Linux & UE 5.6 - Unable to open fab from within engine

TheDarkTrav3l3r · June 11, 2025, 4:12pm

Summary

Unable to use Fab from within UE 5.6

What type of bug are you experiencing?

Plugins

URL where the bug was encountered

Steps to Reproduce

Open Unreal Engine
Click Fab

(also seems to apply to MH Creator)

Expected Result

I would see Fab within UE

Observed Result

Error in screen shot and below

From command line output:

[2025.06.11-16.00.22:098][820]LogFab: Display: Logging in using persist
[2025.06.11-16.00.22:098][820]LogEOSSDK: Warning: LogEOSAuth: No existing persistent auth credentials were found for automatic login.
[2025.06.11-16.00.22:114][820]LogFab: Error: Login failed - error code: EOS_InvalidAuth
[2025.06.11-16.00.22:114][820]LogFab: Display: Logging in using exchange code
[2025.06.11-16.00.22:114][820]LogFab: Display: Reading exchange code from commandline
[2025.06.11-16.00.22:114][820]LogEOSSDK: Error: LogEOSAuth: Invalid parameter EOS_Auth_Credentials.Token reason: must not be null or empty
[2025.06.11-16.00.22:136][821]LogFab: Error: Login failed - error code: EOS_InvalidParameters

Platform

UE 5.6

Operating System

Linux, Arch, Brave

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True_poser · June 14, 2025, 9:10pm

I have the same outcome (ERR_NAME_NOT_RESOLVED when trying to open Fab window in-editor) with slightly different log entries.

LogFab: Display: Logging in using persist
LogWebBrowser: Loaded CEF3 version 128.4.13.3057 from
LogCEFBrowser: CEF GPU acceleration enabled
LogEOSSDK: LogEOSAuth: UpdateUserAuthToken: User ClientId: <deleted_by_me> AccountId: <deleted_by_me> Access[Expires: 2025.06.14-20.47.04 Remaining: 7200.36] Refresh[Expires: 2025-12-11T18:30:35.037Z Remaining: 15551011.04] State: Valid
LogEOSSDK: LogEOSAuth: UserAuthGenerated: Received FUserAuthToken
LogEOSSDK: LogEOSAuth: UserAuthGenerated: Login complete: EOS_Success
LogFab: Display: User logged in
LogFab: Display: User client id: [deleted_by_me]
LogEOSSDK: Warning: LogEOS: Error response received from backend. ServiceName=[Friend], OperationName=[GetBlockList], Url=[], HttpStatus=[403], ErrorCode=[errors.com.epicgames.common.insufficient_scopes], NumericErrorCode=[1056], ErrorMessage=[Insufficient access scopes. Expected: [friends_list]], CorrId=<deleted_by_me>

UE version: 5.6, downloaded from https://www.unrealengine.com/en-US/linux
Fab plugin was downloaded from the same location.
Linux: CachyOS current.

Thank you.

TheDarkTrav3l3r · June 15, 2025, 4:41pm

I compiled from GH so it is universal

TheDarkTrav3l3r · June 16, 2025, 5:19pm

I’ve delved a little deeper. It doesn’t appear to be a permissions issue or at least nothing from Engine/Binaries/Linux/EpicWebHelper

Running a packet capture just says the connection is happening. Still the same result.

I have confirmed it is the same thing when you go into Quixel and try to login.

Really wish I could use UE.

TheDarkTrav3l3r · July 2, 2025, 12:33pm

Unreal Tech - Anything here? I have done everything I can do on my end.

ApellusMagnus · July 11, 2025, 1:30pm

This was a tough one–quite the rabbit hole. I wish I understood the cause better, but I did find a workaround, eventually.

The error was occurring in both Fab and Bridge for me. It presented on my system in the same way. I tried both binary and compiled builds, as well as a main branch build. When the editor was run from my IDE, I got the additional error: ERROR:libc_interceptor.cc(109)] Cannot find getaddrinfo with dlsym. After some digging, I found that this file is housed in the CEF file libcef.so.

The workaround is incredibly simple: change line 16 of Engine/Source/ThirdParty/CEF3/CEF3.build.cs to bool bUseExperimentalVersion = false;

This will revert the used CEF version to the other installed CEF with UE, flag/commit number 4577, which works in my case. I believe this requires a compile of the engine, so this will only work in the case that you’re building from source.

It’s odd that using an older CEF worked. I suspected I would need a newer one, as my system stays rather up to date–perhaps an odd forward incompatibility issue related to glibc or something. Here is some system info perhaps relevant to this at the time of finding the workaround:

OS: Void Linux (rolling release, glibc, XFCE)
Kernel: 6.12.33_1
Architecture: x86_64
glibc Version: 2.41
UE5 Branch: ue5-main

I think a recompile/update of the used CEF may be needed for a full fix–I found that within UE, there is a whole CEF compiling script/environment: Engine/Source/ThirdParty/CEF3/Build. Over the past week, I explored compiling CEF and had poor results, however. I’ll spare you the gory details.

syscrusher · July 15, 2025, 8:25pm

Thanks for the workaround.

I’ve got UE 5.6 from the downloaded binary and (in a different directory tree) a full source build from GitHub.

I’ve built plugins locally for the prebuilt UE installation, but is there a way to use my source build to update just that one .so file in the binary engine installation?

syscrusher · July 15, 2025, 8:28pm

Incidentally, when I was researching this problem during the UE 5.6 preview period, I found that there are multiple discussions online about that CEF3 interaction with libc. It was such a mess that I eventually just filed a bug report to Epic and disabled the plugin here. For that reason, I really appreciate that you found a workaround.

TheDarkTrav3l3r · July 17, 2025, 10:00pm

I wanted to say thank you and confirm that this does work. I had to compile twice to get it to work but I am functional. Thank you.

syscrusher · August 12, 2025, 9:00pm

I notice that in UE 5.6.1 there is a mention of fixing CEF-related crashes, per UE-295044. I can’t find that issue on the bug tracker, so I don’t yet know details. I will test after I get 5.6.1 installed here.

JeppesS_1423 · September 8, 2025, 1:07pm

Can confirm this workaround works as of the 5.6.1-release branch when built from source.

HafManHafAmazin · January 19, 2026, 4:01pm

Worked for me aswell. But…
I can not type the ‘@’ symbol in the login screen of Bridge plugin (or Fab, but will be forwarded to the browser). I am not able to paste anything from outside the login window.

ben-mkiv · February 25, 2026, 6:27am

changing the keyboard layout of my desktop environment to US English helped in the past. (@ is on SHIFT + 2)

PancakeEngineer · March 3, 2026, 4:36am

I changed that boolean and rebuilt the editor from source, but when I open fab it’s on a black screen and in my console after fab crashes 12 times it crashes the whole editor.

PancakeEngineer · March 3, 2026, 4:38am

Or the CEF I should say that is crashing.

PancakeEngineer · March 4, 2026, 6:52pm

That was on Cachyos, now I have Nobara installed, first thing I did was download UE 5.6 from the Linux page, put it in a folder somewhere in my home folder, didn’t try fab plugin yet. But afterwards, I downloaded asset manager studio from the software manager thingy or whatever it’s called (flatpost I think) signed into my epic games account, it automatically recognized that I had UE 5.6 already installed in my home folder, went ahead and installed UE 5.7 from asset manager studio with the fab plugin setting checkbox ticked, created a project after it was done installing and the fab plugin does seem to work now. And on the new home page the web views/CEFs worked there too finally.