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.
Blast Radius
preact📦preact📦preactReal-time download stats are indexed for npm and PyPI packages. This vulnerability affects npm packages — download data is not available via public APIs for these ecosystems.
Description
Impact
Vulnerability Type: HTML Injection via JSON Type Confusion
Affected Versions: Preact 10.26.5 through 10.28.1
Severity: Low to Medium (see below)
Who is Impacted?
Applications using affected Preact versions are vulnerable if they meet all of the following conditions:
- Pass unmodified, unsanitized values from user-modifiable data sources (APIs, databases, local storage, etc.) directly into the render tree
- Assume these values are strings but the data source could return actual JavaScript objects instead of JSON strings
- The data source either:
- Fails to perform type sanitization AND blindly stores/returns raw objects interchangeably with strings, OR
- Is compromised (e.g., poisoned local storage, filesystem, or database)
Technical Details
Preact includes JSON serialization protection to prevent Virtual DOM elements from being constructed from arbitrary JSON. A regression introduced in Preact 10.26.5 caused this protection to be softened. In applications where values from JSON payloads are assumed to be strings and passed unmodified to Preact as children, a specially-crafted JSON payload could be constructed that would be incorrectly treated as a valid VNode. When this chain of failures occurs it can result in HTML injection, which can allow arbitrary script execution if not mitigated by CSP or other means.
Important Notes:
- This regression was never present in
preact-render-to-string - This is primarily an "expanded attack surface" issue rather than a standalone vulnerability
- Exploitation requires either insecure API design (no type validation) or a compromised data source
Patches
Patched Versions:
- 10.26.10 (for 10.26.x users)
- 10.27.3 (for 10.27.x users)
- 10.28.2 (for 10.28.x users)
Users should upgrade to the latest patch version of whatever minor version they are on, which can be done via npm update preact or by installing one of the above versions directly.
The patch versions simply restore the previous strict equality checks that prevent JSON-parsed objects from being treated as valid VNodes.
Mitigations
If you cannot upgrade immediately, implement the following mitigations:
- Validate input types: Don't accept arbitrary objects as inputs in your API and blindly store them. Enforce strict type contracts at API boundaries.
- Cast or validate network data: Don't assume strings are strings if your code got them from the network. Always cast to the expected type or validate before rendering.
- Sanitize external data: Validate that data from external sources (APIs, storage, databases) matches expected types before passing it to preact.
- Use Content Security Policy (CSP): Implement a strict CSP to prevent inline script execution as a defense-in-depth measure.
References
- Reporter: YoungGeun Choi
- Affected Versions: 10.26.5 - 10.28.1
- Patched Versions: 10.26.10, 10.27.3, 10.28.2
Credits
Preact thanks YoungGeun Choi (Xvezda) for the responsible disclosure of this vulnerability and for providing detailed reproduction steps and proof-of-concept demonstrations.
Timeline
- 2026-01-04: Initial vulnerability report received
- 2026-01-05: Clarification requested regarding network/serialization boundary
- 2026-01-06: Network PoC provided demonstrating real-world exploitatibility
- 2026-01-06: Hotfix patches released (10.26.10, 10.27.3, 10.28.2)
Recommendation: All users of Preact 10.26.5 through 10.28.1 should upgrade to the appropriate patched version (10.26.10, 10.27.3, or 10.28.2) as soon as possible, and review their applications for proper input validation and sanitization practices.
Affected Packages
| Ecosystem | Package | Vulnerable range | Fix |
|---|---|---|---|
| 📦npm | preact | ≥ 10.26.5&&< 10.26.10 | 10.26.10 |
| 📦npm | preact | ≥ 10.27.0&&< 10.27.3 | 10.27.3 |
| 📦npm | preact | ≥ 10.28.0&&< 10.28.2 | 10.28.2 |
Detection & mitigation playbook
Open-source dependencyDetect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for preact. O3's reachability analysis confirms whether the vulnerable code path is actually invoked in your application, so you act on real exposure instead of every transitive match.
Fix
Update preact to 10.26.10 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms GHSA-36hm-qxxp-pg3m 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 pinpoints whether GHSA-36hm-qxxp-pg3m is reachable in your code and exactly where to fix it, then blocks exploitation in production at runtime until the patched version is deployed.
Tailored to GHSA-36hm-qxxp-pg3m. 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-36hm-qxxp-pg3m in your dependencies?
O3 detects GHSA-36hm-qxxp-pg3m across npm dependencies and uses function-level reachability to confirm whether the vulnerable code path is actually reachable — not just present. No false positives.