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 insecure HTTP connections.

Since Authelia uses multi-domain cookies to perform single sign-on, an attacker who poisoned 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.

By default the SMTP Notifier implementation does not allow connections that are not secure. As such all connections require the following:

1. TLS Connection (STARTTLS or SMTPS) has been negotiated before authentication or sending emails (unauthenticated connections require it as well)
2. Valid X509 Certificate presented to the client during the TLS handshake

There is an option to disable both of these security measures however they are not recommended. You should only do this in a situation where you control all networks between Authelia and the SMTP server. The following configuration options exist to configure the security level:

By default all connections start as plain text and are upgraded via STARTTLS. SMTPS is supported, however due to the fact it was basically considered deprecated before the turn of the century, there is no way to configure it. It happens automatically when a SMTP notifier is configured with the SMTPS port of 465.

This is a YAML boolean type (true/false, y/n, 1/0, etc). This disables the X509 PKI verification mechanism. We recommend using the trusted_cert option over this, as disabling this security feature makes you vulnerable to MITM attacks.

This is a YAML boolean type (true/false, y/n, 1/0, etc). This disables the requirement that all connections must be over TLS. This is only usable currently with authentication disabled (comment the password) and as such is only an option for SMTP servers that allow unauthenticated relay (bad practice).

This is a YAML string type. This specifies the file location of a pub certificate that can be used to validate the authenticity of a server with a self signed certificate. This can either be the public cert of the certificate authority used to sign the certificate or the public key itself. They must be in the PEM format. The certificate is added in addition to the certificates trusted by the host machine. If the certificate is invalid, inaccessible, or is otherwise not configured; Authelia just uses the hosts certificates.

There are a few reasons for the security measures implemented:

1. Transmitting username's and passwords over plain-text is an obvious vulnerability
2. The emails generated by Authelia, if transmitted in plain-text could allow an attacker to intercept a link used to setup 2FA; which reduces security
3. Not validating the identity of the server allows man-in-the-middle attacks

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.