A cookie itself cannot harm the computer, as it does not and cannot hold code or perform any virus or Trojan execution, However, the cookie can support (or help) malicious actions to be taken on the respective system. Even more, being a plain text file, they are vulnerable, meaning that they can be harvested by other applications or to lead a possible attacker to the respective computer.
An attacker also might harvest information from cookies using either different cookies or from spyware software (for example login information for different websites the customer might use).
Another side effect for the cookies is that over time they can cause computer slow down performance because of the disk space they might cover.
You might also like to read:
- Cookies and Internet Temp Files
- How to manage the Cookies, Cache, Internet temporary files and Internet History?
- Should I delete, scan or disable Cookies, Cache and Internet temporary files?
- What are the benefits of Cookies, Cache and Internet temporary files?