evaluateBlocklist,
+  formatBlocklistReason,
+  isOnBlocklist,
+  getBlocklistPatterns,
+  type BlocklistConfig,
+  type BlocklistResult,
+} from "./exec-blocklist.js";
+
+// Re-export blocklist utilities for consumers
+export {
+  evaluateBlocklist,
+  formatBlocklistReason,
+  isOnBlocklist,
+  getBlocklistPatterns,
+  type BlocklistConfig,
+  type BlocklistResult,
+} from "./exec-blocklist.js";
 
 export type ExecHost = "sandbox" | "gateway" | "node";
 export type ExecSecurity = "deny" | "allowlist" | "full";
@@ -1045,10 +1063,13 @@ export type ExecAllowlistAnalysis = {
   allowlistSatisfied: boolean;
   allowlistMatches: ExecAllowlistEntry[];
   segments: ExecCommandSegment[];
+  /** Blocklist evaluation result (if blocklist check was performed) */
+  blocklistResult?: BlocklistResult;
 };
 
 /**
  * Evaluates allowlist for shell commands (including &&, ||, ;) and returns analysis metadata.
+ * Also checks the security blocklist FIRST to reject dangerous commands.
  */
 export function evaluateShellAllowlist(params: {
   command: string;
@@ -1058,6 +1079,46 @@ export function evaluateShellAllowlist(params: {
   env?: NodeJS.ProcessEnv;
   skillBins?: Set<string>;
   autoAllowSkills?: boolean;
+  /** Configuration for blocklist evaluation (optional) */
+  blocklistConfig?: Partial<BlocklistConfig>;
+  /** Skip blocklist check (use with extreme caution, only for sandbox environments) */
+  skipBlocklist?: boolean;
+}): ExecAllowlistAnalysis {
+  // SECURITY: Check blocklist FIRST before any other evaluation
+  // This ensures dangerous commands are blocked regardless of allowlist
+  if (!params.skipBlocklist) {
+    const blocklistResult = evaluateBlocklist(params.command, params.blocklistConfig);
+    if (blocklistResult.blocked) {
+      return {
+        analysisOk: false, // Treat blocked commands as analysis failure
+        allowlistSatisfied: false,
+        allowlistMatches: [],
+        segments: [],
+        blocklistResult,
+      };
+    }
+    // Even if not blocked, include the result for logging/audit
+    if (blocklistResult.matchedPatterns.length > 0) {
+      // Continue with allowlist evaluation but include blocklist result
+      const result = evaluateShellAllowlistInternal(params);
+      return { ...result, blocklistResult };
+    }
+  }
+
+  return evaluateShellAllowlistInternal(params);
+}
+
+/**
+ * Internal allowlist evaluation (called after blocklist check).
+ */
+function evaluateShellAllowlistInternal(params: {
+  command: string;
+  allowlist: ExecAllowlistEntry[];
+  safeBins: Set<string>;
+  cwd?: string;
+  env?: NodeJS.ProcessEnv;
+  skillBins?: Set<string>;
+  autoAllowSkills?: boolean;
 }): ExecAllowlistAnalysis {
   const chainParts = splitCommandChain(params.command);
   if (!chainParts) {
diff --git a/src/infra/exec-blocklist.test.ts b/src/infra/exec-blocklist.test.ts
new file mode 100644
index 000000000..f31cabc19
--- /dev/null
+++ b/src/infra/exec-blocklist.test.ts
@@ -0,0 +1,320 @@
+import { describe, expect, test } from "vitest";
+import {
+  evaluateBlocklist,
+  isOnBlocklist,
+  formatBlocklistReason,
+  getBlocklistPatterns,
+} from "./exec-blocklist.js";
+
+describe("evaluateBlocklist", () => {
+  describe("critical severity commands", () => {
+    test("blocks rm -rf /", () => {
+      const result = evaluateBlocklist("rm -rf /");
+      expect(result.blocked).toBe(true);
+      expect(result.severity).toBe("critical");
+      expect(result.matchedPatterns.length).toBeGreaterThan(0);
+    });
+
+    test("blocks rm -rf ~", () => {
+      const result = evaluateBlocklist("rm -rf ~");
+      expect(result.blocked).toBe(true);
+      expect(result.severity).toBe("critical");
+    });
+
+    test("blocks dd to disk device", () => {
+      const result = evaluateBlocklist("dd if=/dev/zero of=/dev/sda");
