SSL connection failed (...) routines::certificate verify failed

Support
1

System

  • OS: Ubuntu 25.10
  • Kernel: Linux 6.17.0-22-generic
  • Syncthing version: 2.0.15
  • Syncthing build: 2026-02-11

What’s happening?

I run sudo apt update: everything ok, syncthing is upgradable. When I run sudo apt upgrade, when it tries to update Syncthing I get the following error:

Ign:1 https://apt.syncthing.dev syncthing/stable-v2 amd64 syncthing amd64 2.0.16
Ign:1 https://apt.syncthing.dev syncthing/stable-v2 amd64 syncthing amd64 2.0.16
Ign:1 https://apt.syncthing.dev syncthing/stable-v2 amd64 syncthing amd64 2.0.16
Err:1 https://apt.syncthing.dev syncthing/stable-v2 amd64 syncthing amd64 2.0.16
SSL connection failed: error:0A000086:SSL routines::certificate verify failed / Success \[IP: 104.21.78.103 443\]
Error: Failed to fetch https://apt.syncthing.dev/pool/syncthing_2.0.16_amd64.deb  SSL connection failed: error:0A000086:SSL routines::certificate verify failed / Success \[IP: 104.21.78.103 443\]

Terminal.txt

What have I tried already?

I already made sure ca-certificates is installed (it is) because of what is mentioned in the guide. I already removed everything Syncthing-related from /etc/apt (sources and keyrings) and re-added them following the guide. Rebooted.

2

Are you by any chance on a corporate network with a so-called “security appliance” between you and the Internet? These are known to filter out any traffic related to the term Syncthing, even the APT repo.

Can you access the same URL from a browser on that same machine? Check what it says about the TLS certificate for the connection.

3

I’ve seen reports now and then that sound like there are just some CDN nodes that are bad sometimes (this is cloudflare). That’s the only explanation I can see other than what Andre is suggesting.