GHSA-9w99-78rj-hmxq is a medium-severity (CVSS 6.3) Cross-site Scripting (XSS) vulnerability in decidim. A fix is available for decidim — see the affected versions and patch details below.
Cross-site scripting (XSS) in the dynamic file uploads
Exploitation Status
No confirmed exploitation observed yet
- A successful exploit gives an attacker total control of the affected component, not partial access.
- CISA’s own triage has not observed active exploitation or public proof-of-concept code for this CVE as of its last assessment.
Exploitation and automatability from CISA’s SSVC triage for GHSA-9w99-78rj-hmxq.
EPSS Exploitation Probability
EPSS (Exploit Prediction Scoring System) is a daily probability model maintained by FIRST.org. It estimates the likelihood a CVE will be exploited in production environments within the next 30 days, derived from real-world threat intelligence signals.
How urgent is this, really
GHSA-9w99-78rj-hmxq plotted by exploitation likelihood (EPSS) against impact (CVSS). The shaded corner — EPSS 50%+ and CVSS 7.0+ — is where this CVE doesn't sit, though severity or exploitability alone can still warrant action.
Where this sits among everything scored
Of 377,166 CVEs with a current EPSS score, this one falls in the < 10% band (highlighted). Real counts from FIRST.org, not a sample — log-scaled since the landscape is heavily right-skewed.
Real-World Exposure
decidim💎decidim-coreReal-time download stats are indexed for npm and PyPI packages. This vulnerability affects RubyGems packages — download data is not available via public APIs for these ecosystems.
Description
Impact
The dynamic file upload feature is subject to potential XSS attach in case the attacker manages to modify the file names of the records being uploaded to the server.
This appears in sections where the user controls the file upload dialogs themselves and has the technical knowledge to change the file names through the dynamic upload endpoint. Therefore I believe it would require the attacker to control the whole session of the particular user but in any case, this needs to be fixed.
Successful exploit of this vulneratibility would require the user to have successfully uploaded a file blob to the server with a malicious file name and then have the possibility to direct the other user to the edit page of the record where the attachment is attached.
The users are able to craft the direct upload requests themselves controlling the file name that gets stored to the database as shown here: https://github.com/rails/rails/blob/a967d355c6fee9ad9b8bd115d43bc8b0fc207e7e/activestorage/app/controllers/active_storage/direct_uploads_controller.rb#L14
The attacker is able to change the filename e.g. to <svg onload=alert('XSS')> if they know how to craft these requests themselves. And then enter the returned blob ID to the form inputs manually by modifying the edit page source.
Therefore, anywhere we display these strings, we should properly escape them.
Patches
PR #11612 fixes this problem both for 0.28.dev and 0.27.x.
Workarounds
Disable dynamic uploads for the instance, e.g. from proposals.
References
OWASP ASVS v4.0.3-5.1.3
Credits
This issue was discovered in City of Helsinki's security audit against Decidim 0.27 done during September 2023. The security audit was implemented by Deloitte Finland.
Affected Packages
| Ecosystem | Package | Vulnerable range | Fix |
|---|---|---|---|
| 💎RubyGems | decidim | ≥ 0.27.0&&< 0.27.5 | 0.27.5bundle update decidim --conservative |
| 💎RubyGems | decidim-core | ≥ 0.27.0&&< 0.27.5 | 0.27.5bundle update decidim-core --conservative |
Detection & mitigation playbook
Open-source dependencyDetect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for decidim, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.
Fix
Update decidim to 0.27.5 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms GHSA-9w99-78rj-hmxq is resolved across your whole dependency graph.
Workarounds
If you can't upgrade right away: gate or disable the affected feature, validate untrusted input at the boundary, and avoid passing attacker-controlled data into the vulnerable path. O3's runtime protection blocks exploitation in production as an interim safeguard until the upgrade lands.
How O3 protects you
O3 Security's impact-aware SCA analyses which vulnerable code paths your application actually calls, so a match like GHSA-9w99-78rj-hmxq can be triaged on real exposure rather than presence alone.
Tailored to GHSA-9w99-78rj-hmxq. Runtime protection reduces exposure until a permanent patch is applied and verified — it complements patching, it doesn't replace it.
Frequently Asked Questions
Is GHSA-9w99-78rj-hmxq in your dependencies?
O3 Security finds GHSA-9w99-78rj-hmxq across RubyGems dependencies, including transitive ones, and its impact-aware SCA ranks findings by whether your code actually calls the vulnerable path.