What's the point in providing an MD5 or SHA1 hash along with a downloadable executable?

Question

I thought they were there for security; to check that the file hasn't been tampered with. But surely if someone is capable of modifying the file then they are also capable of modifying the page with the hash!

What security does this actually offer?

Answer 1

They're usually there just to ensure you got the download complete and correct.

It does help with one security-related issue: malicious modification happens as a result of a trojan running on the target machine inserting itself. This isn't a "targeted" attack per se- the trojan just "infects" whatever exe's it can- wouldn't be smart enough to fix the hash.