A major security risk with co-mingling of hosts with different security requirements is:

When systems with different security classifications share the same environment, a compromise or misconfiguration of a lower-security host can expose higher-security systems, directly violating the security policies designed to protect them. Segmentation into separate security zones is the standard architectural control used to prevent this cross-contamination. Co-mingling undermines the foundational principle of least privilege applied to network architecture.

Zombie attacks involve botnet-controlled systems and are a separate threat vector unrelated to the structural risk of co-mingling hosts of different security levels.

Password compromise is a credential-focused threat, not a structural risk specifically caused by mixing hosts with different security requirements on the same segment.

Privilege creep refers to the gradual accumulation of excessive user rights over time, which is an access management issue rather than a network segmentation concern.