Only the latest version of libvoikko and voikko-fi are supported with security updates. For serious issues we may consider providing patches for older versions.
If you think you have found a security vulnerability in any part of Voikko please provide details by email to hatapitk@iki.fi before publicly disclosing the issue. I will respond to you as quickly as possible. If you have not received a reply in a week you may disclose the vulnerability publicly. In that case please also file a issue at https://github.com/voikko/corevoikko/issues .