On the 3rd of October 2025, the latest patch for Cities: Skylines II (version 1.3.5f1) went live. This is a free patch which is released to everyone who owns the game. The patch is live now on Steam. It will be live on Microsoft Game Pass as soon as possible.
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This patch addresses a security vulnerability within the Unity engine used by Cities: Skylines II (and many other games) that was made public yesterday
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This does not affect Cities: Skylines (2015) as the vulnerability is only present in more recent versions of the Unity engine
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Although there are no changes to the underlying code of the game that mods can interact with, mods which specifically target the previous version number of the game (v1.3.3f1) may display errors or warnings
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Although modders have been given access to the update to re-build their mods to target the latest version of the game, due to the urgent nature of security vulnerabilities the patch was published as soon as possible
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Please be patient. Everyone, even Cities: Skylines mod creators, has a personal life and might not be able to update a game mod immediately at launch. Do not bug creators to update their mod.
Please read the links provided by this thread carefully, and completely, to understand what you need to do to get your game working again.
🚨 How to keep yourself safe
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Unity state that there is "no evidence of any exploitation of the vulnerability nor has there been any impact on users or customers"
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Regardless, you should update your game as soon as the patch is made available to you via Steam or Microsoft Game Pass so that you receive the protection against this vulnerability
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The vulnerability can only be exploited if the game is running, so if you're currently playing you should close the game immediately and not re-open it until the update is applied
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You should not attempt to work-around or circumvent Steam or Microsoft's automatic updates to "keep playing with mods" as you will be exposing yourself to this vulnerability if you do so
Seriously, just update your game