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CRITICAL severity

Cross-site Scripting (XSS) in Eclipse TheiaGHSA-gcm9-cc3r-c6vj

CRITICALFix: eclipse-theia/theia#7971

GHSA-gcm9-cc3r-c6vj is a critical-severity (CVSS 9.6) Cross-site Scripting (XSS) vulnerability in @theia/preview. 2 public exploit references exist, so weaponization risk is real. A fix is available for @theia/preview — see the affected versions and patch details below.

Also known asCVE-2020-27224
Published
Updated
Affected
1 pkg
Patched
1 / 1
Exploits
2 known
Exploitation data as of Oct 8, 2026 · OSV.dev, NVD, FIRST.org (EPSS)

EPSS Exploitation Probability

via FIRST.org ↗
2.6%probability of exploitation in next 30 days
Lower Risk0.00%
Lower risk than most CVEs85th percentile — riskier than 85% of all scored CVEsHighest risk
0.39%1.30%2.20%3.10%0.9%2.6%Apr 26Aug 26Oct 26

Probability of exploitation in the next 30 days, from FIRST.org EPSS.

How urgent is this, really

GHSA-gcm9-cc3r-c6vj by exploitation likelihood (EPSS) against impact (CVSS). Outside the shaded patch-first corner.

Where this sits among everything scored

Of 385,738 CVEs with a current EPSS score, this one falls in the < 10% band (highlighted). Counts from FIRST.org, log-scaled.

Real-World Exposure

1 pkg affected

How broadly this vulnerability is actually deployed: weekly install volume shows current usage, a proxy for how much of the ecosystem is exposed.

@theia/previewnpm
10Kdownloads / week

Description

In Eclipse Theia versions up to and including 1.2.0, the Markdown Preview (@theia/preview), can be exploited to execute arbitrary code.

Affected Packages

1 total 1 fixed
EcosystemPackageVulnerable rangeFix
📦npm@theia/previewall versions1.3.0npm install @theia/preview@1.3.0

Affected Products

1 product · 1 configurations
Application
theiaeclipse
≤ 1.2.0
range
Exploits & PoCs
2

Research use only. For defensive security, authorized penetration testing, and academic research only. Never execute exploit code against systems without explicit written authorization.

In Eclipse Theia versions up to and including 1.2.0, the Markdown Previe…

github.comNVDMar 2021

In Eclipse Theia versions up to and including 1.2.0, the Markdown Previe…

omespino.comNVDMar 2021

Detection & mitigation playbook

Open-source dependency

  1. Detect

    Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for @theia/preview, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.

  2. Fix

    Update @theia/preview to 1.3.0 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms GHSA-gcm9-cc3r-c6vj is resolved across your whole dependency graph.

  3. Workarounds

    Escape or sanitise the affected output on the server side rather than relying on client-side filtering, and add a Content-Security-Policy that blocks inline script execution so injected markup cannot run even if it reaches the page.

Frequently Asked Questions

In Eclipse Theia versions up to and including 1.2.0, the Markdown Preview (@theia/preview), can be exploited to execute arbitrary code.
O3 Security · Impact-Aware SCA

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Cross-site Scripting (XSS) in Eclipse Theia (Critical 9.6)