diff --git a/packages/cli/src/config.test.ts b/packages/cli/src/config.test.ts
new file mode 100644
index 0000000..d82eb67
--- /dev/null
+++ b/packages/cli/src/config.test.ts
@@ -0,0 +1,10 @@
+import { describe, expect, it } from "vitest";
+import { getConfigValue, setConfigValue } from "./config.js";
+
+describe("CLI config", () => {
+  it("rejects prototype-polluting path segments", () => {
+    expect(() => setConfigValue("__proto__.polluted", "true", {})).toThrow(/prototype keys/);
+    expect(() => getConfigValue("constructor.prototype.polluted", {})).toThrow(/prototype keys/);
+    expect(({} as Record<string, unknown>).polluted).toBeUndefined();
+  });
+});
diff --git a/packages/cli/src/config.ts b/packages/cli/src/config.ts
index 2b023cb..3c35584 100644
--- a/packages/cli/src/config.ts
+++ b/packages/cli/src/config.ts
@@ -40,14 +40,12 @@ export function writeConfig(config: JsonObject) {
 }
 
 export function getConfigValue(path: string, config = readConfig()) {
-  return path.split(".").reduce<unknown>((current, key) => (isObject(current) ? current[key] : undefined), config);
+  const parts = splitConfigPath(path);
+  return parts.reduce<unknown>((current, key) => (isObject(current) ? current[key] : undefined), config);
 }
 
 export function setConfigValue(path: string, rawValue: string, config = readConfig()) {
-  const parts = path.split(".").filter(Boolean);
-  if (parts.length === 0) {
-    throw new Error("Config path cannot be empty.");
-  }
+  const parts = splitConfigPath(path);
   let current: JsonObject = config;
   for (const part of parts.slice(0, -1)) {
     if (!isObject(current[part])) {
@@ -59,6 +57,18 @@ export function setConfigValue(path: string, rawValue: string, config = readConf
   return config;
 }
 
+function splitConfigPath(path: string) {
+  const parts = path.split(".");
+  const blocked = new Set(["__proto__", "constructor", "prototype"]);
+  if (parts.length === 0 || parts.some((part) => part.length === 0)) {
+    throw new Error("Config path cannot contain empty segments.");
+  }
+  if (parts.some((part) => blocked.has(part))) {
+    throw new Error("Config path cannot contain prototype keys.");
+  }
+  return parts;
+}
+
 export function parseConfigValue(value: string): unknown {
   if (value === "true") return true;
   if (value === "false") return false;