diff --git a/src/__tests__/index.spec.ts b/src/__tests__/index.spec.ts
index 51104e9..966291f 100644
--- a/src/__tests__/index.spec.ts
+++ b/src/__tests__/index.spec.ts
@@ -88,6 +88,18 @@ describe('copyfiles', () => {
       });
     }));
 
+  test('ignores inherited option properties', () =>
+    new Promise((done: any) => {
+      writeFileSync('input/a.txt', 'a');
+      const options = Object.create({ flat: true });
+
+      copyfiles('input/a.txt', 'output', options, () => {
+        expect(existsSync('output/input/a.txt')).toBe(true);
+        expect(existsSync('output/a.txt')).toBe(false);
+        done();
+      });
+    }));
+
   test('copies files using directory pattern with /**', () =>
     new Promise((done: any) => {
       writeFileSync('input/a.txt', 'a');
@@ -789,6 +801,12 @@ describe('copyfiles', () => {
       expect(result.replaceAll('\\', '/').endsWith(posixJoin('dest', 'bar.txt'))).toBe(true);
     });
 
+    it('getDestinationPath ignores inherited option properties', () => {
+      const options = Object.create({ flat: true });
+      const result = getDestinationPath('foo/bar.txt', 'dest', options, false);
+      expect(result.replaceAll('\\', '/')).toBe(posixJoin('dest', 'foo', 'bar.txt'));
+    });
+
     it('getDestinationPath - up === true branch', () => {
       const result = getDestinationPath('foo/bar.txt', 'dest', { up: true }, false);
       expect(result.replaceAll('\\', '/').endsWith(posixJoin('dest', 'bar.txt'))).toBe(true);
diff --git a/src/cli.ts b/src/cli.ts
index 5334508..c926163 100644
--- a/src/cli.ts
+++ b/src/cli.ts
@@ -24,6 +24,57 @@ function handleError(err?: Error) {
 }
 
 try {
+  // cli-nano resolves option names through dynamic property access. Keep this
+  // lookup table free of inherited keys such as "__proto__" and "constructor".
+  const optionDefinitions = {
+    all: {
+      alias: 'a',
+      type: 'boolean',
+      describe: 'Include files & directories begining with a dot (.)',
+    },
+    dryRun: {
+      alias: 'd',
+      type: 'boolean',
+      describe: 'Show what would be copied, but do not actually copy any files',
+    },
+    error: {
+      alias: 'E',
+      type: 'boolean',
+      describe: 'Throw error if nothing is copied',
+    },
+    exclude: {
+      alias: 'e',
+      type: 'array',
+      describe: 'Pattern or glob to exclude (may be passed multiple times)',
+    },
+    flat: {
+      alias: 'f',
+      type: 'boolean',
+      describe: 'Flatten the output',
+    },
+    follow: {
+      alias: 'F',
+      type: 'boolean',
+      describe: 'Follow symbolink links',
+    },
+    stat: {
+      alias: 's',
+      type: 'boolean',
+      describe: 'Show statistics after execution (execution time + file count)',
+    },
+    up: {
+      alias: 'u',
+      type: 'number',
+      describe: 'Slice a path off the bottom of the paths',
+    },
+    verbose: {
+      alias: 'V',
+      type: 'boolean',
+      describe: 'Print more information to console',
+    },
+  } as const;
+  const options = Object.assign(Object.create(null) as typeof optionDefinitions, optionDefinitions);
+
   const config = {
     command: {
       name: 'copyfiles',
@@ -51,53 +102,7 @@ try {
         },
       ],
     },
-    options: {
-      all: {
-        alias: 'a',
-        type: 'boolean',
-        describe: 'Include files & directories begining with a dot (.)',
-      },
-      dryRun: {
-        alias: 'd',
-        type: 'boolean',
-        describe: 'Show what would be copied, but do not actually copy any files',
-      },
-      error: {
-        alias: 'E',
-        type: 'boolean',
-        describe: 'Throw error if nothing is copied',
-      },
-      exclude: {
-        alias: 'e',
-        type: 'array',
-        describe: 'Pattern or glob to exclude (may be passed multiple times)',
-      },
-      flat: {
-        alias: 'f',
-        type: 'boolean',
-        describe: 'Flatten the output',
-      },
-      follow: {
-        alias: 'F',
-        type: 'boolean',
-        describe: 'Follow symbolink links',
-      },
-      stat: {
-        alias: 's',
-        type: 'boolean',
-        describe: 'Show statistics after execution (execution time + file count)',
-      },
-      up: {
-        alias: 'u',
-        type: 'number',
-        describe: 'Slice a path off the bottom of the paths',
-      },
-      verbose: {
-        alias: 'V',
-        type: 'boolean',
-        describe: 'Print more information to console',
-      },
-    },
+    options,
     version: readPackage().version,
   } as const;
 
diff --git a/src/index.ts b/src/index.ts
index 73d8445..fd6248c 100644
--- a/src/index.ts
+++ b/src/index.ts
@@ -40,6 +40,10 @@ function throwOrCallback(err: Error, cb?: (e?: Error) => void) {
   }
 }
 
+function createSafeOptions(options: CopyFileOptions): CopyFileOptions {
+  return Object.assign(Object.create(null) as CopyFileOptions, options);
+}
+
 function callRenameWhenDefined(inFile: string, dest: string, options: CopyFileOptions): string {
   if (typeof options.rename === 'function') {
     return options.rename(inFile, dest);
@@ -51,6 +55,7 @@ function callRenameWhenDefined(inFile: string, dest: string, options: CopyFileOp
  * Calculate the destination path for a given input file and options.
  */
 export function getDestinationPath(inFile: string, outDir: string, options: CopyFileOptions, isSingleFileRename = false): string {
+  options = createSafeOptions(options);
   const fileDir = dirname(inFile);
   const fileName = basename(inFile);
   const srcExt = extname(fileName);
@@ -184,6 +189,9 @@ function getMatchedFiles(
  * @param {(e?: Error) => void} callback - optionally callback that will be executed after copy is finished or when an error occurs
  */
 export function copyfiles(sources: string | string[], outPath: string, options: CopyFileOptions = {}, callback?: (e?: Error) => void) {
+  // Treat options as data rather than inheriting behavior from Object.prototype.
+  // This also safely preserves an own "__proto__" key if options came from JSON.
+  options = createSafeOptions(options);
   const cb = callback || options.callback;
   sources = arrify(sources);