Jacob Appelbaum is an independent computer security researcher and hacker. He is currently employed by the University of Washington, and is a core member of the Tor project. Appelbaum is known for representing Wikileaks at the 2010 Hope conference. He has subsequently been repeatedly targeted by US law enforcement agencies, who obtained a court order for his Twitter account data, detained him 12 times at the US border after trips abroad, and seized a laptop and several mobile phones.

Appelbaum, under the handle "ioerror", has been an active member of the Cult of the Dead Cow hacker collective since 2008, and is the co-founder of the San Francisco hackerspace Noisebridge with Mitch Altman. He has worked for Greenpeace and has volunteered for the Ruckus Society and the Rainforest Action Network. He is also a photographer and ambassador for the art group monochrom.

In 2005, Appelbaum gave two talks at the 22nd Chaos Communication Congress,Personal Experiences: Bringing Technology and New Media to Disaster Areas, and A Discussion About Modern Disk Encryption Systems. The former covered his travels to Iraq - crossing the border by foot, the installing of Internet satellites in Kurdistan, and his visit of New Orleans post-Hurricane Katrina. The latter talk discussed the legal and technical aspects of full disk encryption. At the 2006 23rd Chaos Communication Congress, he gave a talk with Ralf-Philipp Weinmann titled Unlocking FileVault: An Analysis of Apple's Encrypted Disk Storage System. The duo subsequently released the VileFault free software program which broke Apple's FileVault security.