Finite Transducers (FTs) extend the capabilities of Finite Au- tomata (FAs) by enabling the transformation of input strings into output strings. In many practical applications — includ- ing program analysis, string constraint solving, and analysis of security-critical sanitizers — Symbolic FTs (SFTs) and Sym- bolic FAs (SFAs) are used instead of the explicitly represented models. To circumvent to the notorious state-space explo- sion problem caused by an extremely large alphabet size (e.g. UTF-16), SFTs and SFAs allow the representation of the al- phabet as an effective boolean algebra including finite unions of intervals, as well as SMT-Algebras. The security-critical nature of many of these applications demands trustworthy implementations of such systems. To this end, we present the first formalization of SFTs and their most important algo- rithms in Isabelle/HOL. To evaluate the effectiveness of our formalization, we apply the formalized SFTs to two applica- tions: (1) HTMLdecode, a sanitizer used for preventing XSS attacks, and (2) string solving, which increasingly employs intricate string replacement operations. Our experimental results demonstrate that our methods are competitive with the existing unverified implementations.