CVE-2026-42451 Grimmory: Stored XSS via Malicious EPUB Enables Session Token Theft
Attackers can steal session tokens and take over accounts, including admin access, by tricking users into opening malicious EPUB files in the Grimmory reader.
- Exploited in the wild
- Not known
- Login needed
- Any signed-in user
- Attackable
- Local or same network only
- Victim action needed
- A user has to click or open something
- Fix
- None listed
What is CVE-2026-42451?
CVE-2026-42451 is a medium-severity cross-site scripting vulnerability in grimmory-tools grimmory, published on 8 May 2026. It can only be exploited from the same machine or local network, and only if a user opens a file or follows a link.
Official description
Grimmory is a self-hosted digital library. Prior to version 2.3.1, a stored cross-site scripting (XSS) vulnerability in Grimmory's browser-based EPUB reader allows an attacker to embed arbitrary JavaScript in a crafted EPUB file. When a victim opens the book, the script executes in their browser with full access to the Grimmory application's session context. This can enable session token theft and account takeover, including administrative access if an administrator opens the affected book. This issue has been patched in version 2.3.1.
Which versions are affected by CVE-2026-42451?
How do I fix CVE-2026-42451?
No fix has been published for CVE-2026-42451 yet. Limit exposure of the affected software until one is available.
Escape output for the context it lands in and set a strict Content Security Policy. Upgrading the affected component is the real fix. More about Cross-site scripting.
Is CVE-2026-42451 being exploited?
There is no known exploitation of CVE-2026-42451 in the wild and no public exploit code.
Sources
Also known as GHSA-frv6-5wq5-9p24.