+      expect(result.blocked).toBe(true);
+      expect(result.severity).toBe("critical");
+      expect(result.matchedPatterns).toContain("dd to raw disk device");
+    });
+
+    test("blocks mkfs on disk", () => {
+      const result = evaluateBlocklist("mkfs.ext4 /dev/sda1");
+      expect(result.blocked).toBe(true);
+      expect(result.severity).toBe("critical");
+    });
+
+    test("blocks halt command", () => {
+      const result = evaluateBlocklist("halt");
+      expect(result.blocked).toBe(true);
+      expect(result.severity).toBe("critical");
+    });
+
+    test("blocks reboot command", () => {
+      const result = evaluateBlocklist("reboot");
+      expect(result.blocked).toBe(true);
+      expect(result.severity).toBe("critical");
+    });
+
+    test("blocks shutdown command", () => {
+      const result = evaluateBlocklist("shutdown -h now");
+      expect(result.blocked).toBe(true);
+      expect(result.severity).toBe("critical");
+    });
+
+    test("blocks systemctl poweroff", () => {
+      const result = evaluateBlocklist("systemctl poweroff");
+      expect(result.blocked).toBe(true);
+      expect(result.severity).toBe("critical");
+    });
+
+    test("blocks fork bomb", () => {
+      const result = evaluateBlocklist(":(){ :|:& };:");
+      expect(result.blocked).toBe(true);
+      expect(result.severity).toBe("critical");
+      expect(result.matchedPatterns).toContain("fork bomb");
+    });
+  });
+
+  describe("high severity commands", () => {
+    test("blocks sudo", () => {
+      const result = evaluateBlocklist("sudo rm -rf /tmp/test");
+      expect(result.blocked).toBe(true);
+      expect(result.severity).toBe("high");
+      expect(result.matchedPatterns).toContain("sudo (privilege escalation)");
+    });
+
+    test("blocks su to root", () => {
+      const result = evaluateBlocklist("su - root");
+      expect(result.blocked).toBe(true);
+      expect(result.severity).toBe("high");
+    });
+
+    test("blocks passwd", () => {
+      const result = evaluateBlocklist("passwd");
+      expect(result.blocked).toBe(true);
+      expect(result.severity).toBe("high");
+    });
+
+    test("blocks visudo", () => {
+      const result = evaluateBlocklist("visudo");
+      expect(result.blocked).toBe(true);
+      expect(result.severity).toBe("high");
+    });
+
+    test("blocks iptables", () => {
+      const result = evaluateBlocklist("iptables -F");
+      expect(result.blocked).toBe(true);
+      expect(result.severity).toBe("high");
+    });
+
+    test("blocks useradd", () => {
+      const result = evaluateBlocklist("useradd newuser");
+      expect(result.blocked).toBe(true);
+      expect(result.severity).toBe("high");
+    });
+
+    test("blocks userdel", () => {
+      const result = evaluateBlocklist("userdel olduser");
+      expect(result.blocked).toBe(true);
+      expect(result.severity).toBe("high");
+    });
+
+    test("blocks modprobe", () => {
+      const result = evaluateBlocklist("modprobe some_module");
+      expect(result.blocked).toBe(true);
+      expect(result.severity).toBe("high");
+    });
+
+    test("blocks redirect to /etc/passwd", () => {
+      const result = evaluateBlocklist("echo 'malicious' > /etc/passwd");
+      expect(result.blocked).toBe(true);
+      expect(result.severity).toBe("high");
+    });
+  });
+
+  describe("medium severity commands (not blocked by default)", () => {
+    test("does not block command substitution by default", () => {
+      const result = evaluateBlocklist("echo $(date)");
+      expect(result.blocked).toBe(false);
+      expect(result.severity).toBe("medium");
+      expect(result.matchedPatterns).toContain("command substitution $()");
+    });
+
+    test("does not block backtick substitution by default", () => {
