Researchers at the RWTH Aachen University in Germany published a study revealing that tens of thousands of container images hosted on Docker Hub contain confidential secrets, exposing software, online platforms, and users to a massive attack surface.
This isn’t really surprising and isn’t actually a real security issue with Docker itself or any of the popular public images. Docker Hub is a public registry so people inexperienced with Docker accidentally include secrets in their images and upload it to Docker Hub, this is actually pretty well known and the Docker docs specifically warn people about this.
How can you be sure it doesn’t affect popular images? The probability may be lower, but I don’t think you can rule it out.
The most popular images on Docker Hub are official / library images, they are curated and monitored by Docker for best practices and security vulnerabilities. I’m not saying that means you should trust them completely, it’s always best practice to read the source of an image before you use it.
This doesn’t mean that YOUR secrets are exposed by using the image, btw - this means that whomever built that image would be accidentally exposing their secrets.
Unless you built the image and added your secrets to it and then uploaded it to a public Docker registry. But again, that’s not a flaw in Docker.