CVE-2026-52846
Description
A flaw was found in Caddy, an extensible server platform. The stripHTML template function, intended to remove HTML tags from input, fails to reliably process certain malformed HTML. This oversight allows dangerous content to remain in the output, which, if subsequently rendered as HTML, can lead to client-side Cross-Site Scripting (XSS). A remote attacker could exploit this by providing specially crafted input, potentially executing malicious scripts in a user's browser.
Statement
Caddy's stripHTML template function fails to reliably strip malformed HTML, which can lead to client-side XSS if untrusted input is rendered through Caddy's templates directive. Exploitation requires the templates directive to be enabled in a Caddyfile with a template that calls stripHTML on untrusted input; Caddy deployments that only use file_server for static asset serving never reach the vulnerable function. All investigated Red Hat Insights/console.redhat.com frontend services build and deploy through a shared Caddyfile-generation pipeline (insights-frontend-builder-common) or an equivalent static-file-only configuration that does not enable templates, and are therefore not affected. The general-purpose caddy package shipped by Fedora/EPEL may be configured with templates by downstream users and is marked affected.
Mitigation
Users who have enabled Caddy's `templates` directive with untrusted input passed through the stripHTML function should upgrade to Caddy 2.11.4 or later, or disable the `templates` directive if not required.
Common Vulnerability Scoring System (CVSS) Score Details
Info alert:Important note
CVSS scores for open source components depend on vendor-specific factors (e.g. version or build chain). Therefore, Red Hat's score and impact rating can be different from NVD and other vendors. Red Hat remains the authoritative CVE Naming Authority (CNA) source for its products and services (see Red Hat classifications).
The following CVSS metrics and score provided are preliminary and subject to review.
CVSS v3 Score Breakdown
| Red Hat | NVD | cve.org | |
|---|---|---|---|
| Base Score | 4.2 | N/A | 4.2 |
| Attack Vector | Network | N/A | Network |
| Attack Complexity | High | N/A | High |
| Privileges Required | None | N/A | None |
| User Interaction | Required | N/A | Required |
| Scope | Unchanged | N/A | Unchanged |
| Confidentiality | Low | N/A | Low |
| Integrity Impact | Low | N/A | Low |
| Availability Impact | None | N/A | None |
Vector
Red Hat: CVSS:3.1/AV:N/AC:H/PR:N/UI:R/S:U/C:L/I:L/A:N
cve.org: CVSS:3.1/AV:N/AC:H/PR:N/UI:R/S:U/C:L/I:L/A:N
Understanding the Weakness (CWE)
Access Control,Confidentiality
Technical Impact: Bypass Protection Mechanism; Read Application Data
The most common attack performed with cross-site scripting involves the disclosure of private information stored in user cookies, such as session information. Typically, a malicious user will craft a client-side script, which -- when parsed by a web browser -- performs some activity on behalf of the victim to an attacker-controlled system (such as sending all site cookies to a given E-mail address). This could be especially dangerous to the site if the victim has administrator privileges to manage that site. This script will be loaded and run by each user visiting the web site. Since the site requesting to run the script has access to the cookies in question, the malicious script does also.
Integrity,Confidentiality,Availability
Technical Impact: Execute Unauthorized Code or Commands
In some circumstances it may be possible to run arbitrary code on a victim's computer when cross-site scripting is combined with other flaws, for example, "drive-by hacking."
Confidentiality,Integrity,Availability,Access Control
Technical Impact: Execute Unauthorized Code or Commands; Bypass Protection Mechanism; Read Application Data
The consequence of an XSS attack is the same regardless of whether it is stored or reflected. The difference is in how the payload arrives at the server. XSS can cause a variety of problems for the end user that range in severity from an annoyance to complete account compromise. Some cross-site scripting vulnerabilities can be exploited to manipulate or steal cookies, create requests that can be mistaken for those of a valid user, compromise confidential information, or execute malicious code on the end user systems for a variety of nefarious purposes. Other damaging attacks include the disclosure of end user files, installation of Trojan horse programs, redirecting the user to some other page or site, running "Active X" controls (under Microsoft Internet Explorer) from sites that a user perceives as trustworthy, and modifying presentation of content.
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