Description:
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The php-fpm systemd service, php-fpm.service, should use be hardened as much as possible against potential attacks. Besides reducing the likelihood of an attack, if php does get compromised, there will be less damage possible.

I suggest these features be added to the systemd units:
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ProtectHome=true
PrivateTmp=true
ProtectSystem=full
NoNewPrivileges=true
PrivateDevices=true
# Required for dropping privileges and running as a different user
CapabilityBoundingSet=CAP_SETGID CAP_SETUID
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I tested these settings and didn't experience any problems in my (admitted limited) setup. I think they should be fine for anyone except for exceptional and odd situations (ex, php-fpm is setup to read files from /tmp that another services writes to /tmp). For the (very rare) impacted user, they can always override the systemd service - but a secure configuration should be the default.

Test script:
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n/a

Expected result:
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n/a

Actual result:
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n/a