This is a zero day flaw, and was recently reported to Tails folks by Exodus Intelligence. Following is a short excerpt from Exodus blog:
We publicized the fact that we’ve discovered these issues for a very
simple reason: no user should put full trust into any particular
security solution. By bringing to light the fact that we have found
verifiable flaws in such a widely trusted piece of code, we hope to
remind the Tails user base that no software is infallible. Even when
the issues we’ve found are fixed by the Tails team, the community
should keep in mind that there are most certainly other flaws still
present and likely known to others.
My view is that, it is best to refrain from using Tails until dust settles. Be aware that this flaw existed for very long time and maybe used by adversaries for ages, so cover yourself if you can. COMPUTERWORLD have an interesting article on this subject. http://blogs.computerworld.com/malware-and-vulnerabilities/24177/zero-day-broker-exploits-vulnerability-i2p-de-anonymize-tails-users
It estimates that 30000 users of I2P are unmasked.
The zero-day is in the Invisible Internet Project, or I2P, networking
component that comes bundled with Tails to encrypt web traffic and
hide a user’s real IP address. The 30,000 I2P users who previously
felt anonymous could be unmasked, their true IP address revealed, by
visiting a booby-trapped website.
Your best option at this point is to use Tor browser bundle, But be careful, more revelations will going to be uncovered.
P.S. There are other issues beside this zero day flaw too.
On July 4 2014 we found a group of relays that we assume were trying to de-anonymize users. They appear to have been targeting people who operate or access Tor hidden services. The attack involved modifying Tor protocol headers to do traffic confirmation attacks.
Read Tor security advisory for detail.