+      const result = evaluateBlocklist("echo `whoami`");
+      expect(result.blocked).toBe(false);
+      expect(result.severity).toBe("medium");
+    });
+
+    test("does not block eval by default", () => {
+      const result = evaluateBlocklist("eval 'echo hello'");
+      expect(result.blocked).toBe(false);
+      expect(result.severity).toBe("medium");
+    });
+
+    test("does not block curl POST by default", () => {
+      const result = evaluateBlocklist("curl -d 'data' https://example.com");
+      expect(result.blocked).toBe(false);
+      expect(result.severity).toBe("medium");
+    });
+
+    test("does not block killall by default", () => {
+      const result = evaluateBlocklist("killall node");
+      expect(result.blocked).toBe(false);
+      expect(result.severity).toBe("medium");
+    });
+
+    test("does not block chmod 777 by default", () => {
+      const result = evaluateBlocklist("chmod 777 /tmp/test");
+      expect(result.blocked).toBe(false);
+      expect(result.severity).toBe("medium");
+    });
+
+    test("blocks medium severity when configured", () => {
+      const result = evaluateBlocklist("echo $(date)", { blockMedium: true });
+      expect(result.blocked).toBe(true);
+      expect(result.severity).toBe("medium");
+    });
+  });
+
+  describe("safe commands", () => {
+    test("does not block ls", () => {
+      const result = evaluateBlocklist("ls -la");
+      expect(result.blocked).toBe(false);
+      expect(result.severity).toBe(null);
+    });
+
+    test("does not block cat", () => {
+      const result = evaluateBlocklist("cat /etc/hosts");
+      expect(result.blocked).toBe(false);
+      expect(result.severity).toBe(null);
+    });
+
+    test("does not block grep", () => {
+      const result = evaluateBlocklist("grep -r pattern .");
+      expect(result.blocked).toBe(false);
+      expect(result.severity).toBe(null);
+    });
+
+    test("does not block normal rm", () => {
+      const result = evaluateBlocklist("rm /tmp/test.txt");
+      expect(result.blocked).toBe(false);
+      expect(result.severity).toBe(null);
+    });
+
+    test("does not block npm install", () => {
+      const result = evaluateBlocklist("npm install");
+      expect(result.blocked).toBe(false);
+      expect(result.severity).toBe(null);
+    });
+
+    test("does not block git operations", () => {
+      const result = evaluateBlocklist("git push origin main");
+      expect(result.blocked).toBe(false);
+      expect(result.severity).toBe(null);
+    });
+
+    test("does not block curl GET", () => {
+      const result = evaluateBlocklist("curl https://example.com");
+      expect(result.blocked).toBe(false);
+      expect(result.severity).toBe(null);
+    });
+  });
+
+  describe("configuration options", () => {
+    test("can disable critical blocking", () => {
+      const result = evaluateBlocklist("rm -rf /", { blockCritical: false });
+      expect(result.blocked).toBe(false);
+      expect(result.severity).toBe("critical");
+      expect(result.matchedPatterns.length).toBeGreaterThan(0);
+    });
+
+    test("can disable high blocking", () => {
+      const result = evaluateBlocklist("sudo ls", { blockHigh: false });
+      expect(result.blocked).toBe(false);
+      expect(result.severity).toBe("high");
+    });
+
+    test("can enable medium blocking", () => {
+      const result = evaluateBlocklist("eval 'test'", { blockMedium: true });
+      expect(result.blocked).toBe(true);
+      expect(result.severity).toBe("medium");
+    });
+  });
+});
+
+describe("isOnBlocklist", () => {
+  test("returns true for blocked commands", () => {
+    expect(isOnBlocklist("rm -rf /")).toBe(true);
+    expect(isOnBlocklist("sudo ls")).toBe(true);
+    expect(isOnBlocklist("echo $(date)")).toBe(true);
+  });
+
+  test("returns false for safe commands", () => {
