From Bruce Schneier:
Major newspapers do their best to verify the authenticity of leaked documents they receive from sources. They only publish the ones they know are authentic. The newspapers consult experts, and pay attention to forensics. They have tense conversations with governments, trying to get them to verify secret documents they’re not actually allowed to admit even exist. This is only possible because the news outlets have ongoing relationships with the governments, and they care that they get it right. There are lots of instances where neither of these two things are true, and lots of ways to leak documents without any independent verification at all.
No one is talking about this, but everyone needs to be alert to the possibility. Sooner or later, the hackers who steal an organization’s data are going to make changes in them before they release them. If these forgeries aren’t questioned, the situations of those being hacked could be made worse, or erroneous conclusions could be drawn from the documents. When someone says that a document they have been accused of writing is forged, their arguments at least should be heard.
As someone who routinely receives, and consults on, leaked documents I can emphatically say this is a serious issue. And that journalists are generally very cautious these days about publishing based on mysteriously sourced documents.