TYPO3 is open source software as well as all TYPO3 extensions published in the TYPO3 Extension Repository (TER). This means, everyone can download and investigate the code base. From a security perspective, this usually improves the software, simply because more people review the code, not only a few core developers. Currently, there are hundreds of developers actively involved in the TYPO3 community and if someone discovers and reports a security issue, he/she will be honored by being credited in the appropriate security bulletin.
The open source concept also implies that everyone can compare the old version with the new version of the software after a vulnerability became public. This may give an insight to anyone who has programming knowledge, how to exploit the vulnerability and therefore it is understandable how important it is, to react quickly and fix the issue before someone else compromises it. In other words, it is not enough to receive and read the security bulletins, it is also essential to react as soon as possible and to update the software or deinstall the affected component.
The security bulletins may also include specific advice such as configuration changes or similar. Check your individual TYPO3 instance and follow these recommendations.