Prohibiting the use of identical keys in different cryptographic algorithms and protocols is a prerequisite for the security of a wide variety of information systems. However, developers of such systems sometimes ignore this requirement since underestimate the threat. This paper addresses practical attack scenarios against electronic document management systems in a situation where the signature key is also used for client authentication in the TLS 1.2 protocol. As a result of one of the attacks, an adversary forms a signature for a selected PDF file up to 16 MB in size, which is correctly displayed by a number of popular applications. An analysis of the reasons for the feasibility of these attacks leads to the conclusion that a property exists in the TLS 1.2 protocol that leads to a vulnerability when the client authentication key is used as a signature key in electronic document management systems.