How severe is CVE-2026-40478?

CVE-2026-40478 has a CVSS score of 9/10, rated CRITICAL. Immediate patching is strongly recommended.

Which packages are affected by CVE-2026-40478?

CVE-2026-40478 affects the following packages: org.thymeleaf:thymeleaf (Maven), org.thymeleaf:thymeleaf-spring5 (Maven), org.thymeleaf:thymeleaf-spring6 (Maven). Ecosystems affected: Maven.

How do I fix CVE-2026-40478?

Update org.thymeleaf:thymeleaf to 3.1.4.RELEASE or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2026-40478 is resolved across your whole dependency graph.

How do I detect CVE-2026-40478 in my Maven dependencies?

Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for org.thymeleaf:thymeleaf, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.

How do I mitigate CVE-2026-40478 if there is no patch (or I can't update yet)?

Stop passing untrusted input into the interpreter or shell: call the affected binary with an argument array rather than a composed command string, reject anything outside a strict allowlist of expected values, and run the component under an account that cannot reach beyond the work it legitimately does.

Is CVE-2026-40478 actively exploited in the wild?

No public exploit code has been indexed for CVE-2026-40478 yet. This does not mean the vulnerability cannot be exploited — absence of public exploits does not imply safety. Apply the recommended fix and use O3 Security to monitor your exposure.

What is the EPSS score for CVE-2026-40478?

CVE-2026-40478 has an EPSS (Exploit Prediction Scoring System) score of 1.1%, placing it in the 66th percentile of all CVEs. EPSS is maintained by FIRST.org and estimates the probability that a vulnerability will be exploited in the wild within the next 30 days. This score indicates relatively lower exploitation probability, though the CVSS severity should still guide your patching priority.

What type of vulnerability is CVE-2026-40478?

CVE-2026-40478 is classified as Expression Language Injection (CWE-917), CWE-1336 (CWE-1336). These weakness types describe the underlying flaw category, which helps determine the potential impact and the right class of mitigation. This is a high-impact weakness class that often enables remote code execution or data exposure.

When was CVE-2026-40478 published, and has it been updated?

CVE-2026-40478 was published on April 17, 2026 and was last updated on October 28, 2026. Advisory data evolves as severity scores, affected ranges, and exploit intelligence are revised — always check the latest version of the advisory before acting.

☕

☕ Maven

Not in CISA KEV

CRITICAL severity

CVE-2026-40478 — thymeleaf

CRITICAL

CVE-2026-40478 is a critical-severity (CVSS 9) Expression Language Injection vulnerability in org.thymeleaf:thymeleaf. A fix is available for org.thymeleaf:thymeleaf — see the affected versions and patch details below.

Improper neutralization of specific syntax patterns for unauthorized expressions in Thymeleaf

Also known asGHSA-xjw8-8c5c-9r79

Published

Apr 17, 2026

Updated

Oct 28, 2026

Affected

3 pkgs

Patched

3 / 3

Exploits

None indexed

Exploitation data as of Nov 9, 2026 · OSV.dev, NVD, FIRST.org (EPSS)

Exploitation Status

No confirmed exploitation observed yet

A successful exploit gives an attacker total control of the affected component, not partial access.
CISA’s own triage has not observed active exploitation or public proof-of-concept code for this CVE as of its last assessment.

Exploitation and automatability from CISA’s SSVC triage for CVE-2026-40478.

EPSS Exploitation Probability

via FIRST.org ↗

1.1%probability of exploitation in next 30 days

Lower Risk0.00%

Lower risk than most CVEs66th percentile — riskier than 66% of all scored CVEsHighest risk

Probability of exploitation in the next 30 days, from FIRST.org EPSS.

How urgent is this, really

Where this sits among everything scored

Real-World Exposure

3 pkgs affected

☕org.thymeleaf:thymeleaf☕org.thymeleaf:thymeleaf-spring5☕org.thymeleaf:thymeleaf-spring6

Real-time download stats are indexed for npm and PyPI packages. This vulnerability affects Maven packages — download data is not available via public APIs for these ecosystems.

Description

Impact

A security bypass vulnerability exists in the expression execution mechanisms of Thymeleaf up to and including 3.1.3.RELEASE. Although the library provides mechanisms to prevent expression injection, it fails to properly neutralize specific syntax patterns that allow for the execution of unauthorized expressions. If an application developer passes unvalidated user input directly to the template engine, an unauthenticated remote attacker can bypass the library's protections to achieve Server-Side Template Injection (SSTI).

Patches

This has been fixed in Thymeleaf 3.1.4.RELEASE.

Workarounds

No workaround is available beyond ensuring applications do not pass unvalidated user input directly to the template engine. Upgrading to 3.1.4.RELEASE is strongly recommended in any case.

Credits

Thanks to Dawid Bakaj (VIPentest.com) for responsible disclosure.

Affected Packages

3 total 3 fixed

Ecosystem	Package	Vulnerable range	Fix
☕Maven	`org.thymeleaf:thymeleaf`	all versions	3.1.4.RELEASE`org.thymeleaf:thymeleaf:3.1.4.RELEASE`
☕Maven	`org.thymeleaf:thymeleaf-spring5`	all versions	3.1.4.RELEASE`org.thymeleaf:thymeleaf-spring5:3.1.4.RELEASE`
☕Maven	`org.thymeleaf:thymeleaf-spring6`	all versions	3.1.4.RELEASE`org.thymeleaf:thymeleaf-spring6:3.1.4.RELEASE`

Affected Products

1 product · 1 configurations

Application

thymeleafthymeleaf

< 3.1.4

range

Detection & mitigation playbook

Open-source dependency

Detect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for org.thymeleaf:thymeleaf, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.
Fix
Update org.thymeleaf:thymeleaf to 3.1.4.RELEASE or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2026-40478 is resolved across your whole dependency graph.
Workarounds
Stop passing untrusted input into the interpreter or shell: call the affected binary with an argument array rather than a composed command string, reject anything outside a strict allowlist of expected values, and run the component under an account that cannot reach beyond the work it legitimately does.

Fixing This On Your OS

If you run this on a Linux distribution, patch through your package manager against the distro's own security advisory below — it tracks the exact backported fix for your release, which can ship on a different timeline (and sometimes a different severity) than the upstream project.

Red HatImportant

Workaround published by Red Hat

The vulnerability arises when unvalidated user input is directly passed to the Thymeleaf template engine. To mitigate this, application developers should implement robust input validation and sanitization for all user-supplied data before it is processed by the Thymeleaf template engine. This ensures that malicious expressions cannot be executed.

Source: Red Hat security advisory for CVE-2026-40478 (CC BY 4.0)

Product	Fixed in	Advisory
Red Hat OpenShift Dev Spaces 3.28	`devspaces/openvsx-rhel9:1779528224`	RHSA-2026:21772

Frequently Asked Questions

### Impact A security bypass vulnerability exists in the expression execution mechanisms of Thymeleaf up to and including 3.1.3.RELEASE. Although the library provides mechanisms to prevent expression injection, it fails to properly neutralize specific syntax patterns that allow for the execution of unauthorized expressions. If an application developer passes unvalidated user input directly to the template engine, an unauthenticated remote attacker can bypass the library's protections to achieve Server-Side Template Injection (SSTI). ### Patches This has been fixed in Thymeleaf 3.1.4.RELEASE.