+    expect(isOnBlocklist("ls -la")).toBe(false);
+    expect(isOnBlocklist("cat file.txt")).toBe(false);
+    expect(isOnBlocklist("npm install")).toBe(false);
+  });
+});
+
+describe("formatBlocklistReason", () => {
+  test("formats blocked command reason", () => {
+    const result = evaluateBlocklist("rm -rf /");
+    const formatted = formatBlocklistReason(result);
+    expect(formatted).toContain("blocked");
+    expect(formatted).toContain("critical");
+    expect(formatted).toContain("catastrophic");
+  });
+
+  test("formats allowed command", () => {
+    const result = evaluateBlocklist("ls -la");
+    const formatted = formatBlocklistReason(result);
+    expect(formatted).toContain("allowed");
+  });
+});
+
+describe("getBlocklistPatterns", () => {
+  test("returns all patterns", () => {
+    const patterns = getBlocklistPatterns();
+    expect(Array.isArray(patterns)).toBe(true);
+    expect(patterns.length).toBeGreaterThan(0);
+
+    // Check structure
+    const first = patterns[0];
+    expect(first).toHaveProperty("pattern");
+    expect(first).toHaveProperty("description");
+    expect(first).toHaveProperty("severity");
+  });
+
+  test("includes critical patterns", () => {
+    const patterns = getBlocklistPatterns();
+    const critical = patterns.filter((p) => p.severity === "critical");
+    expect(critical.length).toBeGreaterThan(0);
+  });
+
+  test("includes high patterns", () => {
+    const patterns = getBlocklistPatterns();
+    const high = patterns.filter((p) => p.severity === "high");
+    expect(high.length).toBeGreaterThan(0);
+  });
+
+  test("includes medium patterns", () => {
+    const patterns = getBlocklistPatterns();
+    const medium = patterns.filter((p) => p.severity === "medium");
+    expect(medium.length).toBeGreaterThan(0);
+  });
+});
+
+describe("edge cases", () => {
+  test("handles empty command", () => {
+    const result = evaluateBlocklist("");
+    expect(result.blocked).toBe(false);
+    expect(result.matchedPatterns.length).toBe(0);
+  });
+
+  test("handles command with spaces only", () => {
+    const result = evaluateBlocklist("   ");
+    expect(result.blocked).toBe(false);
+  });
+
+  test("handles multiline command", () => {
+    const result = evaluateBlocklist("ls -la\nrm -rf /");
+    expect(result.blocked).toBe(true);
+  });
+
+  test("handles case variations", () => {
+    const result = evaluateBlocklist("SUDO ls");
+    expect(result.blocked).toBe(true);
+  });
+});
diff --git a/src/infra/exec-blocklist.ts b/src/infra/exec-blocklist.ts
new file mode 100644
index 000000000..c6cc6ad7b
--- /dev/null
+++ b/src/infra/exec-blocklist.ts
@@ -0,0 +1,514 @@
+/**
+ * Command Execution Blocklist
+ *
+ * This module provides a security layer that blocks dangerous commands before
+ * they reach the allowlist evaluation. Commands matching blocklist patterns
+ * are ALWAYS blocked regardless of allowlist settings or elevated permissions.
+ *
+ * The blocklist is designed to prevent catastrophic system damage from
+ * commands that should almost never be run via an AI assistant.
+ */
+
+import { createSubsystemLogger } from "../logging/subsystem.js";
+
+const log = createSubsystemLogger("exec").child("blocklist");
+
+/** Result of blocklist evaluation */
+export type BlocklistResult = {
+  blocked: boolean;
+  reason: string | null;
+  matchedPatterns: string[];
+  severity: "critical" | "high" | "medium" | null;
+};
+
+/** Blocklist entry with pattern and metadata */
+type BlocklistEntry = {
+  pattern: RegExp;
+  description: string;
+  severity: "critical" | "high" | "medium";
+  /** If true, requires explicit user approval even in elevated mode */
+  requiresExplicitApproval: boolean;
+};
+
+/**
+ * CRITICAL: Commands that can cause catastrophic, irreversible system damage.
