CVE-2026-33937: Handlebars.js has JavaScript Injection via AST Type Confusion

Should you act?

Critical and remotely reachable

No known exploitation yet, but it is network-reachable without credentials. This signal is computed from CISA KEV, EPSS and CVSS data; it is not an editorial verdict.

• Not in CISA KEV, but CISA’s SSVC assessment records a public proof-of-concept, so attack code exists.
• EPSS 1.7%, percentile 77: FIRST’s model estimates a 1.7% chance of exploitation activity in the next 30 days, higher than 77% of all scored CVEs (moderate). EPSS is a probability, not a verdict.
• Remotely reachable with no login and no user interaction. If the vulnerable service is exposed to the internet, prioritize it.
• Only applies if your dependency tree includes handlebars (npm) >= 4.0.0, <= 4.7.8, fixed in 4.7.9. Check your lockfile, including transitive dependencies.

What it is

Handlebars provides the power necessary to let users build semantic templates. In versions 4.0.0 through 4.7.8, Handlebars.compile() accepts a pre-parsed AST object in addition to a template string. The value field of a NumberLiteral AST node is emitted directly into the generated JavaScript without quoting or sanitization. An attacker who can supply a crafted AST to compile() can therefore inject and execute arbitrary JavaScript, leading to Remote Code Execution on the server. Version 4.7.9 fixes the issue. Some workarounds are available. Validate input type before calling Handlebars.compile(); ensure the argument is always a string, never a plain object or JSON-deserialized value. Use the Handlebars runtime-only build (handlebars/runtime) on the server if templates are pre-compiled at build time; compile() will be unavailable.

From the GitHub advisory

Summary

Handlebars.compile() accepts a pre-parsed AST object in addition to a template string. The value field of a NumberLiteral AST node is emitted directly into the generated JavaScript without quoting or sanitization. An attacker who can supply a crafted AST to compile() can therefore inject and execute arbitrary JavaScript, leading to Remote Code Execution on the server.

Description

Handlebars.compile() accepts either a template string or a pre-parsed AST. When an AST is supplied, the JavaScript code generator in lib/handlebars/compiler/javascript-compiler.js emits NumberLiteral values verbatim:

Because the value is not wrapped in quotes or otherwise sanitized, passing a string such as {},{})) + process.getBuiltinModule('childprocess').execFileSync('id').toString() // as the value of a NumberLiteral causes the generated eval-ed code to break out of its intended context and execute arbitrary commands.

Weakness type: CWE-843, CWE-94.

Product: handlebars (npm) · Assigned by: GitHub_M · Published: 2026-03-27

Affected and fixed versions

Severity scores

• Attack vector: network (remotely reachable)
• Attack complexity: low
• Privileges required: none (no login)
• User interaction: none
• Scope: unchanged

CVSS measures how bad a flaw is if exploited, not how likely exploitation is. Read it next to the exploitation data below.

Exploitation and urgency

• CISA KEV: not listed (no confirmed in-the-wild exploitation known to CISA).
• CISA SSVC assessment: exploitation poc, automatable yes, technical impact total.
• FIRST EPSS (model of 2026-10-21): 1.74% probability of exploitation activity in the next 30 days; percentile 77.

Timeline

• 2026-03-27 — CVE record published by GitHub_M
• 2026-03-27 — GitHub-reviewed advisory GHSA-2w6w-674q-4c4q published (critical)
• 2026-09-04 — CVE record last updated

Sources and references

Original sources first. Version and fix details should always be confirmed against the vendor or project advisory.

Vendor and project advisories, patches

1. access.redhat.com access.redhat.com
2. access.redhat.com access.redhat.com

Vulnerability records

1. GitHub Advisory GHSA-2w6w-674q-4c4q github.com
2. CVE.org record CVE-2026-33937 cve.org
3. NVD entry CVE-2026-33937 nvd.nist.gov

Other references from the CVE record and advisory

1. github.com github.com
2. github.com github.com
3. github.com github.com
4. access.redhat.com access.redhat.com
5. RHBZ#2452523 bugzilla.redhat.com
6. security.access.redhat.com security.access.redhat.com

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