Symfony version(s) affected

=5.1

Description

5.1 introduced the X-Debug-Exception header when in debug mode.
Because this header is not truncated and because web servers don't allow infinite header size (single-header / all headers limitations), the web server might return a native 5xx error page instead of proper symfony error page.

See #36003 (comment) first issue report (this is the initial PR, not an issue)

How to reproduce

Just throw a big exception, with debug mode enabled :

throw new \Exception(str_repeat('0', 32000));

In my case, it's a DB query with some JOIN and a lot of fields that fail for instance when a migration has not been executed yet.

Possible Solution

Truncate the header, for instance to 2kB to have some space left for other headers, because a web server might have a limitation for the size of all the headers, and not only per-header-line size limit.

Additional Context

Typical nginx log (502) :

upstream sent too big header while reading response header from upstream