Troubleshooting broken image signatures in Roundcube with cPanel and CloudLinux

I recently investigated a common webmail issue: every rich email signature containing an image (JPG, PNG, WebP, uploaded directly or pasted via clipboard) resulted in a blank white box with a broken image icon upon saving in Roundcube.

On Linux servers running cPanel on CloudLinux, this symptom is rarely a frontend interface bug. The failure usually resides in intermediary layers: PHP image processing extensions, CageFS isolation restrictions, CloudLinux LVE memory ceilings, or SQLite database locks.

Here is the technical breakdown, terminal diagnostic checklist, and corrective steps to restore image signatures.

1. Why does Roundcube display a broken image box?#

A broken image placeholder indicates that Roundcube received the incoming payload (multipart data or Base64 string), but failed to process raw binary bytes or generate the internal inline preview.

The primary breakdown points include:

1. Missing or sandboxed PHP extensions (gd, imagick, fileinfo): Roundcube relies on GD or ImageMagick to sanitize and re-encode images. If CageFS lacks access to shared image libraries (libjpeg, libpng) or if fileinfo is disabled (preventing MIME type detection), image processing fails silently, generating a 0-byte buffer.
2. LVE or PHP memory exhaustion: Re-encoding images requires transient RAM spikes. If memory_limit is set too low or if the user reaches their CloudLinux PMEM threshold, the kernel terminates the worker before saving completes.
3. Payload truncation and storage quotas: Base64 encoding introduces a ~33% data overhead. If post_max_size or upload_max_filesize are constrained, or if the user's SQLite database is locked in read-only mode due to disk quotas, data is truncated.

2. Terminal diagnostic workflow#

To inspect each subsystem directly from the CLI:

Step 1: Inspect Roundcube error logs#

cPanel's Roundcube installation is centrally hosted and writes execution exceptions to /usr/local/cpanel/base/roundcube/logs/errors.log (as /home/USERNAME/logs/ only captures domain vhost logs):

tail -f /usr/local/cpanel/base/roundcube/logs/errors.log

Check for image manipulation errors or database constraint exceptions.

Step 2: Check CloudLinux LVE memory faults#

Verify whether CloudLinux terminated the PHP process during upload handling:

lveinfo --user USERNAME --period 10m

If the PMemF (Physical Memory Faults) column shows non-zero counts, the user's RAM allocation is insufficient.

Step 3: Audit active PHP extensions in system PHP#

Roundcube on cPanel executes under the system's default internal PHP runtime (EasyApache 4 / System Default), not under the user domain's PHP version (CloudLinux alt-php / PHP Selector). Validate modules directly against the system PHP binary:

/usr/local/bin/php -m | grep -E "gd|imagick|fileinfo"

Both fileinfo and gd (or imagick) in the system PHP runtime are mandatory for rich signature processing.

Step 4: Check cpsrvd and ModSecurity logs#

Because cPanel webmail is served via cpsrvd (ports 2095/2096) rather than Apache, connection aborts, timeouts, or WAF triggers appear in the cpsrvd log:

tail -f /usr/local/cpanel/logs/error_log | grep USERNAME

3. Corrective actions#

After isolating the bottlenecks, apply the following adjustments:

A. Adjust system PHP runtime directives#

In cPanel's MultiPHP INI Editor (selecting System Default or editing the system php.ini), expand limits to accommodate conversion overhead — since webmail runs under the system PHP rather than the user's per-domain runtime:

• upload_max_filesize = 32M
• post_max_size = 32M
• memory_limit = 256M

B. CageFS maintenance and temp directories#

Ensure the user's caged environment has updated library visibility:

# Update skeleton and remount the user sandbox
cagefsctl --update && cagefsctl --remount USERNAME

Verify that Roundcube's temporary workspace (/var/cpanel/roundcube/temp) has write permissions.

C. Quota validation and SQLite database repair#

On modern cPanel deployments, Roundcube stores SQLite databases inside the user's /home directory. If the account exceeds 100% of its allocated disk quota, SQLite locks into read-only mode, blocking signature payload saves:

# Check user disk quota
quota -v USERNAME

# Repair and update Roundcube database
/usr/local/cpanel/bin/update-roundcube-db --user USERNAME

Practical isolation and diagnostic takeaways#

A reliable diagnostic strategy: test plain text before testing images.

If saving a plain text signature works without error, you immediately rule out database corruptions and base account permission issues. The root cause is strictly confined to the binary image processing pipeline (GD/Fileinfo modules, POST limits, and CageFS mount permissions).

After enabling gd and fileinfo, remounting CageFS, and adjusting PHP memory ceilings, image signature saving returns to normal operation without broken placeholders.

Conclusion#

In this article, I shared the workflow I used to isolate the issue: testing plain text before images, validating system PHP extensions, checking CloudLinux LVE faults, and raising runtime limits. With gd and fileinfo active in the system PHP runtime and the CageFS sandbox refreshed, image signatures resumed saving normally.

See also

Email Roundcube on cPanel failing with "DB error: [14] unable to open database file

Email Deep diagnosis of request timeout in cPanel webmail with Roundcube, cpsrvd, and SQLite

Email Anatomy of a cascading failure: from spam IP blocks to Roundcube debugging in cPanel

This post is licensed under CC BY-NC.