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I am running a LEMP server (Ubuntu 20.04 server) with fast cgi enabled, and am getting the following error message in the logs for one of my virtual hosts (one of my websites). Here is the full error with my website and IP address redacted:

[error] 2041#2041: *38 FastCGI sent in stderr: "PHP message: PHP Warning:  hash_equals(): Expected known_string to be a string, null given in /var/www/example.com/wp-includes/pluggable.php on line 2508" while reading response header from upstream, client: 123.456.789.123, server: example.com, request: "POST /wp-login.php HTTP/2.0", upstream: "fastcgi://unix:/var/run/php/php7.4-fpm.sock:", host: "example.com", referrer: "https://example.com/wp-login.php"

Could someone please explain to me this error and how I might go about fixing this?

DanRan
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1 Answers1

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The message has all the information you need:

hash_equals(): Expected known_string to be a string, null given

This tells that hash_equals() function is expecting known_string to be a string, however null value is given.

/var/www/example.com/wp-includes/pluggable.php on line 2508

This tells which file contains the error and on what line it is in the file.

You need to look into the code in that line and see why the function receives ǹull` as the argument.

If you cannot determine the reason yourself, you can ask in StackOverflow about it.

Tero Kilkanen
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