Data injection attacks (DIAs) pose a significant cybersecurity threat to the Smart Grid by enabling an attacker to compromise the integrity of data acquisition and manipulate estimated states without triggering bad data detection procedures. To mitigate this vulnerability, the moving target defense (MTD) alters branch admittances to mismatch the system information that is available to an attacker, thereby inducing an imperfect DIA construction that results in degradation of attack performance. In this paper, we first analyze the existence of stealth attacks for the case in which the MTD strategy only changes the admittance of a single branch. Equipped with this initial insight, we then extend the results to the case in which multiple branches are protected by the MTD strategy. Remarkably, we show that stealth attacks can be constructed with information only about which branches are protected, without knowledge about the particular admittance value changes. Furthermore, we provide a sufficient protection condition for the MTD strategy via graph-theoretic tools that guarantee that the system is not vulnerable to DIAs. Numerical simulations are implemented on IEEE test systems to validate the obtained results.