Certificate is singed by an unrecognised certificate authority. If a browser receives a self-signed certificate, it pops up a warning, and the burden falls to the user to confirm the identity. Pushing this decision to the user is ultimately what opens up the possibility of a man-in-the-middle (MITM) attack. The security issue is not with self-signed certificates, but with the way users interact with them in the browser.
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You can purchase SSL certificates from Symantec