Skip to content
Fencer Vulns
high 7.5

CVE-2026-33939 json

Handlebars.js has Denial of Service via Malformed Decorator Syntax in Template Compilation

published 2026-03-27 7 mo ago modified 2026-10-26 13 d ago also GHSA-9cx6-37pm-9jff , EUVD-2026-16858

cvss
7.5
version 3.1, CNA
cisa kev
Not listed
not in CISA's catalog
epss
0.6%
EPSS 0.6% chance of exploitation in 30 days, percentile 50%
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, when a Handlebars template contains decorator syntax referencing an unregistered decorator (e.g. `{{*n}}`), the compiled template calls `lookupProperty(decorators, "n")`, which returns `undefined`. The runtime then immediately invokes the result as a function, causing an unhandled `TypeError: ... is not a function` that crashes the Node.js process. Any application that compiles user-supplied templates without wrapping the call in a `try/catch` is vulnerable to a single-request Denial of Service. Version 4.7.9 fixes the issue. Some workarounds are available. Wrap compilation and rendering in `try/catch`. Validate template input before passing it to `compile()`; reject templates containing decorator syntax (`{{*...}}`) if decorators are not used in your application. Use the pre-compilation workflow; compile templates at build time and serve only pre-compiled templates; do not call `compile()` at request time.

Patch / fix

  • npm handlebars 4.7.9
  • Red Hat Cluster Observability Operator 1.5.0 1782839279, 1782839996, 1782840539
  • Red Hat Red Hat OpenShift Dev Spaces 3.27 1776744110
  • node-handlebars Debian 14 3:4.7.9-1

debian advisories DEBIAN-CVE-2026-33939

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-2026-33939 updated cvss 7.5

handlebars.js: Handlebars.js: Denial of Service via malformed decorator syntax in template compilation

  • 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
    affected: workaround, no fix planned “Will not fix”
  • Red Hat Enterprise Linux 8 mozjs60, 389-ds-base, cockpit-389-ds, python3-lib389, 389-ds-base-libs, 389-ds-base-snmp, +1 more
    affected: workaround, no fix available, no fix planned “Will not fix”
  • Red Hat Enterprise Linux 9 firefox, firefox-x11
    affected: workaround, no fix available
  • Red Hat Cluster Observability Operator 1.5 cluster-observability-operator/logging-console-plugin-pf4-rhel9 1782839279, cluster-observability-operator/logging-console-plugin-pf5-rhel9 1782840539, cluster-observability-operator/troubleshooting-panel-console-plugin-pf6-rhel9 1782839996
    fixed: vendor fix RHSA-2026:34342
  • Red Hat OpenShift Dev Spaces 3.27 devspaces 1776744110
    fixed: vendor fix RHSA-2026:10175
  • Red Hat Cluster Observability Operator 1.5 cluster-observability-operator/perses-rhel9 1782839109, cluster-observability-operator/thanos-rhel9 1782840280, cluster-observability-operator/korrel8r-rhel9 1782839157, cluster-observability-operator/prometheus-rhel9 1782840780, cluster-observability-operator/alertmanager-rhel9 1782840761, cluster-observability-operator/perses-rhel9-operator 1782839191, +16 more
    not affected: vulnerable code not present
  • Red Hat Cryostat 4 handlebars
    not affected: component not present
  • Red Hat Data Grid 8 handlebars
    not affected: vulnerable code cannot be controlled by adversary
show 4 more
  • Red Hat Developer Hub 1 rhdh/rhdh-hub-rhel9
    not affected: vulnerable code not present
  • Red Hat Enterprise Linux 7 firefox Red Hat Enterprise Linux 8 firefox, grafana, thunderbird, grafana-loki, grafana-mssql, grafana-mysql, +10 more Red Hat Enterprise Linux 9 thunderbird Red Hat Enterprise Linux 10 firefox, thunderbird
    not affected: vulnerable code not in execute path
  • Red Hat OpenShift AI (RHOAI) rhoai/odh-workbench-codeserver-datascience-cpu-py312-rhel9 Red Hat OpenShift Dev Spaces 3.27 devspaces 1776716842, devspaces 1776717247, devspaces 1776718438, devspaces 1776718450, devspaces 1776718480, devspaces 1776718585, +9 more
    not affected: vulnerable code not present
  • Red Hat Process Automation 7 handlebars
    not affected: component not present

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
yes
Technical impact
partial
Assessed
7 mo ago

Exploit prediction

0.63% chance of exploitation in the next 30 days, higher than 50% of all scored CVEs (FIRST EPSS, 2026-11-08).

Exploits and checks

  • github.com https://github.com/handlebars-lang/handlebars.js/security/advisories/GHSA-9cx6-37pm-9jff referenced PoC, tagged "exploit" by a source, unverified

CVSS scores

assigned by version score vector
CNA 3.1 7.5 CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
CISA ADP 3.1 7.5 CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
GitHub 3.1 7.5 CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H

Affected packages

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

Affected products

  • handlebars-lang handlebars.js
  • Red Hat Cluster Observability Operator 1.5.0
  • Red Hat Red Hat OpenShift Dev Spaces 3.27
  • Red Hat Cryostat 4
  • Red Hat Logging Subsystem for Red Hat OpenShift
  • Red Hat Red Hat Data Grid 8
  • Red Hat Red Hat Enterprise Linux 10
  • Red Hat Red Hat Enterprise Linux 7
  • Red Hat Red Hat Enterprise Linux 8
  • Red Hat Red Hat Enterprise Linux 9
  • Red Hat Red Hat OpenShift AI (RHOAI)
  • Red Hat Red Hat Process Automation 7

Classification

Weakness
  • CWE-754 Improper Check for Unusual or Exceptional Conditions
  • CWE-248 Uncaught Exception

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.