+ * These are ALWAYS blocked without any override option.
+ */
+const CRITICAL_BLOCKLIST: BlocklistEntry[] = [
+  // Destructive filesystem operations
+  {
+    pattern: /\brm\s+(-[a-zA-Z]*r[a-zA-Z]*\s+)?(-[a-zA-Z]*f[a-zA-Z]*\s+)?[\/~]\s*$/i,
+    description: "rm -rf / (root filesystem deletion)",
+    severity: "critical",
+    requiresExplicitApproval: true,
+  },
+  {
+    pattern: /\brm\s+(-[a-zA-Z]*[rf]+[a-zA-Z]*\s+)+\s*\/\s*$/i,
+    description: "rm -rf / (root filesystem deletion)",
+    severity: "critical",
+    requiresExplicitApproval: true,
+  },
+  {
+    pattern: /\brm\s+(-[a-zA-Z]*[rf]+[a-zA-Z]*\s+)+\s*~\s*$/i,
+    description: "rm -rf ~ (home directory deletion)",
+    severity: "critical",
+    requiresExplicitApproval: true,
+  },
+
+  // Disk/filesystem destruction
+  {
+    pattern: /\bdd\s+.*\bof\s*=\s*\/dev\/(sd[a-z]|hd[a-z]|nvme\d+n\d+|disk\d+)\b/i,
+    description: "dd to raw disk device",
+    severity: "critical",
+    requiresExplicitApproval: true,
+  },
+  {
+    pattern: /\bmkfs(\.[a-z0-9]+)?\s+.*\/dev\/(sd[a-z]|hd[a-z]|nvme\d+n\d+|disk\d+)/i,
+    description: "mkfs on disk device (filesystem destruction)",
+    severity: "critical",
+    requiresExplicitApproval: true,
+  },
+  {
+    pattern: /\bfdisk\s+.*\/dev\/(sd[a-z]|hd[a-z]|nvme\d+n\d+|disk\d+)/i,
+    description: "fdisk partition manipulation",
+    severity: "critical",
+    requiresExplicitApproval: true,
+  },
+  {
+    pattern: /\bparted\s+.*\/dev\/(sd[a-z]|hd[a-z]|nvme\d+n\d+|disk\d+)/i,
+    description: "parted partition manipulation",
+    severity: "critical",
+    requiresExplicitApproval: true,
+  },
+
+  // System control commands
+  {
+    pattern: /\b(halt|poweroff|reboot|shutdown)\b/i,
+    description: "system halt/reboot command",
+    severity: "critical",
+    requiresExplicitApproval: true,
+  },
+  {
+    pattern: /\binit\s+[0-6]\b/i,
+    description: "init runlevel change",
+    severity: "critical",
+    requiresExplicitApproval: true,
+  },
+  {
+    pattern: /\bsystemctl\s+(halt|poweroff|reboot|suspend|hibernate)\b/i,
+    description: "systemctl power control",
+    severity: "critical",
+    requiresExplicitApproval: true,
+  },
+
+  // Fork bomb patterns
+  {
+    pattern: /:\s*\(\s*\)\s*\{\s*:\s*\|\s*:\s*&\s*\}\s*;\s*:/,
+    description: "fork bomb",
+    severity: "critical",
+    requiresExplicitApproval: true,
+  },
+  {
+    pattern: /\bwhile\s+true\s*;\s*do\s*:\s*;\s*done\s*&/i,
+    description: "infinite loop fork",
+    severity: "critical",
+    requiresExplicitApproval: true,
+  },
+];
+
+/**
+ * HIGH: Commands that can cause significant security issues or data loss.
+ * These require explicit user approval.
