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Vulnerability record · CVE-2021-34435 · published 1 September 2021

CVE-2021-34435: Eclipse theia origin validation error vulnerability

Eclipse · Theia

In Eclipse Theia 0.3.9 to 1.8.1, the "mini-browser" extension allows a user to preview HTML files in an iframe inside the IDE. But with the way it is made it is possible for a previewed HTML file to trigger an RCE. This exploit only happens if a user previews a malicious file..

8.8 CVSS 3.1 High EPSS 0.60% · top 53.7% CWE-942 · CWE-942CWE-346 · Origin validation error
8.8CVSS 3.1 base score, v2 6.8
0.60%EPSS exploitation probability, 30 days
NoNot in CISA KEV
1Affected product versions listed by NVD
2References, 2 tagged exploit
17 Jun 2026Last modified by NVD

Description

In Eclipse Theia 0.3.9 to 1.8.1, the "mini-browser" extension allows a user to preview HTML files in an iframe inside the IDE. But with the way it is made it is possible for a previewed HTML file to trigger an RCE. This exploit only happens if a user previews a malicious file..

CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:H

Affected products

1 vulnerable configurations from NVD's CPE data, grouped by vendor and product.

References

LinkTags
https://bugs.eclipse.org/bugs/show_bug.cgi?id=568018 ExploitPatchVendor Advisory
https://bugs.eclipse.org/bugs/show_bug.cgi?id=568018 ExploitPatchVendor Advisory

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Related vulnerabilities

Same products first, then exploited flaws of the same weakness class.

9.8CVE-2021-34436Eclipse theia path traversal vulnerabilityIn Eclipse Theia 0.1.1 to 0.2.0, it is possible to exploit the default build to obtain remote code execution (and XXE) via the theia-xml-extension. T…EPSS 2.2%9.6CVE-2020-27224Eclipse theia cross-site scripting vulnerabilityIn Eclipse Theia versions up to and including 1.2.0, the Markdown Preview (@theia/preview), can be exploited to execute arbitrary code.EPSS 2.6%8.8CVE-2026-60009Eclipse theia path traversal vulnerabilityIn Eclipse Theia versions up to and including 1.73.1, the `@theia/filesystem` backend binds `POST /file-upload` in every filesystem-enabled deploymen…EPSS 0.44%8.4CVE-2026-44691Eclipse theia inclusion from untrusted sphere vulnerabilityIn Eclipse Theia versions prior to 1.69.0, custom task definitions in workspace files (e.g. .theia/tasks.json, .vscode/tasks.json) could be executed …EPSS 0.41%8.4CVE-2026-46580Eclipse theia inclusion from untrusted sphere vulnerabilityIn Eclipse Theia versions prior to 1.71.0, files matching the pattern .prompts/*.prompttemplate in a workspace were automatically loaded and could ov…EPSS 0.51%8.4CVE-2026-44688Eclipse theia inclusion from untrusted sphere vulnerabilityIn Eclipse Theia versions prior to 1.71.0, the AI chat agent processed workspace file and directory names as part of its prompt context without disti…EPSS 0.51%8.1CVE-2019-17636Eclipse theia insufficient verification of data authenticity vulnerabilityIn Eclipse Theia versions 0.3.9 through 0.15.0, one of the default pre-packaged Theia extensions is "Mini-Browser", published as "@theia/mini-browser…EPSS 0.60%7.5CVE-2026-61891Eclipse theia path traversal vulnerabilityIn Eclipse Theia versions up to and including 1.73.1, the `@theia/filesystem` backend exposes HTTP file-download endpoints (`GET /file`, `GET /files/…EPSS 0.53%

Source: NIST National Vulnerability Database (record CVE-2021-34435), CISA KEV, FIRST EPSS (scores of 2026-09-27). This page is refreshed as NVD updates the record.