In case you're wondering, the exposed social security numbers belong to...wait for it...transactions involving 527 groups.

National Journal:

Every so often, 527s have to file tax forms to the IRS, which then get added to a database. The database itself is hardly a secret; the IRS has been sending updated records routinely to Public.Resource.org and other public-interest groups, and it's a favorite among political reporters. But when the IRS told the group's founder, Carl Malamud, to disregard the Form 990-Ts included in the agency's January release, he took a closer look at the files in question.

After analyzing the breach, Malamud wrote a letter to the IRS pointing out 10 instances where a social security number was accidentally revealed on the government's website--just a small sample of the larger breach. 

Just the day before, Malamud had filed another letter to the agency describing a problem with the 990-Ts. Of over 3,000 tax returns contained in the January update, 319 contained sensitive data the agency should have scrubbed, Malamud wrote in the July 1 report that he filed to the inspector general's office. In that mixup, some 2,319 social security numbers--perhaps more--were revealed.

To determine the extent of the exposure, we've analyzed our logs and have also analyzed the data received from the IRS. We maintain a privacy registry based on any clicks made on the privacy cover sheet on the top of each return. That registry indicates that 8 clicks were made from 4 unique IP addresses. However, none of those resulted in privacy complaints and could have been made by an automated process.

In addition, we examined our FTP and HTTP logs. We only maintain a 7-day window for HTTP logs and did not see any HTTP-based access that was not from a search engine crawler. For the FTP logs (which indicates bulk download activity), we did not see extensive activity for the January directory, but it was clear that at least one copy of the DVD ISO image (the image of the original DVD) had been transferred.

Public.Resource.org took down its copy of the compromised 990-Ts and replaced them with a clean version that the IRS had sent. But it was another day before the IRS removed the files from public view on their end, on July 3.

Public Resource.org called the IRS miscue "unprofessional and amateur." Gee, ya think? No doubt identity theives had a field day and a lot of Americans are going to get some unpleasant surprises as a result of this fiasco.

Yeah...I know. More political chicanery from our friends at the IRS? It's possible, but far more likely to be simple unprofessionalism and amateurish actions. Why posit malevolence when incompetence is more likely?