+ */
+const HIGH_BLOCKLIST: BlocklistEntry[] = [
+  // Privilege escalation
+  {
+    pattern: /\bsudo\s+/i,
+    description: "sudo (privilege escalation)",
+    severity: "high",
+    requiresExplicitApproval: true,
+  },
+  {
+    pattern: /\bsu\s+(-\s+)?root\b/i,
+    description: "su to root",
+    severity: "high",
+    requiresExplicitApproval: true,
+  },
+  {
+    pattern: /\bdoas\s+/i,
+    description: "doas (privilege escalation)",
+    severity: "high",
+    requiresExplicitApproval: true,
+  },
+
+  // Credential manipulation
+  {
+    pattern: /\bpasswd\b/i,
+    description: "passwd (password change)",
+    severity: "high",
+    requiresExplicitApproval: true,
+  },
+  {
+    pattern: /\bchpasswd\b/i,
+    description: "chpasswd (bulk password change)",
+    severity: "high",
+    requiresExplicitApproval: true,
+  },
+  {
+    pattern: /\bvisudo\b/i,
+    description: "visudo (sudoers modification)",
+    severity: "high",
+    requiresExplicitApproval: true,
+  },
+
+  // Firewall and network security
+  {
+    pattern: /\biptables\s+/i,
+    description: "iptables (firewall rules)",
+    severity: "high",
+    requiresExplicitApproval: true,
+  },
+  {
+    pattern: /\bip6tables\s+/i,
+    description: "ip6tables (IPv6 firewall rules)",
+    severity: "high",
+    requiresExplicitApproval: true,
+  },
+  {
+    pattern: /\bnft\s+/i,
+    description: "nftables (firewall rules)",
+    severity: "high",
+    requiresExplicitApproval: true,
+  },
+  {
+    pattern: /\bufw\s+(disable|reset|delete)/i,
+    description: "ufw firewall disable/reset",
+    severity: "high",
+    requiresExplicitApproval: true,
+  },
+  {
+    pattern: /\bfirewall-cmd\s+/i,
+    description: "firewalld manipulation",
+    severity: "high",
+    requiresExplicitApproval: true,
+  },
+
+  // User/group manipulation
+  {
+    pattern: /\buseradd\b/i,
+    description: "useradd (user creation)",
+    severity: "high",
+    requiresExplicitApproval: true,
+  },
+  {
+    pattern: /\buserdel\b/i,
+    description: "userdel (user deletion)",
+    severity: "high",
+    requiresExplicitApproval: true,
+  },
+  {
+    pattern: /\busermod\b/i,
+    description: "usermod (user modification)",
+    severity: "high",
+    requiresExplicitApproval: true,
+  },
+  {
+    pattern: /\bgroupadd\b/i,
+    description: "groupadd (group creation)",
+    severity: "high",
+    requiresExplicitApproval: true,
+  },
+  {
+    pattern: /\bgroupdel\b/i,
+    description: "groupdel (group deletion)",
+    severity: "high",
+    requiresExplicitApproval: true,
+  },
+
+  // SSH key manipulation
+  {
+    pattern: /\bssh-keygen\s+.*(-[a-zA-Z]*f[a-zA-Z]*\s+)?~\/\.ssh\/(id_|authorized_keys)/i,
+    description: "SSH key overwrite",
+    severity: "high",
+    requiresExplicitApproval: true,
+  },
+
+  // Kernel/boot manipulation
+  {
+    pattern: /\bmodprobe\s+/i,
+    description: "modprobe (kernel module loading)",
+    severity: "high",
+    requiresExplicitApproval: true,
+  },
+  {
+    pattern: /\binsmod\s+/i,
+    description: "insmod (kernel module insertion)",
+    severity: "high",
+    requiresExplicitApproval: true,
+  },
+  {
+    pattern: /\brmmod\s+/i,
+    description: "rmmod (kernel module removal)",
+    severity: "high",
+    requiresExplicitApproval: true,
+  },
+
+  // Cron manipulation (persistence)
+  {
+    pattern: /\bcrontab\s+-[a-zA-Z]*e/i,
+    description: "crontab edit",
+    severity: "high",
+    requiresExplicitApproval: true,
+  },
+
+  // System config files
+  {
+    pattern: />\s*\/etc\/(passwd|shadow|sudoers|hosts|fstab|ssh)/i,
+    description: "redirect to system config file",
+    severity: "high",
+    requiresExplicitApproval: true,
+  },
+
+  // macOS specific
+  {
+    pattern: /\bcsrutil\s+(disable|enable)\b/i,
+    description: "csrutil (SIP modification)",
+    severity: "high",
+    requiresExplicitApproval: true,
+  },
+  {
+    pattern: /\bspctl\s+/i,
+    description: "spctl (Gatekeeper manipulation)",
+    severity: "high",
+    requiresExplicitApproval: true,
+  },
+  {
+    pattern: /\bdscl\s+.*-passwd/i,
+    description: "dscl password change",
+    severity: "high",
+    requiresExplicitApproval: true,
+  },
+];
+
+/**
+ * MEDIUM: Commands that should be reviewed but may have legitimate uses.
