Discourse session cookies (400 Request Header Or Cookie Too Large)

Why does Discourse have such large cookies? Is there a reason for it? Is there a way to decrease their size?

I was poking around in the Discourse admin dashboard, and I suddenly got a 400 Bad Request error back from /sidekiq/retries

400 Bad Request
Request Header Or Cookie Too Large

I checked the request and, sure enough, my browser sent a huge Cookie header to my Discourse server. Why?

Cookie: _t=403b8203003bdaf522679e0b6c17605f; rack.session=BAh7C0kiD3Nlc3Npb25faWQGOgZFVG86HVJhY2s6OlNlc3Npb246OlNlc3Np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%0AYTI1NTUGOwBG%0A--e602081dddcd88bdb269034e7acb8c582665be0e; _forum_session=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--325ca2e886afaefffeb6174c99776f91fefd4292

The above Discourse cookie string is 1,962 characters long!

For comparison, the cookie header sent to my Mediawiki site is 122 characters long, including my username, user id, and session id.

The Mediawiki session id is 32 characters long, but Discourse appears to have two session ids: rack.session is 813 characters long and _forum_session is 1,075 characters long. Please help me to understand why a simple uid needs to be so long. Can I configure Discourse to use a shorter session uid? Does such a request make sense?

What is stored in that string? Is it possible to make it smaller?

Specifically, what component of the Discourse software is rack.session used for? And what is _forum_session used for?

Of course I can just bump up the nginx config limits, but I’d like to keep them reasonably low unless there’s a strong requirement to increase them.


I support auditing our session cookies and maybe just moving to default use our redis backed session that auto expires which is better anyway. @david any thoughts here?


This shouldn’t be a problem for someone using our official install, just for people using badly configured proxies right?

Also HTTP/2 header compression makes for this headers to be uploaded just once in the entire user visit.


Depends, plugins that use session heavily can be impacted, its also a hygiene thing, nicer to keep all this secret info on the server instead of encrypted on the client.


Yikes, this tuning was done intentionally for security; it’s not a “badly configured” proxy. Tuning down the following nginx directives is commonly done as part of hardening nginx (to address availability, rate limiting, DOS, etc):

add_header # SAMEORIGIN/XSS-Protection/CORS/CSP

The settings we use in our nginx config work fine on all our existing sites. The problem here appears to be that Discourse stores unnecessary client data in the cookie, where imho the only thing stored in the cookie should be just a few unique identifiers that the server can use to access that data server-side.

Thanks Sam. By “moving to default use redis” are you suggesting that it’s currently possible to move the data stored in the session cookie to redis with a configuration change? Or would moving this data out of the session cookie to the server necessarily require a code change?

Nope, requires a code change

Why is 10k worth of headers a problem? They are compressed, if 10k of headers is a problem why is 10k html payload allowed?


Note: I’ve also had to override my hardened nginx config’s large_client_header_buffers directive to get Discourse working.

Specifically, I use large_client_header_buffers 2 1k for all my other sites’ nginx configs. But this causes a 414 Request-URI Too Large error from /admin/reports/bulk?XYZ – where XYZ is actually 1,019 characters long!

This was fixed by setting large_client_header_buffers 4 8k; in my Discourse nginx config’s server{} block, which overrides the global directive and restores the directive to its default value for Discourse

For better interoperability of Discourse installs through popular hardened web servers, network firewalls, and web application firewalls, I urge the Discourse developers to consider using POST for such long query strings.

That is a very very narrow use case… specific to admin.