noun
illegal character
Computing term for a character that is not allowed in a given context, such as in a filename, password, or data field.
ファイル名に不正文字が含まれていると保存できません。
If the filename contains an illegal character, it cannot be saved.
パスワードに不正文字を使うとエラーになります。
Using an illegal character in the password will cause an error.
禁則文字 refers specifically to characters prohibited by typographic or formatting rules (e.g., line-start punctuation), while 不正文字 is a broader computing term for any disallowed character.
Compound of 不正 (illegal, improper) and 文字 (character). A straightforward modern technical term.