# GHSA-442j-39wm-28r2: Handlebars.js has a Property Access Validation Bypass in container.lookup

> Attackers can bypass prototype access controls in Handlebars.js when compat mode is enabled, potentially reading restricted properties from context objects.

- **Severity:** Not scored
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Over the internet
- **Victim action needed:** Not needed
- **Fix available:** Yes
- **Published:** 29 Mar 2026

## What is GHSA-442j-39wm-28r2?

GHSA-442j-39wm-28r2 is a vulnerability in handlebars, published on 29 Mar 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.

### Official description

## Summary

In `lib/handlebars/runtime.js`, the `container.lookup()` function uses `container.lookupProperty()` as a gate check to enforce prototype-access controls, but then discards the validated result and performs a second, unguarded property access (`depths[i][name]`). This Time-of-Check Time-of-Use (TOCTOU) pattern means the security check and the actual read are decoupled, and the raw access bypasses any sanitization that `lookupProperty` may perform.

Only relevant when the **compat** compile option is enabled (`{compat: true}`), which activates `depthedLookup` in `lib/handlebars/compiler/javascript-compiler.js`.

## Description

The vulnerable code in `lib/handlebars/runtime.js` (lines 137–144):

```javascript
lookup: function (depths, name) {
  const len = depths.length;
  for (let i = 0; i < len; i++) {
    let result = depths[i] && container.lookupProperty(depths[i], name);
    if (result != null) {
      return depths[i][name];  // BUG: should be `return result;`
    }
  }
},
```

`container.lookupProperty()` (lines 119–136) enforces `hasOwnProperty` checks and `resultIsAllowed()` prototype-access controls. However, `container.lookup()` only uses `lookupProperty` as a boolean gate — if the gate passes (`result != null`), it then performs an independent, raw `depths[i][name]` access that circumvents any transformation or wrapped value that `lookupProperty` may have returned.

## Workarounds

- Avoid enabling `{ compat: true }` when rendering templates that include untrusted data.
- Ensure context data objects are plain JSON (no Proxies, no getter-based accessor properties).

## Which versions are affected by GHSA-442j-39wm-28r2?

2 open source packages ship an affected version, including handlebars.


| Package | Ecosystem | Affected | Fixed in |
| --- | --- | --- | --- |
| `handlebars` | npm | From 4.0.0 | 4.7.9 |
| `handlebars` | npm | From 4.0.0 | 4.7.9 |

## How do I fix GHSA-442j-39wm-28r2?

Upgrade handlebars to 4.7.9 or later.

## Is GHSA-442j-39wm-28r2 being exploited?

There is no known exploitation of GHSA-442j-39wm-28r2 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for GHSA-442j-39wm-28r2 and fix it if it is affected.

What it is: GHSA-442j-39wm-28r2 is a vulnerability in handlebars, published on 29 Mar 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.
Exploitation: There is no known exploitation of GHSA-442j-39wm-28r2 in the wild and no public exploit code.

Affected packages and fixed versions:
- handlebars (npm): affected from 4.0.0; fixed in 4.7.9
- handlebars (npm): affected from 4.0.0; fixed in 4.7.9

Task:
1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies.
2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it.
3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead.
4. Run the tests and summarise what changed and why.
5. If nothing in the project is affected, say so and change nothing.

Reference: https://insights.sbomify.com/GHSA-442j-39wm-28r2/ (Markdown: https://insights.sbomify.com/GHSA-442j-39wm-28r2/index.md)
```

## Sources

- OSV: https://osv.dev/vulnerability/GHSA-442j-39wm-28r2

---
From sbomify Insights: https://insights.sbomify.com/GHSA-442j-39wm-28r2/
Summaries are generated and may contain mistakes; the official description above is the record.