GHSA-7rx3-28cr-v5wh — handlebars
MEDIUMGHSA-7rx3-28cr-v5wh is a medium-severity (CVSS 4.8) vulnerability in handlebars. A fix is available for handlebars — see the affected versions and patch details below.
Handlebars.js has a Prototype Method Access Control Gap via Missing __lookupSetter__ Blocklist Entry
Real-World Exposure
How broadly this vulnerability is actually deployed: weekly install volume shows current usage, and reverse-dependency count shows how many other packages break if it stays unpatched.
handlebarsnpmDescription
Summary
The prototype method blocklist in lib/handlebars/internal/proto-access.js blocks constructor, __defineGetter__, __defineSetter__, and __lookupGetter__, but omits the symmetric __lookupSetter__. This omission is only exploitable when the non-default runtime option allowProtoMethodsByDefault: true is explicitly set — in that configuration __lookupSetter__ becomes accessible while its counterparts remain blocked, creating an inconsistent security boundary.
4.6.0 is the version that introduced protoAccessControl and the allowProtoMethodsByDefault runtime option.
Description
In lib/handlebars/internal/proto-access.js:
const methodWhiteList = Object.create(null);
methodWhiteList['constructor'] = false;
methodWhiteList['__defineGetter__'] = false;
methodWhiteList['__defineSetter__'] = false;
methodWhiteList['__lookupGetter__'] = false;
// __lookupSetter__ intentionally blocked in CVE-2021-23383,
// but omitted here — creating an asymmetric blocklist
All four legacy accessor helpers (__defineGetter__, __defineSetter__, __lookupGetter__, __lookupSetter__) were involved in the exploit chain addressed by CVE-2021-23383. Three of the four were explicitly blocked; __lookupSetter__ was left out.
When allowProtoMethodsByDefault: true is set, any prototype method not present in methodWhiteList is permitted by default. Because __lookupSetter__ is absent from the list, it passes the checkWhiteList check and is accessible in templates, while __lookupGetter__ (its sibling) is correctly denied.
Workarounds
- Do not set
allowProtoMethodsByDefault: true. The default configuration is not affected. - If
allowProtoMethodsByDefaultmust be enabled, ensure templates do not reference__lookupSetter__through untrusted input.
Affected Packages
| Ecosystem | Package | Vulnerable range | Fix |
|---|---|---|---|
| 📦npm | handlebars | ≥ 4.6.0&&< 4.7.9 | 4.7.9npm install handlebars@4.7.9 |
Detection & mitigation playbook
Open-source dependencyDetect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for handlebars, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.
Fix
Update handlebars to 4.7.9 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms GHSA-7rx3-28cr-v5wh 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-7rx3-28cr-v5wh can be triaged on real exposure rather than presence alone.
Tailored to GHSA-7rx3-28cr-v5wh. 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-7rx3-28cr-v5wh in your dependencies?
O3 Security finds GHSA-7rx3-28cr-v5wh across npm dependencies, including transitive ones, and its impact-aware SCA ranks findings by whether your code actually calls the vulnerable path.