fix(auth): avoid SameSite CookieError on Python < 3.8 (login 500)#32
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On Python 3.6/3.7 the XSRF cookie's 'samesite' attribute raises
http.cookies.CookieError ("Invalid attribute 'samesite'") — samesite was only
added to http.cookies.Morsel in Python 3.8 — which 500s the login page.
Only include samesite='Lax' in xsrf_cookie_kwargs when the interpreter supports
it (detected via http.cookies.Morsel._reserved). On older Python the cookie is
set without SameSite (the double-submit XSRF token protection still applies);
on 3.8+ behaviour is unchanged. Extracted as build_xsrf_cookie_kwargs() with
unit tests for both branches.
Note: the project's documented minimum is Python 3.9+, but this avoids a hard
500 on older interpreters.
Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
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Symptom
On the user's server the login page fails with:
Root cause
The XSRF cookie is configured with
samesite: 'Lax'(inxsrf_cookie_kwargs). Thesamesiteattribute was only added tohttp.cookies.Morselin Python 3.8. On Python 3.6/3.7, Tornado'sset_cookie→morsel['samesite']raisesCookieError, which 500s every response that sets the XSRF cookie — including login.(The server in question runs Python 3.6; the project's documented minimum is 3.9+, but a hard 500 is a poor failure mode.)
Fix
Only include
samesite='Lax'when the interpreter supports it, detected viahttp.cookies.Morsel._reserved. On older Python the XSRF cookie is set without SameSite — the double-submit XSRF token protection still applies, so CSRF protection is preserved; the SameSite defense-in-depth is simply dropped where the runtime can't express it. On 3.8+ behaviour is unchanged.Extracted as
build_xsrf_cookie_kwargs(cookie_secure, samesite_supported=None)so both branches are unit-tested.Testing
pytest src/tests/— 1748 passed (4 new tests covering supported/unsupported/securepass-through/auto-detection).CookieError; SameSite still applied on 3.8+ (no regression).🤖 Generated with Claude Code