The function check_is_login_page() uses headers for the IP check, which can be easily spoofed.
[
{
"vendor": "Unknown",
"product": "Login Block IPs",
"collectionURL": "https://wordpress.org/plugins",
"versions": [
{
"status": "affected",
"versionType": "custom",
"version": "0",
"lessThanOrEqual": "1.0.0"
}
],
"defaultStatus": "affected"
}
]