4. of Repudiation

An attacker can alter digital signatures because the digital signature system you’re implementing is weak or uses MACs where it should use a signature.

Threat
	Your system uses shared keys to generate message authentication codes (MACs). These shared keys mean that both the sender and the receiver have the same key and, therefore, you cannot rely on this as a means to identify the source of the message. This poses a number of issues. As both you (the sender) and the receiver need the key, you will need a secure channel on which to share the key. You will also need to have a key for every recipient of the message and create a separate signature for each of them.
CAPEC	CAPEC-151 - Identity Spoofing CAPEC-195 - Principal Spoof CAPEC-194 - Fake the Source of Data
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