diff --git a/src/PhpImap/Mailbox.php b/src/PhpImap/Mailbox.php
index 011b497d..76beb8e8 100644
--- a/src/PhpImap/Mailbox.php
+++ b/src/PhpImap/Mailbox.php
@@ -1440,6 +1440,14 @@ protected function hasAttachmentDisposition(object $partStructure): bool
         return \is_string($disposition) && 'attachment' === \mb_strtolower($disposition);
     }
 
+    protected function sanitizeAttachmentFileSystemName(string $fileName): string
+    {
+        return \strtr($fileName, [
+            '\\' => '_',
+            '/' => '_',
+        ]);
+    }
+
     /**
      * Download attachment.
      *
@@ -1519,7 +1527,7 @@ public function downloadAttachment(DataPartInfo $dataInfo, array $params, object
 
         if (null != $attachmentsDir) {
             if (true == $this->getAttachmentFilenameMode()) {
-                $fileSysName = $attachment->name;
+                $fileSysName = $this->sanitizeAttachmentFileSystemName($attachment->name);
             } else {
                 $fileSysName = \bin2hex(\random_bytes(16)).'.bin';
             }
diff --git a/tests/unit/MailboxTest.php b/tests/unit/MailboxTest.php
index 30f39b7f..29b6b002 100644
--- a/tests/unit/MailboxTest.php
+++ b/tests/unit/MailboxTest.php
@@ -453,6 +453,62 @@ public function testDownloadAttachmentTreatsMixedCaseRfc822DispositionAsEmlAttac
         $this->assertFalse($attachment->emlOrigin);
     }
 
+    /**
+     * @return array<string, array{0:string, 1:string}>
+     */
+    public function unsafeAttachmentFilenameProvider(): array
+    {
+        return [
+            'forward slash' => ['foo/bar.txt', 'foo_bar.txt'],
+            'backslash' => ['foo\\bar.txt', 'foo_bar.txt'],
+        ];
+    }
+
+    /**
+     * @dataProvider unsafeAttachmentFilenameProvider
+     */
+    public function testDownloadAttachmentSanitizesFilePathWhenUsingOriginalFilenameMode(string $unsafeName, string $expectedFileName): void
+    {
+        $attachmentsDir = \sys_get_temp_dir().DIRECTORY_SEPARATOR.'php-imap-attachment-name-'.\bin2hex(\random_bytes(8));
+        $mailbox = new class ($this->imapPath, $this->login, $this->password, $attachmentsDir, $this->serverEncoding, true, true) extends Fixtures\Mailbox {
+            public function decodeMimeStr(string $string): string
+            {
+                return $string;
+            }
+        };
+        $dataInfo = new Fixtures\DataPartInfo($mailbox, 1, '2', 0, 0);
+        $dataInfo->setData('attachment body');
+        $partStructure = (object) [
+            'type' => 3,
+            'subtype' => 'OCTET-STREAM',
+            'bytes' => 15,
+            'encoding' => 0,
+            'ifid' => 0,
+            'ifsubtype' => 1,
+            'ifdescription' => 0,
+        ];
+        $attachmentPath = null;
+
+        \mkdir($attachmentsDir);
+
+        try {
+            $attachment = $mailbox->downloadAttachment($dataInfo, ['filename' => $unsafeName], $partStructure);
+            $attachmentPath = $attachment->filePath;
+
+            $this->assertSame($unsafeName, $attachment->name);
+            $this->assertSame($attachmentsDir.DIRECTORY_SEPARATOR.$expectedFileName, $attachmentPath);
+            $this->assertFileExists($attachmentPath);
+        } finally {
+            if (\is_string($attachmentPath) && \file_exists($attachmentPath)) {
+                \unlink($attachmentPath);
+            }
+
+            if (\is_dir($attachmentsDir)) {
+                \rmdir($attachmentsDir);
+            }
+        }
+    }
+
     /**
      * Provides test data for testing encoding.
      *