Authelia uses two mechanisms to protect against cookie theft:

1. session attribute httpOnly set to true make client-side code unable to read the cookie.
2. session attribute secure ensure the cookie will never be sent over an unsecure HTTP connections.

Since Authelia uses multi-domain cookies to perform single sign-on, an attacker who poisonned a user's DNS cache can easily retrieve the user's cookies by making the user send a request to one of the attacker's IPs.

To mitigate this risk, it's advisable to only use HTTPS connections with valid certificates and enforce it with HTTP Strict Transport Security (HSTS) so that the attacker must also require the certificate to retrieve the cookies.

Note that using HSTS has consequences. That's why you should read the blog post nginx has written on HSTS.

You can also apply the following headers to your nginx configuration for improving security. Please read the documentation of those headers before applying them blindly.

# We don't want any credentials / TOTP secret key / QR code to be cached by
# the client
add_header Cache-Control "no-store";
add_header Pragma "no-cache";

# Clickjacking / XSS protection

# We don't want Authelia's login page to be rendered within a <frame>, 
# <iframe> or <object> from an external website.
add_header X-Frame-Options "SAMEORIGIN";

# Block pages from loading when they detect reflected XSS attacks.
add_header X-XSS-Protection "1; mode=block";

If you find possible vulnerabilities or threats, do not hesitate to contribute either by writing a test case demonstrating the possible attack and if possible some solutions to prevent it or submit a PR.