+ * These log warnings but don't block by default.
+ */
+const MEDIUM_BLOCKLIST: BlocklistEntry[] = [
+  // Shell operators that could enable injection
+  {
+    pattern: /\$\([^)]+\)/,
+    description: "command substitution $()",
+    severity: "medium",
+    requiresExplicitApproval: false,
+  },
+  {
+    pattern: /`[^`]+`/,
+    description: "backtick command substitution",
+    severity: "medium",
+    requiresExplicitApproval: false,
+  },
+  {
+    pattern: /\beval\s+/i,
+    description: "eval (arbitrary code execution)",
+    severity: "medium",
+    requiresExplicitApproval: false,
+  },
+
+  // Network data exfiltration
+  {
+    pattern: /\bcurl\s+.*-[a-zA-Z]*d\s+/i,
+    description: "curl POST data",
+    severity: "medium",
+    requiresExplicitApproval: false,
+  },
+  {
+    pattern: /\bwget\s+.*--post/i,
+    description: "wget POST",
+    severity: "medium",
+    requiresExplicitApproval: false,
+  },
+
+  // Process manipulation
+  {
+    pattern: /\bkillall\s+/i,
+    description: "killall (mass process termination)",
+    severity: "medium",
+    requiresExplicitApproval: false,
+  },
+  {
+    pattern: /\bpkill\s+-9\s+/i,
+    description: "pkill -9 (force kill)",
+    severity: "medium",
+    requiresExplicitApproval: false,
+  },
+
+  // File permission changes
+  {
+    pattern: /\bchmod\s+777\s+/i,
+    description: "chmod 777 (world-writable)",
+    severity: "medium",
+    requiresExplicitApproval: false,
+  },
+  {
+    pattern: /\bchmod\s+-[a-zA-Z]*R[a-zA-Z]*\s+/i,
+    description: "chmod recursive",
+    severity: "medium",
+    requiresExplicitApproval: false,
+  },
+  {
+    pattern: /\bchown\s+-[a-zA-Z]*R[a-zA-Z]*\s+/i,
+    description: "chown recursive",
+    severity: "medium",
+    requiresExplicitApproval: false,
+  },
+];
+
+/** Combined blocklist for evaluation */
+const BLOCKLIST = [...CRITICAL_BLOCKLIST, ...HIGH_BLOCKLIST, ...MEDIUM_BLOCKLIST];
+
+/**
+ * Configuration for blocklist evaluation behavior.
+ */
+export type BlocklistConfig = {
+  /** Block critical severity commands (default: true) */
+  blockCritical: boolean;
+  /** Block high severity commands (default: true) */
+  blockHigh: boolean;
+  /** Block medium severity commands (default: false, just warn) */
+  blockMedium: boolean;
+  /** Log all blocklist matches */
+  logMatches: boolean;
+  /** Allow bypass for explicitly approved commands */
+  allowExplicitApproval: boolean;
+};
+
+const DEFAULT_BLOCKLIST_CONFIG: BlocklistConfig = {
+  blockCritical: true,
+  blockHigh: true,
+  blockMedium: false,
+  logMatches: true,
+  allowExplicitApproval: false, // Even explicit approval cannot bypass critical/high by default
+};
+
+/**
+ * Evaluates a command against the blocklist.
