Security issues of text watermarking are greatly different from those of other multimedia watermarking, in terms of its specific requirements and characteristics of text watermarking. The security theory of text watermarking is proposed in this paper, and the following security topics are discussed: (i) the classification and application of text watermarking; (ii) the classification and analysis of attacks; (iii) the watermarking model and security countermeasures. Other open issues and further challenges related to text watermarking are also addressed.