CVE-2026-13601: Yelp Information Disclosure Vulnerability

CVE-2026-13601 Overview

CVE-2026-13601 is an information disclosure vulnerability in GNOME Yelp caused by an overly permissive Content Security Policy (CSP) shipped through yelp-xsl. A malicious Flatpak application can invoke the OpenURI portal to open crafted help content in Yelp. By embedding an untrusted CSS stylesheet inside a structured SVG document, an attacker bypasses Flatpak sandbox isolation. Yelp then evaluates local XML inclusions and exfiltrates arbitrary user-readable host files through remote CSS resource requests. The weakness is tracked under [CWE-693: Protection Mechanism Failure].

Critical Impact
A sandboxed Flatpak application can escape isolation and read arbitrary user files from the host, resulting in unauthorized disclosure of sensitive data.

Affected Products

GNOME Yelp help viewer
yelp-xsl stylesheets shipping the permissive CSP
Linux distributions bundling Yelp with Flatpak and the OpenURI portal (see Red Hat CVE-2026-13601 Advisory)

Discovery Timeline

2026-06-29 - CVE-2026-13601 published to NVD
2026-06-30 - Last updated in NVD database
Vendor coordination and fix details are tracked in GNOME Yelp Work Item #238 and the GNOME Yelp Commit Details.

Technical Details for CVE-2026-13601

Vulnerability Analysis

Yelp renders help documents using XSLT transformations provided by yelp-xsl. The stylesheets set a Content Security Policy that is too permissive, allowing external resource loads from rendered content. When Yelp opens an attacker-supplied document, the loose CSP lets embedded CSS and SVG references reach out to attacker-controlled hosts. This turns Yelp into a confused deputy that operates outside the Flatpak sandbox and reads local files on behalf of the malicious application. The vulnerability chains a CSP weakness, XML inclusion evaluation, and CSS resource fetches to achieve file disclosure.

Root Cause

The root cause is a Protection Mechanism Failure [CWE-693]. The CSP delivered through yelp-xsl does not restrict external stylesheet or resource origins tightly enough to block untrusted documents from initiating network fetches. Combined with Yelp's willingness to process XML inclusions in content passed through the OpenURI portal, the trust boundary between a Flatpak sandbox and the host user session collapses.

Attack Vector

A local, sandboxed Flatpak application initiates the chain by calling the OpenURI portal with a crafted help document. Yelp, running outside the sandbox, loads the document and processes an embedded SVG that references an attacker-controlled CSS stylesheet. The stylesheet triggers XML inclusion of local files that the invoking user can read, then leaks their contents through URL parameters on remote CSS resource requests. Additional detail is available in the GNOME Blog on Flatpak Escape and Red Hat Bug Report #2494110.

No verified public exploit code is available. The vulnerability mechanism is described in prose above; refer to the GNOME advisory and commit for technical details.

Detection Methods for CVE-2026-13601

Indicators of Compromise

Outbound HTTP or HTTPS requests originating from the Yelp process to unfamiliar domains, especially with long query strings containing file-like content.
Yelp opening .page, .xml, or SVG documents that were written by a Flatpak application into shared portal paths.
OpenURI portal invocations from Flatpak apps that immediately precede Yelp network activity.

Detection Strategies

Monitor process ancestry where xdg-desktop-portal or a Flatpak-launched binary hands off a URI to yelp, followed by network egress from Yelp.
Alert on Yelp reading files outside expected help documentation paths, such as /home/<user>/.ssh/, /home/<user>/.config/, or dotfiles.
Inspect DNS and TLS SNI logs for unfamiliar destinations contacted by the Yelp binary.

Monitoring Recommendations

Enable audit rules on execve for yelp and correlate with open syscalls against sensitive user directories.
Log xdg-desktop-portal OpenURI calls and retain the requesting Flatpak application ID for review.
Baseline Yelp network behavior; under normal use Yelp performs limited outbound traffic, so deviations are high-signal.

How to Mitigate CVE-2026-13601

Immediate Actions Required

Update Yelp and yelp-xsl to the fixed versions provided by your distribution once available. Track fix status via the Red Hat CVE-2026-13601 Advisory and Red Hat CSAF VEX Document.
Audit installed Flatpak applications and remove untrusted sources from flatpak remotes.
Restrict which applications may call the OpenURI portal by reviewing portal configuration for each user.

Patch Information

The upstream fix tightens the CSP delivered through yelp-xsl and adjusts Yelp's handling of XML inclusions. See the GNOME Yelp Commit Details and GNOME Yelp Work Item #238 for the source-level changes. Apply distribution updates that ship both yelp and yelp-xsl at or above the fixed versions.

Workarounds

Avoid installing Flatpak applications from untrusted remotes until patched packages are deployed.
Do not open help content or URIs delivered by third-party Flatpak applications.
Consider unsetting Yelp as the default handler for help: and related MIME types on multi-user systems until the fix is applied.

bash

# Configuration example: check installed versions and Flatpak remotes
rpm -q yelp yelp-xsl || dpkg -l | grep -E 'yelp|yelp-xsl'
flatpak remotes --show-details
flatpak update