A popular Bitcoin discussion forum warned on Monday some user passwords may have been intercepted after the site's DNS (Domain Name System) registrar was breached.
An attacker found a flaw in Bitcointalk.org's domain name registrar, a company based in Japan called Anonymous Speech, wrote an administrator for the forum who goes by the nickname "Theymos." Anonymous Speech could not immediately be reached for comment.
Tampering with a DNS server is a powerful attack. A hacker can direct traffic destined for one website to one that he or she controls, even if a person types in the correct domain name in the URL address bar of a browser.
The attacker could then execute a man-in-the-middle attack on Bitcointalk.org, intercepting encrypted forum submissions, passwords sent during login sessions, authentication cookies, private messages and more, Theymos wrote.
The attack occurred between 06:00 UTC on Sunday and 20:00 UTC Monday. People who logged into the site during this period should change their passwords, Theymos wrote.
Since many people tend to re-use passwords, hackers usually try to see if the captured login credentials are valid on other services.
People who had the "Remember me" login feature enabled should not be at risk, Theymos wrote. Security codes used to let people login without entering their passwords again have now been invalidated.
A Bitcointalk.org user noticed the change and "immediately transferred Bitcointalk.org to a different registrar," Theymos wrote. But DNS changes can take up to a day to be updated on servers around the world.
In the meantime, Theymos wrote that users want to make sure they're communicating with the right server. Users can add this IP address to their hosts file: "184.108.40.206 bitcointalk.org" and remove it later when the updated settings have been distributed through the DNS.
Bitcointalk.org's TLS certificate -- which allows users to cryptographically verify they're talking to the correct server -- has a SHA1 fingerprint of "29:0E:CC:82:2B:3C:CE:0A:73:94:35:A0:26:15:EC:D3:EB:1F:46:6B," Theymos wrote.
Additionally, Bitcointalk.org was targeted by a DDoS (distributed denial-of-service attack), Theymos wrote. "These two events are probably related, though I'm not yet sure why an attacker would do both of these things at once."
Bitcointalk.org was defaced in early October and taken offline after hackers apparently found a flaw in the Simple Machines forum software the site uses.
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