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ethyca-fides has a DOM-based XSS vulnerability in fides.js via fides_description override

High severity GitHub Reviewed Published May 14, 2026 in ethyca/fides • Updated May 14, 2026

Package

pip ethyca-fides (pip)

Affected versions

>= 2.33.0, < 2.84.5

Patched versions

2.84.5

Description

Summary

fides.js is the script that renders Fides's consent banner on customer websites. It lets the embedding page override the banner's description text at runtime via a URL query parameter, a JavaScript global, or a cookie. On sites that have opted into HTML-formatted descriptions, the overridden value is rendered as live HTML without passing through the server-side sanitiser the rendering path was designed to trust.

The result is a DOM-based XSS that any visitor can trigger with a crafted link, no authentication required. The cookie source lets the payload persist, so a single click can plant a payload that fires on every subsequent banner render across all subdomains until cookies are cleared.

Am I affected?

This vulnerability affects Fides Enterprise deployments that use fides.js for consent management, a.k.a. "Janus", and have HTML-formatted banner descriptions enabled (the FIDES_PRIVACY_CENTER__ALLOW_HTML_DESCRIPTION Privacy Center container env var set to true ). Deployments that do not load fides.js, or that leave HTML descriptions off (the default), are not affected. The vulnerable code lives in the open-source ethyca-fides PyPI package, which is why this is published as a GHSA and CVE against Fides OSS, but only Fides Enterprise consent management platform deployments exercise the affected path.

To self-test, visit any page where your site loads the consent banner, replacing <your-site>:

https://<your-site>/?fides_override_language=en&fides_description=<img src=x onerror="alert(`DOM XSS in fides_description. Origin: ${document.domain}`)">

If a browser alert() appears showing the embedding origin when the banner renders, the deployment is affected. If the banner does not auto-open, click the manage privacy preferences link to display it.

dom-xss-localhost

If no alert appears, the deployment is not affected via this path.

The fides_override_language=en parameter is required because experience-translation overrides only apply when the override language matches the active locale. Adjust to match the active locale on the target site if en is not it.

Details

The vulnerability is a trust gap between two features that are safe in isolation but unsafe together.

The first feature is the override mechanism. fides.js lets the embedding page replace certain banner fields at runtime by reading them from the URL, a JavaScript global, or a cookie on the page origin. The description text is one of the overrideable fields.

The second feature is HTML-formatted descriptions. When the allowHTMLDescription server-controlled flag is on, the rendering path writes the description into the DOM as live HTML rather than plain text. This mode relies on a server-side sanitiser (nh3) to clean any HTML before it reaches the client, so the rendering sink can trust its input.

Combining the two breaks the assumption. The override mechanism feeds the description directly from a client-controlled source, never reaching the server, so the sanitiser the rendering path was designed to trust never runs. On a deployment that has both features active, an attacker-supplied value travels straight from URL or cookie to the HTML sink and executes as script.

The cookie source compounds the impact. Cookies set by JavaScript on the apex domain are visible to every subdomain on subsequent loads, so a payload delivered via URL can write a fides_description cookie that fires on every later banner render across the entire site, until the cookie is cleared.

Impact

A successful exploit gives the attacker arbitrary JavaScript execution in the embedding site's origin, with the same authority as the site's own scripts. The payload can read and modify any data the page can access, issue requests on behalf of the visitor, and render content that appears to come from the site itself. The actual consequences on a given deployment depend on what the embedding origin holds and what other defences (CSP, cookie flags, origin isolation) are in place.

The cookie-based persistence variant raises the impact ceiling, since a single click can convert into a payload that fires on every subsequent banner render across the apex and all subdomains until the visitor clears their cookies.

Patches

This vulnerability has been patched in Fides OSS version 2.84.5, specifically ethyca/fides-privacy-center:2.84.5 Users are advised to upgrade to this version or later to secure their systems against these threats.

Fides Enterprise (fidesplus) version 2.84.6 contains the same patch.

Workarounds

Set FIDES_PRIVACY_CENTER__ALLOW_HTML_DESCRIPTION=false on the Privacy Center container(s) and redeploy. The XSS sink relies on the flag being on, so turning it off makes fides.js strip HTML from the description before rendering, regardless of which source the value came from.

Severity

This vulnerability has been assigned a severity of HIGH with a CVSS v4 vector of CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:N/VI:N/VA:N/SC:H/SI:H/SA:N (7.0, High).

The score reflects the worst realistic exploitation path: an attacker who can plant a fides_description cookie on the target origin gets persistent script execution that fires on every visitor's banner render, with no further interaction required. Cookie-write capability is not always available to a remote attacker, and the score accounts for that as a precondition, but when it is available the impact is script execution in the embedding site's origin every time the banner or modal renders.

The more common delivery path (a crafted link the victim clicks) scores 6.3, Medium, with a vector of CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:A/VC:N/VI:N/VA:N/SC:H/SI:H/SA:N. Per the FIRST CVSS 4.0 User Guide, library-level vulnerabilities should be scored against the "reasonable worst-case scenario", since the analyst cannot anticipate every deployment context. We publish the higher score on that basis.

The published score is the base severity of the vulnerability itself. Real-world impact on a given deployment can be higher or lower depending on the embedding site's own defences. Sites with HttpOnly session cookies, strict Content-Security-Policy, and authentication flows isolated from the consent experience origin reduce the surface a payload can reach. Sites with permissive CSP, JavaScript-readable session tokens, or same-origin OIDC/social-login flows expose more. Operators should assess the score against their own environment.

