CVE-2026-4800

Workarounds

Do not pass untrusted input as key names in options.imports. Only use developer-controlled, static key names.

The fix for CVE-2021-23337 added validation for the variable option in _.template but did not apply the same validation to options.imports key names. Both paths flow into the same Function() constructor sink.

When an application passes untrusted input as options.imports key names, an attacker can inject default-parameter expressions that execute arbitrary code at template compilation time.

Additionally, _.template uses assignInWith to merge imports, which enumerates inherited properties via for..in. If Object.prototype has been polluted by any other vector, the polluted keys are copied into the imports object and passed to Function().

Untrusted input is evaluated as executable code within the application's runtime environment. Typical impact: arbitrary code execution within the application's privilege context.

CVE-2026-4800 has a CVSS score of 8.1 (High). The vector is network-reachable, no privileges required, and no user interaction. A CVSS score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether this affects your application depends on whether the vulnerable code is present and reachable in your environment. A fixed version is available (4.18.0); upgrading removes the vulnerable code path.

Kodem intelligence

Users should upgrade to version 4.18.0.

The fix applies two changes:

1. Validate importsKeys against the existing reForbiddenIdentifierChars regex (same check already used for the variable option)
2. Replace assignInWith with assignWith when merging imports, so only own properties are enumerated

1. What is CVE-2026-4800? CVE-2026-4800 is a high-severity code injection vulnerability in lodash (npm), affecting versions >= 4.0.0, <= 4.17.23. It is fixed in 4.18.0. Untrusted input is evaluated as executable code within the application's runtime environment.
2. How severe is CVE-2026-4800? CVE-2026-4800 has a CVSS score of 8.1 (High). This score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether it represents real risk in your environment depends on whether the vulnerable code is present and reachable.
3. Which packages are affected by CVE-2026-4800?
   • lodash (npm) (versions >= 4.0.0, <= 4.17.23)
   • lodash-es (npm) (versions >= 4.0.0, <= 4.17.23)
   • lodash-amd (npm) (versions >= 4.0.0, <= 4.17.23)
   • lodash.template (npm) (versions >= 4.0.0, < 4.18.0)
4. Is there a fix for CVE-2026-4800? Yes. CVE-2026-4800 is fixed in 4.18.0. Upgrade to this version or later.
5. Is CVE-2026-4800 exploitable, and should I be worried? Whether CVE-2026-4800 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
6. What actually determines whether CVE-2026-4800 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
7. How do I fix CVE-2026-4800?
   • Upgrade lodash to 4.18.0 or later
   • Upgrade lodash-es to 4.18.0 or later
   • Upgrade lodash-amd to 4.18.0 or later
   • Upgrade lodash.template to 4.18.0 or later