CVE-2026-42451

Session token theft via unsafe iframe sandbox

Published May 8, 2026 · Updated May 8, 2026

Stored XSS in Grimmory's browser-based EPUB reader before 2.3.1 allows attackers to steal session tokens through a crafted EPUB file. Grimmory embeds foliate-js in a same-origin iframe with both allow-scripts and allow-same-origin, letting EPUB JavaScript reach the parent DOM and localStorage. Exploitation requires a victim to open the book; an uploader needs library access, while a third-party EPUB can be imported by the victim, and stolen tokens grant the victim's account permissions.

CVSS severity6.3
Medium
EPSS probability0.14%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

Stored XSS in Grimmory's browser-based EPUB reader before 2.3.1 allows attackers to steal session tokens through a crafted EPUB file. Grimmory embeds foliate-js in a same-origin iframe with both allow-scripts and allow-same-origin, letting EPUB JavaScript reach the parent DOM and localStorage. Exploitation requires a victim to open the book; an uploader needs library access, while a third-party EPUB can be imported by the victim, and stolen tokens grant the victim's account permissions.

The record

CVE
CVE-2026-42451
Published
May 8, 2026
Updated
May 8, 2026
Vendor
Grimmory
Product
Grimmory
Classifications
CWE-80, CWE-79, T1204.002
Attack vector
Unavailable
Privileges
Unavailable

Timeline

How it unfolded

  1. May 8, 2026CVE publishedPublication date reported by the CVE source.
  2. May 8, 2026Record updatedLatest update available in the CVE record.

Exploitability

Present is not the same as exploitable

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Is a vulnerable build present?

Compare these published version ranges with your installed build and any vendor patches.

  1. Affected versionversion=< 2.3.1

What conditions does exploitation require?

Attack vectorUnavailable in this record.
Required privilegesUnavailable in this record.

What is affected?

Grimmory · Grimmoryversion=< 2.3.1

Published CVSS scores

6.3GitHubCVSS:3.1/AV:L/AC:H/PR:L/UI:R/S:U/C:H/I:H/A:L

CVSS describes severity. EPSS estimates exploitation probability.

Attacks

What attackers are doing with it

Daily unique IPs observed by Shadowserver honeypots for known exploited vulnerabilities (KEVs). Missing observations do not establish an absence of attacks.

Daily unique IPsNo honeypot observations are available for this CVE in the selected window.

No observations available

Sep 10, 2026Sep 16, 2026
Latest reporting daySep 16, 2026
Latest daily unique IPsUnavailable
Prior 30-day averageUnavailable
SourceShadowserver honeypots (KEV)
VectorUnavailable
PrivilegesUnavailable
Known exploitationUnconfirmed
Public exploitUnconfirmed

Weakness, pattern, technique

CWE-80Improper Neutralization of Script-Related HTML Tags in a Web Page (Basic XSS)
CWE-79Improper Neutralization of Input During Web Page Generation ('Cross-site Scripting')
T1204.002Malicious File

Public exploit references

No public exploit references are available in this record.

Labels summarize the accepted research assessment. They do not indicate a test against your environment.

Technologies

Your stack

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Your stack

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