GitHub Security Advisory: GHSA-gmfg-3v4q-9qr4
Package: github.com/authelia/authelia/v4 (GO)
Severity: LOW (CVSS 0.0)
CWE: CWE-79: Improper Neutralization of Input During Web Page Generation ('Cross-site Scripting')
Vulnerable versions: = 4.39.15
Patched in: 4.39.16

Summary: Authelia: Improper Neutralization of Input During Web Page Generation Leads to Potential Cross-site Scripting

Details:
### Impact

**Official Weighted Severity Rating:** Low

This exploit is very unlikely to be the case for most users as it requires configuration of the Content Security Policy template value. Below represents a safe value, **_any other value_** other than unconfigured should be **_very carefully evaluated_** regardless of the fix.

```yaml
server:
  headers:
    csp_template: ''
```

```env
AUTHELIA_SERVER_HEADERS_CSP_TEMPLATE=
```

Provided the following conditions are met:

1. The Content Security Policy:
    1. Has been disabled or modified from the entirely safe default value; and
    2. Has been completely disabled by the Administrator by omitting the header explicitly at the proxy (worst practice); or
    3. Has been effectively disabled by modifying `script-src` allowing unsafe inline scripts rather than using hashes AND effectively disabled by modifying `connect-src` allowing connections to arbitrary websites.
2. Authelia is being hosted on a domain that has other applications that can write to the cookie for the Authelia domain.
3. One of the other applications noted in 2 has an vulnerability that can be exploited to execute malicious javascript with similar requirements to 1.
4. The attacker can exploit the javascript in 3 to delete the existing `language` cookie scoped to the fully qualified domain name of Authelia with the same site value of `strict` (which is not possible in most scenarios unless the application in 3 has the exact same domain or a subdomain of the Authelia domain).
5. The attacker can exploit the javascript in 3 to write a new `language` cookie scoped for a domain that Authelia is sent cookies for.
6. The attacker can get a user to meet the conditions required to execute the javascript in 3.
7. You are running Authelia 4.39.15.

An attacker may potentially be able to inject javascript into the Authelia login page. Unless both the `script-src` and `connect-src` directives have been modified it's almost impossible for this to have a meaningful impact. However if both of these are and they are done so without consideration to their potential impact; there is a are situations where this vulnerability could be exploited.

This is caused to the lack of neutralization of the `langauge` cookie value when rendering the HTML template.

This vulnerability is likely difficult to discover though fingerprinting due to the way Authelia is designed but it should not be considered impossible. The additional requirement to identify the secondary application is however likely to be significantly harder to identify along side this, but also likely easier to fingerprint.

### Patches

Upgrade to 4.39.16 or Downgrade to 4.39.14.

### Proof of Concept

No current proof of concept exists that does not require manual manipulation of the browser which is effectively a local attack where all privileges have been compromised without the need for this attack vector (i.e. installation of userscripts or a browser plugin which would be able to comp