Privacy Violation: BREACH

The application may leak sensitive data contained in HTTP responses transferred over an SSL/TLS-enabled channel.

The Browser Reconnaissance and Exfiltration via Adaptive Compression of Hypertext (BREACH) attack is a type of side-channel attack that allows attackers to steal sensitive information contained in HTTP responses transferred over an SSL/TLS-enabled channel. All existing SSL/TLS versions and ciphers are susceptible to this attack. If the following three conditions are met, an attacker may successfully conduct a BREACH attack against the application:

1. The application transmits a secret (for example, an anti-CSRF token) in the response.
2. The application reflects a user input in the same response containing a secret.
3. The web server is configured to use HTTP compression.

The size of a response compressed using gzip is determined by the amount of repetitions observed within its content. Thus when characters are repeated more often, the smaller the response size becomes. This allows an attacker to use the HTTP compression as an oracle which can indicate whether a guess supplied by an attacker to the application through the reflected parameter value is correct or not. If the guessed character matches the corresponding character in the secret, the size of the compressed response is smaller. The attacker may use this process to obtain the entire secret character-by-character through the analysis of the compression ratio for each request.

An attacker may use this technique to extract a secret from the HTTP responses even if the application is configured to serve content over a secure channel.

To evaluate whether the application is vulnerable to the BREACH attack, first review whether the application is configured to enable HTTP compression. If so, for each response containing a secret to be protected, evaluate whether a user input is included inside it.

This issue is reported because the Django application is configured to use CSRF tokens and GZip compression:

MIDDLEWARE = (
  ...
  'django.middleware.csrf.CsrfViewMiddleware',
  'django.middleware.gzip.GZipMiddleware',
  ...
)