+ * Returns whether the command should be blocked and why.
+ */
+export function evaluateBlocklist(
+  command: string,
+  config: Partial<BlocklistConfig> = {},
+): BlocklistResult {
+  const cfg = { ...DEFAULT_BLOCKLIST_CONFIG, ...config };
+  const matchedPatterns: string[] = [];
+  let highestSeverity: "critical" | "high" | "medium" | null = null;
+  let blocked = false;
+  let reason: string | null = null;
+
+  for (const entry of BLOCKLIST) {
+    if (entry.pattern.test(command)) {
+      matchedPatterns.push(entry.description);
+
+      // Track highest severity
+      if (
+        highestSeverity === null ||
+        entry.severity === "critical" ||
+        (entry.severity === "high" && highestSeverity !== "critical")
+      ) {
+        highestSeverity = entry.severity;
+      }
+
+      // Determine if we should block
+      if (entry.severity === "critical" && cfg.blockCritical) {
+        blocked = true;
+        reason = `BLOCKED: ${entry.description} (critical severity)`;
+      } else if (entry.severity === "high" && cfg.blockHigh) {
+        blocked = true;
+        reason = `BLOCKED: ${entry.description} (high severity)`;
+      } else if (entry.severity === "medium" && cfg.blockMedium) {
+        blocked = true;
+        reason = `BLOCKED: ${entry.description} (medium severity)`;
+      }
+    }
+  }
+
+  // Log matches if configured
+  if (cfg.logMatches && matchedPatterns.length > 0) {
+    const logLevel = blocked ? "error" : highestSeverity === "medium" ? "warn" : "error";
+    const preview = command.length > 80 ? command.slice(0, 80) + "..." : command;
+
+    if (logLevel === "error") {
+      log.error("Command blocklist match", {
+        blocked,
+        severity: highestSeverity,
+        patterns: matchedPatterns,
+        command: preview,
+      });
+    } else {
+      log.warn("Command blocklist match", {
+        blocked,
+        severity: highestSeverity,
+        patterns: matchedPatterns,
+        command: preview,
+      });
+    }
+  }
+
+  return {
+    blocked,
+    reason,
+    matchedPatterns,
+    severity: highestSeverity,
+  };
+}
+
+/**
+ * Quick check if a command is on the blocklist (any severity).
+ * Use evaluateBlocklist for full analysis.
+ */
+export function isOnBlocklist(command: string): boolean {
+  return BLOCKLIST.some((entry) => entry.pattern.test(command));
+}
+
+/**
+ * Returns a human-readable explanation of why a command was blocked.
+ */
+export function formatBlocklistReason(result: BlocklistResult): string {
+  if (!result.blocked) {
+    return "Command is allowed";
+  }
+
+  const lines = [
+    `Command blocked due to security policy.`,
+    `Severity: ${result.severity}`,
+    `Matched patterns:`,
+    ...result.matchedPatterns.map((p) => `  - ${p}`),
+  ];
+
+  if (result.severity === "critical") {
+    lines.push("");
+    lines.push("Critical severity commands cannot be approved and are always blocked.");
+    lines.push("These commands can cause catastrophic, irreversible system damage.");
+  } else if (result.severity === "high") {
+    lines.push("");
+    lines.push("High severity commands require explicit user approval.");
+    lines.push("If you need to run this command, please do so directly in your terminal.");
+  }
+
+  return lines.join("\n");
+}
+
+/**
+ * Get all blocklist patterns for documentation/display purposes.
+ */
+export function getBlocklistPatterns(): Array<{
+  pattern: string;
+  description: string;
+  severity: "critical" | "high" | "medium";
+}> {
+  return BLOCKLIST.map((entry) => ({
+    pattern: entry.pattern.source,
+    description: entry.description,
+    severity: entry.severity,
+  }));
+}