# Terminal sandbox fails on first start in 2.5.26: "loopback: failed to look up lo interface"

**URL:** <https://forum.cursor.com/t/terminal-sandbox-fails-on-first-start-in-2-5-26-loopback-failed-to-look-up-lo-interface/153013>\
**Category:** Bug Reports\
**Tags:** sandbox, terminal, linux\
**Created:** [February 26, 2026, 10:08am UTC](https://forum.cursor.com/t/terminal-sandbox-fails-on-first-start-in-2-5-26-loopback-failed-to-look-up-lo-interface/153013 "2026-02-26T10:08:00Z")\
**Posts on this page:** 3\
**Page:** 1

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**Author:** ![Jesko\_Schwarzer](https://avatars.discourse-cdn.com/v4/letter/j/e8c25b/32.png) [@Jesko\_Schwarzer](https://forum.cursor.com/u/Jesko_Schwarzer)\
**Post date:** [February 26, 2026, 10:08am UTC](https://forum.cursor.com/t/terminal-sandbox-fails-on-first-start-in-2-5-26-loopback-failed-to-look-up-lo-interface/153013/1 "2026-02-26T10:08:00Z")

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### Where does the bug appear (feature/product)?

Cursor IDE

### Describe the Bug

Body:

## SummaryAfter upgrading to Cursor 2.5.26, the terminal sandbox fails to start. The error appears on first launch. Previous versions worked without this issue.## Environment- **Cursor version:** 2.5.26- **OS:** Ubuntu (Linux)- **Kernel:** 6.17.0-14-generic- **Environment:** VMware guest- **Installation:** System package (deb) at /usr/share/cursor## ErrorThe sandbox preflight fails with:

Error: Failed to apply sandbox: IO error: Step 2.5/7 (loopback setup) failed: IO error: loopback: failed to look up lo interface

## Log locationFull error in: `~/.config/Cursor/logs/*/window*/exthost/exthost.log`Example log lines:

[shell-exec:sandbox] [isSandboxHelperSupported] Preflight failed: Command failed: /usr/share/cursor/resources/app/resources/helpers/cursorsandbox … --preflight-only – /bin/true  
Error: Failed to apply sandbox: IO error: Step 2.5/7 (loopback setup) failed: IO error: loopback: failed to look up lo interface

## Additional context- The default loopback interface (lo) exists and is UP in the host namespace.- The renderer reports `sandboxSupported=false` after the preflight fails, so Cursor falls back to non-sandbox mode.- The user sees the “terminal sandbox cannot be started” message during startup.- This started in 2.5.26; earlier versions did not show this error.- User namespaces: `kernel.apparmor_restrict_unprivileged_userns = 1`## Expected behaviorThe sandbox should initialize successfully, as it did in previous versions.## Reproduction1. Upgrade to Cursor 2.5.262. Start Cursor3. Observe “terminal sandbox cannot be started” and the preflight failure in the logs

### Steps to Reproduce

Start cursor 2.5.26 in ubuntu LTS 24.04 in VMWare Workstation 17.0

### Expected Behavior

Start without error

### Operating System

Linux

### Version Information

Version: 2.5.26  
VSCode Version: 1.105.1  
Commit: 7d96c2a03bb088ad367615e9da1a3fe20fbbc6a0  
Date: 2026-02-26T04:57:56.825Z  
Build Type: Stable  
Release Track: Default  
Electron: 39.4.0  
Chromium: 142.0.7444.265  
Node.js: 22.22.0  
V8: 14.2.231.22-electron.0  
OS: Linux x64 6.17.0-14-generic

### Does this stop you from using Cursor

Yes - Cursor is unusable

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<div class="post-metadata">

**Author:** ![deanrie](https://sea3.discourse-cdn.com/cursor1/user_avatar/forum.cursor.com/deanrie/32/1263_2.png) [@deanrie](https://forum.cursor.com/u/deanrie)\
**Post date:** [February 26, 2026, 10:30am UTC](https://forum.cursor.com/t/terminal-sandbox-fails-on-first-start-in-2-5-26-loopback-failed-to-look-up-lo-interface/153013/3 "2026-02-26T10:30:29Z")

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Hey, thanks for the detailed report. This is a known class of issues with the sandbox and AppArmor on newer Linux kernels, especially when `apparmor_restrict_unprivileged_userns = 1`.

The loopback lookup failure at step 2.5/7 is likely happening because the sandbox can’t set up a network namespace under your current AppArmor restrictions. A few things to try:

1. Create an AppArmor profile for Cursor at `/etc/apparmor.d/cursor-system`:

```plaintext
abi <abi/4.0>,
include <tunables/global>
profile cursor-system "/usr/share/cursor/cursor" flags=(unconfined) {
  userns,
  include if exists <local/cursor>
}

```

Then load it:

```bash
sudo apparmor_parser -r /etc/apparmor.d/cursor-system

```

1. Check for and remove any stale sandbox profile:

```bash
sudo aa-status | grep cursor

```

If `cursor_sandbox` shows up:

```bash
sudo apparmor_parser -R /etc/apparmor.d/cursor_sandbox
sudo rm /etc/apparmor.d/cursor_sandbox
sudo systemctl restart apparmor

```

1. Set the setuid bit on the sandbox binary:

```bash
sudo chmod 4755 /usr/share/cursor/resources/app/resources/helpers/cursorsandbox

```

Restart Cursor after all steps.

This has worked for several other Linux users with similar setups. More context here:

- [`cursor-sandbox` binary missing setuid bit breaks extensions](https://forum.cursor.com/t/cursor-sandbox-binary-missing-setuid-bit-breaks-extensions/147166)
- [Terminal Sandbox Issue Linux](https://forum.cursor.com/t/terminal-sandbox-issue-linux/152979)

I flagged the specific loopback error with the team. It’s a new variant we haven’t seen before in VMware environments. Let me know if the above helps.

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<div class="post-metadata">

**Author:** ![system](https://sea3.discourse-cdn.com/cursor1/user_avatar/forum.cursor.com/system/32/99069_2.png) [@system](https://forum.cursor.com/u/system)\
**Post date:** [March 20, 2026, 10:31am UTC](https://forum.cursor.com/t/terminal-sandbox-fails-on-first-start-in-2-5-26-loopback-failed-to-look-up-lo-interface/153013/5 "2026-03-20T10:31:57Z")

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