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It's about UID mappings between namespaces. When you are UID=0 in namespace X and manage to get out of namespace X, then you are still UID=0 outside X, so you're root.

It's possible to remap UIDs such that root in namespace X has UID=12340, and when root gets out of X, then he's nobody.

it's not quite as straightforward as just UID mapping. Assuming a standard install of Docker, the container processes only have a limited set of capabilities, have an AppArmor/SELinux profile applied and have a seccomp filter also applied, which makes it harder to break out the the underlying host.

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