In the present E-healthcare industry, data breaches result in substantial economic losses due to cyber-attacks and hence create a trust deficit between the industry and users. The healthcare industry has rapidly adopted IoT frameworks but the trust deficit and privacy concerns limit its utilization among the masses. Along with privacy protection, content authentication is an important requirement in a number of critical applications and fragile watermarking provides an effective solution. However, existing fragile watermarking techniques lack the accuracy of tamper detection and hence are not reliable enough in terms of security and privacy of the data. This paper presents a novel low-complexity block-based fragile watermarking technique with high security against cyber-security attacks. This is achieved by embedding a fragile watermark in the host image using pixel domain blocking approach. The security of embedded watermark has been taken care of by using Cellular Automata and DNA based ENcryption (CADEN) framework to scramble the watermark bits using various secret keys. Experimental investigations show that besides being highly secure, the proposed technique is fragile to various signal processing and geometric attacks. The comparative analysis shows that the proposed scheme, despite having lower complexity, offers better efficiency in terms of imperceptibility, tamper detection and localization compared to other state-of-the-art techniques. Besides, the fragile watermark embedding makes the system capable to preserve the secret information in case of an attack with an average BER of 40%.