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Fencer Vulns
medium 4.7

CVE-2026-33916 json

Handlebars.js has Prototype Pollution Leading to XSS through Partial Template Injection

published 2026-03-26 7 mo ago modified 2026-10-26 13 d ago also GHSA-2qvq-rjwj-gvw9 , EUVD-2026-16847

cvss
4.7
version 3.1, CNA
cisa kev
Not listed
not in CISA's catalog
epss
0.3%
EPSS 0.3% chance of exploitation in 30 days, percentile 20%
public exploits
0
none known

Summary

Handlebars provides the power necessary to let users build semantic templates. In versions 4.0.0 through 4.7.8, `resolvePartial()` in the Handlebars runtime resolves partial names via a plain property lookup on `options.partials` without guarding against prototype-chain traversal. When `Object.prototype` has been polluted with a string value whose key matches a partial reference in a template, the polluted string is used as the partial body and rendered without HTML escaping, resulting in reflected or stored XSS. Version 4.7.9 fixes the issue. Some workarounds are available. Apply `Object.freeze(Object.prototype)` early in application startup to prevent prototype pollution. Note: this may break other libraries, and/or use the Handlebars runtime-only build (`handlebars/runtime`), which does not compile templates and reduces the attack surface.

Patch / fix

  • npm handlebars 4.7.9
  • handlebarsjs handlebars 4.7.9
  • node-handlebars Debian 14 3:4.7.9-1

debian advisories DEBIAN-CVE-2026-33916

Vendor advisories

What the vendors themselves say about their products: affected, fixed or not affected, and what to do.

red hat product security vex CVE-2026-33916 updated cvss 4.7

handlebars.js: Handlebars: Cross-Site Scripting (XSS) via prototype pollution in partial resolution

  • Red Hat Cryostat 4 handlebars Red Hat Logging Subsystem for Red Hat OpenShift openshift-logging/kibana6-rhel8, openshift-logging/elasticsearch6-rhel9, openshift-logging/logging-curator5-rhel9, openshift-logging/elasticsearch-proxy-rhel9, openshift-logging/elasticsearch-rhel9-operator, openshift-logging/elasticsearch-operator-bundle Red Hat Data Grid 8 handlebars Red Hat Enterprise Linux 7 firefox
    affected: workaround, no fix available “Fix deferred”
  • Red Hat Enterprise Linux 8 firefox, grafana, mozjs60, 389-ds-base, thunderbird, grafana-loki, +17 more
    affected: workaround, no fix available, no fix planned “Out of support scope”
  • Red Hat Enterprise Linux 9 firefox, firefox-x11, thunderbird Red Hat Enterprise Linux 10 firefox, thunderbird Red Hat OpenShift AI (RHOAI) rhoai/odh-workbench-codeserver-datascience-cpu-py312-rhel9 Red Hat OpenShift Dev Spaces devspaces/code-rhel9 Red Hat Process Automation 7 handlebars
    affected: workaround, no fix available “Fix deferred”

Red Hat CSAF VEX © Red Hat, Inc., from https://security.access.redhat.com/data/csaf/v2/vex/; changed: reduced to product statuses and remediations. CC BY 4.0 · csaf

CISA SSVC

Exploitation
Proof of concept is public
Automatable
no
Technical impact
partial
Assessed
7 mo ago

Exploit prediction

0.28% chance of exploitation in the next 30 days, higher than 20% of all scored CVEs (FIRST EPSS, 2026-11-08).

Exploits and checks

  • github.com https://github.com/handlebars-lang/handlebars.js/security/advisories/GHSA-2qvq-rjwj-gvw9 referenced PoC, tagged "exploit" by a source, unverified

CVSS scores

assigned by version score vector
CNA 3.1 4.7 CVSS:3.1/AV:N/AC:H/PR:N/UI:R/S:C/C:L/I:L/A:N
NVD 3.1 4.7 CVSS:3.1/AV:N/AC:H/PR:N/UI:R/S:C/C:L/I:L/A:N
GitHub 3.1 4.7 CVSS:3.1/AV:N/AC:H/PR:N/UI:R/S:C/C:L/I:L/A:N

Affected packages

ecosystem package affected fixed in
npm handlebars
>=4.0.0, <4.7.9
4.7.9

Affected products

  • handlebars-lang handlebars.js

Classification

Weakness
  • CWE-79 Improper Neutralization of Input During Web Page Generation
  • CWE-1321 Improperly Controlled Modification of Object Prototype Attributes

References

Sources

Fencer Vulns keeps one entry per vulnerability: the CVE id wins, then GHSA, and records that alias each other are merged, except that two CVEs always keep separate entries. The same data is available as JSON.