References

@daveqnet daveqnet published to ethyca/fides May 14, 2026
Published to the GitHub Advisory Database May 14, 2026
Reviewed May 14, 2026
Last updated May 14, 2026

Severity

High

CVSS overall score

This score calculates overall vulnerability severity from 0 to 10 and is based on the Common Vulnerability Scoring System (CVSS).
/ 10

CVSS v4 base metrics

Exploitability Metrics
Attack Vector Network
Attack Complexity Low
Attack Requirements Present
Privileges Required None
User interaction None
Vulnerable System Impact Metrics
Confidentiality None
Integrity None
Availability None
Subsequent System Impact Metrics
Confidentiality High
Integrity High
Availability None

CVSS v4 base metrics

Exploitability Metrics
Attack Vector: This metric reflects the context by which vulnerability exploitation is possible. This metric value (and consequently the resulting severity) will be larger the more remote (logically, and physically) an attacker can be in order to exploit the vulnerable system. The assumption is that the number of potential attackers for a vulnerability that could be exploited from across a network is larger than the number of potential attackers that could exploit a vulnerability requiring physical access to a device, and therefore warrants a greater severity.
Attack Complexity: This metric captures measurable actions that must be taken by the attacker to actively evade or circumvent existing built-in security-enhancing conditions in order to obtain a working exploit. These are conditions whose primary purpose is to increase security and/or increase exploit engineering complexity. A vulnerability exploitable without a target-specific variable has a lower complexity than a vulnerability that would require non-trivial customization. This metric is meant to capture security mechanisms utilized by the vulnerable system.
Attack Requirements: This metric captures the prerequisite deployment and execution conditions or variables of the vulnerable system that enable the attack. These differ from security-enhancing techniques/technologies (ref Attack Complexity) as the primary purpose of these conditions is not to explicitly mitigate attacks, but rather, emerge naturally as a consequence of the deployment and execution of the vulnerable system.
Privileges Required: This metric describes the level of privileges an attacker must possess prior to successfully exploiting the vulnerability. The method by which the attacker obtains privileged credentials prior to the attack (e.g., free trial accounts), is outside the scope of this metric. Generally, self-service provisioned accounts do not constitute a privilege requirement if the attacker can grant themselves privileges as part of the attack.
User interaction: This metric captures the requirement for a human user, other than the attacker, to participate in the successful compromise of the vulnerable system. This metric determines whether the vulnerability can be exploited solely at the will of the attacker, or whether a separate user (or user-initiated process) must participate in some manner.
Vulnerable System Impact Metrics
Confidentiality: This metric measures the impact to the confidentiality of the information managed by the VULNERABLE SYSTEM due to a successfully exploited vulnerability. Confidentiality refers to limiting information access and disclosure to only authorized users, as well as preventing access by, or disclosure to, unauthorized ones.
Integrity: This metric measures the impact to integrity of a successfully exploited vulnerability. Integrity refers to the trustworthiness and veracity of information. Integrity of the VULNERABLE SYSTEM is impacted when an attacker makes unauthorized modification of system data. Integrity is also impacted when a system user can repudiate critical actions taken in the context of the system (e.g. due to insufficient logging).
Availability: This metric measures the impact to the availability of the VULNERABLE SYSTEM resulting from a successfully exploited vulnerability. While the Confidentiality and Integrity impact metrics apply to the loss of confidentiality or integrity of data (e.g., information, files) used by the system, this metric refers to the loss of availability of the impacted system itself, such as a networked service (e.g., web, database, email). Since availability refers to the accessibility of information resources, attacks that consume network bandwidth, processor cycles, or disk space all impact the availability of a system.
Subsequent System Impact Metrics
Confidentiality: This metric measures the impact to the confidentiality of the information managed by the SUBSEQUENT SYSTEM due to a successfully exploited vulnerability. Confidentiality refers to limiting information access and disclosure to only authorized users, as well as preventing access by, or disclosure to, unauthorized ones.
Integrity: This metric measures the impact to integrity of a successfully exploited vulnerability. Integrity refers to the trustworthiness and veracity of information. Integrity of the SUBSEQUENT SYSTEM is impacted when an attacker makes unauthorized modification of system data. Integrity is also impacted when a system user can repudiate critical actions taken in the context of the system (e.g. due to insufficient logging).
Availability: This metric measures the impact to the availability of the SUBSEQUENT SYSTEM resulting from a successfully exploited vulnerability. While the Confidentiality and Integrity impact metrics apply to the loss of confidentiality or integrity of data (e.g., information, files) used by the system, this metric refers to the loss of availability of the impacted system itself, such as a networked service (e.g., web, database, email). Since availability refers to the accessibility of information resources, attacks that consume network bandwidth, processor cycles, or disk space all impact the availability of a system.
CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:N/VI:N/VA:N/SC:H/SI:H/SA:N

EPSS score

Weaknesses

Improper Neutralization of Input During Web Page Generation ('Cross-site Scripting')

The product does not neutralize or incorrectly neutralizes user-controllable input before it is placed in output that is used as a web page that is served to other users. Learn more on MITRE.

CVE ID

CVE-2026-44541

GHSA ID

GHSA-5qrq-9645-g5g2

Source code

Credits

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