The alleged plan, published on the Taliban's website late Sunday, features a satellite map of the venue where the event will take place, mobile phone numbers of top security officials and details of security force deployments.
But Afghan government officials and the international military say it is a fake. The assembly, or loya jirga, is due to start in Kabul on Wednesday and will feature around 2,000 elders discussing Afghanistan's long-term relationship with the United States plus efforts to make peace in the warring country.
The Taliban have already threatened to target the meeting, and struck at Afghanistan's last loya jirga in 2010.
"The vigilant mujahideen of the Islamic emirate (the Taliban's name for itself) have acquired the security plans, maps and other documents related to the upcoming supposed loya jirga," the militant group said in a statement sent to media.
The Taliban claim the documents were seized "by means of its personnel embedded inside the enemy ranks."
But Afghanistan's interior ministry dismissed the report as a "big lie".
"This claim is false and is part of Taliban psychological war aimed at disrupting the loya jirga," the ministry said in a statement.
"It's a big lie," the ministry's spokesman Sediq Sediqqi told AFP. The NATO-led International Security Assistance Force (ISAF) said via its official Twitter account that the document "doesn't appear authentic".
"If this doc (document) were real, why in the world would you let us know you have it?" it added in response to a comment from an apparent Taliban supporter.