How to Fix Codex Sandbox Errors on Ubuntu 24.04

TL;DR: Codex sandbox errors on Ubuntu 24.04 almost always trace back to one thing on freshly installed boxes: AppArmor blocking bwrap. A five-line /etc/apparmor.d/bwrap profile fixed it on my system. If you’re hitting the same wall, paste your error into Claude Code and let it walk you through. 💪

The Symptom: Codex Sandbox Hangs on Ubuntu 24.04

My Codex setup on a fresh Ubuntu 24.04 VM was unusable. Every codex exec call burned 35K to 54K tokens over 2 or 3 minutes retrying fallback paths, then died. The MCP Codex tool connected in about 3 seconds but could not read local files or fetch URLs. My workaround was cat-ing every file into the prompt. Slow and goofy.

The actual error, once I ran the simplest possible repro:

$ bwrap --dev-bind / / --unshare-net echo ok
bwrap: loopback: Failed RTM_NEWADDR: Operation not permitted

If you see that line on your system, the AppArmor userns restriction is worth checking before anything else.

The Fix: Five Lines of AppArmor

Create /etc/apparmor.d/bwrap with this body:

abi <abi/4.0>,
include <tunables/global>

profile bwrap /usr/bin/bwrap flags=(unconfined) {
  userns,
  include if exists <local/bwrap>
}

Then load it:

sudo apparmor_parser -r /etc/apparmor.d/bwrap

That’s it. The profile shape is the same one Ubuntu ships for flatpak and chrome, scoped to /usr/bin/bwrap.

If you would rather not touch AppArmor at all, codex-cli 0.117.0 has a Landlock-based sandbox you can opt into: codex --enable use_legacy_landlock sandbox linux <cmd>. That uses the kernel’s Landlock LSM instead of bwrap. The catch: on codex-cli 0.117.0 this only applies to the codex sandbox linux subcommand, not to codex exec sandbox modes. For codex exec to work, you still want the AppArmor profile above.

Verify

$ bwrap --dev-bind / / --unshare-net echo ok
ok
$ sudo aa-status | grep bwrap
   bwrap

If the canary still fails, look at the kernel log for AppArmor events on bwrap. Ubuntu logs userns transitions as AUDIT events, not DENIED, so the grep needs to cover both:

sudo journalctl -k | grep -E 'apparmor.*(DENIED|userns_create)'

Why Ubuntu 24.04 Breaks Codex Sandboxes

Ubuntu 23.10+ sets kernel.apparmor_restrict_unprivileged_userns=1 by default. Bubblewrap needs unprivileged user namespaces to build its sandbox, and the restriction blocks unshare(CLONE_NEWUSER) unless a permitted AppArmor profile applies. The upstream AppArmor project keeps a bwrap-userns-restrict profile in its extras/ directory. Ubuntu’s apparmor package added then reverted then partially reworked it (the dropped step is documented in LP #2072811, where the bwrap restrict profile was removed after it broke Flatpak app saves), so a current 24.04 install has no bwrap profile out of the box. The five-line profile above is what fills that gap.

If you want tighter confinement than a compatibility stub, there is an upstream bwrap-userns-restrict profile that strips capability from bwrap‘s children via profile stacking. Heavier to install and maintain, so I’m leaving it for a later session.

One Gotcha After the Fix

After installing the profile, MCP Codex started reading my local files again. Good. Fetching URLs still failed inside sandbox=read-only with “Could not resolve host.” That was not a profile bug. The read-only preset on my Codex version uses bwrap --unshare-net, which isolates the sandbox from the network on purpose. Switching the call to sandbox=workspace-write made URL fetches work. Your Codex build may map the presets slightly differently, so try both before you blame the profile.

Bottom Line

The fix itself took 30 seconds. Getting to the right five lines was a few hours of Claude Code and Codex trading hypotheses: reading Ubuntu’s userns spec, pulling the upstream profile for comparison, flipping the sysctl as a diagnostic, and verifying each success criterion one at a time. Same CC and Codex loop I use for real work.

If you hit codex sandbox errors on your Ubuntu 24.04 box, hand the error to CC and let it run. You’ll learn the AppArmor model along the way. I hope the fix lands on yours as fast as it did on mine. 👍

Sources and Further Reading

• Ubuntu spec on unprivileged user namespace restrictions via AppArmor: Canonical’s rationale for the default restriction.
• Launchpad bug 2046844: AppArmor userns creation restrictions causing bwrap failures; where the community landed on the per-binary profile fix.
• Upstream bubblewrap issue #632: bwrap broke on Ubuntu 24.04 (tracking the same symptom).
• Upstream AppArmor bwrap-userns-restrict profile: the tighter two-profile design if you want more than a compatibility stub.

Drafted with Claude and Codex. Sources cited against primary docs where possible. If something here does not match what you are seeing, drop a comment and I will update the post.