Question nginx cache "Cache requests with cookies" generates broken regex — all whitelisted cookies cause BYPASS

[dima]

Server operating system version

Almalinux 10.1

Plesk version and microupdate number

Plesk Obsidian 18.0.77.1

Environment​

• Plesk Obsidian 18.0.77.1 (AlmaLinux 10.1)
• nginx 1.28.3
• WordPress site with nginx caching enabled (Web Server Settings → Smart Static Files Processing / nginx cache)

Problem​

In the domain's Apache & nginx settings → "nginx caching" → "Cache requests with cookies" I added common analytics cookies (_ga, _gid, _gat, _fbp, _fbc, _ga_N68HH2MJ1V, PHPSESSID, plesk_technical_domain) so that requests carrying only these cookies would still be served from cache.

However, every request that contains any cookie — even a cookie that is in the whitelist — still returns x-cache-status: BYPASS. Only requests with no Cookieheader at all are cached (HIT).

Test results​

# No cookies
$ curl -sI https://example.tld/ | grep x-cache
x-cache-status: HIT

# Cookie that IS in the whitelist
$ curl -sI -b "_ga=test" https://example.tld/ | grep x-cache
x-cache-status: BYPASS

# Another whitelisted cookie
$ curl -sI -b "PHPSESSID=test" https://example.tld/ | grep x-cache
x-cache-status: BYPASS

Root cause (debug)​

I added two temporary debug headers to the generated vhost config:

add_header X-Debug-Cookie "$cache_cookie" always;
add_header X-Debug-NoCache "$no_cache" always;

Result for Cookie: _ga=test:

x-debug-cookie: _ga=test
x-debug-nocache: 1

So $cache_cookie is not stripped — none of the if ($cache_cookie ~ "...") blocks match, and $no_cache is set to 1, causing bypass.

The generated regexes look correct but don't match​

The Plesk-generated config at /etc/nginx/plesk.conf.d/vhosts/<domain>.conf contains:

set $cache_cookie $http_cookie;
if ($cache_cookie ~ "(.*)(?:^|\\s*_ga=[^;]+(?:$|(.*)") {
set $cache_cookie $1$2;
}
if ($cache_cookie ~ "(.*)(?:^|\\s*PHPSESSID=[^;]+(?:$|(.*)") {
set $cache_cookie $1$2;
}
...
if ($cache_cookie !~ "^\s*$") {
set $no_cache 1;
}

That regex matches _ga=test perfectly in PCRE/Python:

>>> import re
>>> re.match(r"(.*)(?:^|\s*_ga=[^;]+(?:$|(.*)", "_ga=test").groups()
('', '')

But in nginx the if block never triggers. I verified by temporarily replacing one of the blocks with a simple if ($cache_cookie ~* "_ga") { set $cache_cookie "STRIPPED"; } — that one does fire, so the if mechanism itself works. Only the Plesk-generated complex regex does not match.

Suspected issue​

The $ inside (?:$| in a double-quoted nginx string may be consumed by nginx's variable interpolation (since $ followed by a non-identifier falls into an unclear/version-dependent behavior), resulting in the compiled regex being different from what is written.

Also noted​

Entering .* as a cookie name in the Plesk UI is escaped to \.\* in the generated config, producing:

if ($cache_cookie ~ "(.*)(?:^|\\s*\\.\\*=[^;]+(?:$|(.*)") { ... }

— i.e. it looks for a cookie literally named .*. So there is no way through the UI to whitelist all cookies.

Question​

1. Is this a known bug with the cookie-stripping regex in the generated nginx vhost config?
2. Is there a supported way to effectively disable cookie-based cache bypass (so that only specific cookies like wordpress_logged_in bypass the cache)?
3. Can the "Cache requests with cookies" field be changed to accept a wildcard / regex pattern, or output a working regex?

Any workaround through Additional nginx directives or vhost_nginx.conf that persists across Plesk config regeneration would also be appreciated.

Thanks.

 

[Kaspar]

I doubt there is a bug, as the cache functionality hasn't changed much for the past few years. Besides, I am unable replicate the behavior you're seeing. For me, on my test server, the cache status is always hit when whitelisting cookies (and actually sending those along with the request).

What you could do, to bypass cookies (or some cookies) when using nginx caching, is to use your own custom virtual hosts template(s) to create modified nginx configuration. That way you can bypass the Plesk default caching behavior.

Hope this helps.

 

[dima]

For me, on my test server, the cache status is always hit when whitelisting cookies (and actually sending those along with the request).

@Kaspar - you are talking about HTML page cache, not a CSS/JS/image, right?
Your cache is HIT for HTML pages, and you are using latest version Wordpress?

 

[Kaspar]

@Kaspar - you are talking about HTML page cache, not a CSS/JS/image, right?
Your cache is HIT for HTML pages, and you are using latest version Wordpress?

I tested this on an empty PHP page.

 

[dima]

I created a custom templates - now it works better.