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Stored XSS via Photo Description in RSS/Atom/JSON Feed (No Sanitization on Public Endpoint)

Moderate
ildyria published GHSA-5574-7f3r-hm9j Mar 23, 2026

Package

composer lychee-org/lychee (Composer)

Affected versions

<= 7.5.2

Patched versions

> 7.5.2

Description

Vulnerability Details

CWE-79: Stored Cross-Site Scripting (XSS) via RSS/Atom/JSON Feed

The photo description field is stored without HTML sanitization and rendered using {!! $item->summary !!} (Blade unescaped output) in the RSS, Atom, and JSON feed templates. The /feed endpoint is publicly accessible without authentication, allowing any RSS reader to execute attacker-controlled JavaScript.

Data Flow

  1. Source: EditPhotoRequest.php — DescriptionRule allows any string up to 1000 chars with no HTML sanitization
  2. Storage: PhotoController.php:154 — $photo->description = $request->description() (stored as-is)
  3. RSS Generation: Actions/RSS/Generate.php:57 — 'summary' => $data->description ?? ''
  4. Sink (unescaped):
    • resources/views/vendor/feed/rss.blade.php:24: {!! $item->summary !!}
    • resources/views/vendor/feed/atom.blade.php:36: {!! $item->summary !!}
    • resources/views/vendor/feed/json.blade.php:14-15: {!! str_replace('"', '\\"', $item->summary) !!}

Vulnerable Code

resources/views/vendor/feed/rss.blade.php:24:

<description><![CDATA[...{!! $item->summary !!}]]></description>

The {!! Blade syntax explicitly outputs unescaped HTML. Combined with CDATA, any HTML/JavaScript in the description is rendered by RSS readers.

Steps to Reproduce

  1. Log in as any authenticated user
  2. Upload a photo to a public album
  3. Edit the photo description via PATCH /api/v2/Photo:
{
  "photo_id": "TARGET_PHOTO_ID",
  "description": "<img src=x onerror=alert(document.domain)>",
  "title": "test", "tags": [], "license": "none",
  "upload_date": "2026-03-23", "taken_at": null, "from_id": null
}
  1. Visit /feed (no authentication required) — the XSS payload is in the RSS <description> field

Verified Output (Docker PoC)

Tested on Lychee v7.5.2 (lycheeorg/lychee:v7.5.2):

<description><![CDATA[<img src="..."/><br/><br/><img src=x onerror=alert(document.domain)>]]></description>

Impact

  • Attacker: Any authenticated user who can upload/edit photos
  • Victim: Anyone consuming the public RSS feed (RSS readers, aggregators, browsers)
  • Impact: Session theft, credential harvesting, phishing via RSS readers that render HTML content
  • The /feed endpoint requires no authentication, so the XSS payload is served to all feed consumers

CVSS v3.1: 5.4 Medium (AV:N/AC:L/PR:L/UI:R/S:C/C:L/I:L/A:N)

Proposed Fix

Escape the summary in the feed templates:

// Before (vulnerable)
{!! $item->summary !!}

// After (safe)
{{ $item->summary }}

Or sanitize HTML in DescriptionRule / Generate.php before output.

Severity

Moderate

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 Low
User interaction Passive
Vulnerable System Impact Metrics
Confidentiality Low
Integrity Low
Availability None
Subsequent System Impact Metrics
Confidentiality High
Integrity Low
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:L/UI:P/VC:L/VI:L/VA:N/SC:H/SI:L/SA:N

CVE ID

CVE-2026-33738

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.

Credits