Direct answer

Update the public Stable client and server to 42.20.4. The 26 August release is an emergency security hotfix, not a content update; official notes say 42.20.x saves should continue, but mods that used the removed loadstring or loadstream methods can fail until their authors update them.

Do this first

  1. Back up the world, server settings, and working mod list.
  2. Leave Steam Betas on None for Stable and confirm 42.20.4 in the main menu.
  3. Update a dedicated server before reopening joins and confirm 42.20.4 in its startup log.
  4. Test the save without mods if the menu, startup, or load process fails.
  5. Disable only the identified incompatible mod and wait for its author rather than restoring removed Lua methods.

Current branches after the security hotfix

The same-day patch moved all maintained branches. Match the branch to the world that created it; the version number alone does not convert saves.

  • None / default Stable: 42.20.4 for current play.
  • 42.19 beta: 42.19.2 for worlds created on the pre-Stable 42.19 line.
  • legacy41: 41.78.21 for Build 41 worlds.

What changed in 42.20.4

The public changelog identifies security fixes and removal of the Lua loadstring and loadstream methods. That can break Workshop items that compiled or executed strings dynamically. The patch does not add a town, animal, recipe tier, or new save-migration path.

  • Players: stay patched and test the affected mod on a disposable world.
  • Hosts: update the executable before clients reconnect.
  • Mod authors: replace removed calls with explicit commands and supported client methods.

Save failure versus mod failure

Official notes say the 42.20.4 hotfix should not break 42.20.x saves. If a known world stops loading, copy it first, confirm the exact branch, then try a fresh no-mod world and a backed-up copy without the Workshop preset. A world that loads cleanly without one mod is not evidence that the underlying save was converted or repaired.

Do not weaken the security boundary

Do not restore removed Lua methods, stay on an unpatched public build, or disable checksum protection as a blanket connection fix. Preserve logs and report a reproducible vanilla failure through official support; send a mod-only failure to the item author with the exact build and first relevant error.

FAQ

Quick answers

Is 42.20.4 the current Project Zomboid Stable version?

Yes. As of 29 August 2026, the official site lists Stable and IWBUMS at 42.20.4.

Will 42.20.4 delete a 42.20.3 save?

Official notes say the hotfix should not break saves on the 42.20.x line. Back up first and separate mod failures from save failures.

Why did mods break after 42.20.4?

Some mods depended on loadstring or loadstream, which were removed as part of the security fix. Those mods need an author update.

Evidence log

Sources checked

Official sources establish universal facts; forum and specialist sources are used for symptoms, vocabulary, and cross-checking.

Official42.20.4 Stable Security Hotfix

Official 26 August security hotfix for Stable 42.20.4, beta 42.19.2, and legacy41 41.78.21, including removal of loadstring and loadstream.

OfficialProject Zomboid Build 42.20 Released

Official Stable status, legacy41 instructions, and the explicit Build 41/42.19 save boundaries.

Community report42.20.4 UI Missing With Mods

Recent symptom evidence that a specific UI mod can remove the menu after the hotfix; not proof that every UI failure has the same cause.