--- lib/tmp.js
@@ -525,6 +525,19 @@ function _generateTmpName(opts) {
   return path.join(tmpDir, opts.dir, name);
 }
 
+/**
+ * Check the prefix and postfix options
+ *
+ * @private
+ */
+function _assertPath(path) {
+  if (path.includes("..")) {
+    throw new Error("Relative value not allowed");
+  }
+
+  return path;
+}
+
 /**
  * Asserts and sanitizes the basic options.
  *
@@ -539,8 +552,9 @@ function _assertOptionsBase(options) {
 
     // must not fail on valid .<name> or ..<name> or similar such constructs
     const basename = path.basename(name);
-    if (basename === '..' || basename === '.' || basename !== name)
+    if (basename === '..' || basename === '.' || basename !== name) {
       throw new Error(`name option must not contain a path, found "${name}".`);
+    }
   }
 
   /* istanbul ignore else */
@@ -561,8 +575,9 @@ function _assertOptionsBase(options) {
   options.unsafeCleanup = !!options.unsafeCleanup;
 
   // for completeness' sake only, also keep (multiple) blanks if the user, purportedly sane, requests us to
-  options.prefix = _isUndefined(options.prefix) ? '' : options.prefix;
-  options.postfix = _isUndefined(options.postfix) ? '' : options.postfix;
+  options.prefix = _isUndefined(options.prefix) ? '' : _assertPath(options.prefix);
+  options.postfix = _isUndefined(options.postfix) ? '' : _assertPath(options.postfix);
+  options.template = _isUndefined(options.template) ? undefined : _assertPath(options.template);
 }
 
 /**
@@ -578,7 +593,7 @@ function _getRelativePath(option, name, tmpDir, cb) {
 
     const relativePath = path.relative(tmpDir, resolvedPath);
 
-    if (!resolvedPath.startsWith(tmpDir)) {
+    if (relativePath.startsWith('..') || path.isAbsolute(relativePath)) {
       return cb(new Error(`${option} option must be relative to "${tmpDir}", found "${relativePath}".`));
     }
 
@@ -597,7 +612,7 @@ function _getRelativePathSync(option, name, tmpDir) {
   const resolvedPath = _resolvePathSync(name, tmpDir);
   const relativePath = path.relative(tmpDir, resolvedPath);
 
-  if (!resolvedPath.startsWith(tmpDir)) {
+  if (relativePath.startsWith('..') || path.isAbsolute(relativePath)) {
     throw new Error(`${option} option must be relative to "${tmpDir}", found "${relativePath}".`);
   }
 

--- test/GHSA-ph9p-34f9-6g65-test.js
@@ -0,0 +1,24 @@
+const assert = require('assert');
+const tmp = require('../lib/tmp');
+
+describe('GHSA-ph9p-34f9-6g65', function () {
+  describe('#fileSync with attacker `prefix`', function () {
+    it('should not allow such prefixes', function (done) {
+      assert.throws(function () {
+        tmp.fileSync({ prefix: "../foo"});
+      }, new RegExp('^Error: Relative value not allowed'));
+
+      done();
+    });
+  });
+
+  describe('#fileSync with attacker `postfix`', function () {
+    it('should not allow such prefixes', function (done) {
+      assert.throws(function () {
+        tmp.fileSync({ postfix: "../foo"});
+      }, new RegExp('^Error: Relative value not allowed'));
+
+      done();
+    });
+  });
+});