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CRITICAL severity

CVE-2026-40477 — thymeleaf

CRITICAL

CVE-2026-40477 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 restriction of the scope of accessible objects in Thymeleaf expressions

Also known asGHSA-r4v4-5mwr-2fwr
Published
Apr 17, 2026
Updated
Aug 18, 2026
Affected
3 pkgs
Patched
3 / 3
Exploits
None indexed
Exploitation data as of Sep 21, 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-2026-40477.

EPSS Exploitation Probability

via FIRST.org ↗
0.9%probability of exploitation in next 30 days
Lower Risk0.00%
Lower risk than most CVEs56th percentile — riskier than 56% of all scored CVEsHighest risk

EPSS (Exploit Prediction Scoring System) is a daily probability model maintained by FIRST.org. It estimates the likelihood a CVE will be exploited in production environments within the next 30 days, derived from real-world threat intelligence signals.

How urgent is this, really

CVE-2026-40477 plotted by exploitation likelihood (EPSS) against impact (CVSS). The shaded corner — EPSS 50%+ and CVSS 7.0+ — is where this CVE doesn't sit, though severity or exploitability alone can still warrant action.

Where this sits among everything scored

Of 377,333 CVEs with a current EPSS score, this one falls in the < 10% band (highlighted). Real counts from FIRST.org, not a sample — log-scaled since the landscape is heavily right-skewed.

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

Thymeleaf is a server-side Java template engine for web and standalone environments. Versions 3.1.3.RELEASE and prior contain a security bypass vulnerability in the expression execution mechanisms. Although the library provides mechanisms to prevent expression injection, it fails to properly restrict the scope of accessible objects, allowing specific potentially sensitive objects to be reached from within a template. 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). This issue has ben fixed in version 3.1.4.RELEASE.

Affected Packages

3 total 3 fixed
EcosystemPackageVulnerable rangeFix
☕Mavenorg.thymeleaf:thymeleafall versions3.1.4.RELEASEorg.thymeleaf:thymeleaf:3.1.4.RELEASE
☕Mavenorg.thymeleaf:thymeleaf-spring5all versions3.1.4.RELEASEorg.thymeleaf:thymeleaf-spring5:3.1.4.RELEASE
☕Mavenorg.thymeleaf:thymeleaf-spring6all versions3.1.4.RELEASEorg.thymeleaf:thymeleaf-spring6:3.1.4.RELEASE

Detection & mitigation playbook

Open-source dependency

  1. 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.

  2. 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-2026-40477 is resolved across your whole dependency graph.

  3. Workarounds

    If you can't upgrade right away: gate or disable the affected feature, validate untrusted input at the boundary, and avoid passing attacker-controlled data into the vulnerable path. O3's runtime protection blocks exploitation in production as an interim safeguard until the upgrade lands.

  4. How O3 protects you

    O3 Security's impact-aware SCA analyses which vulnerable code paths your application actually calls, so a match like CVE-2026-40477 can be triaged on real exposure rather than presence alone.

Tailored to CVE-2026-40477. Runtime protection reduces exposure until a permanent patch is applied and verified — it complements patching, it doesn't replace it.

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
ProductFixed inAdvisory
Red Hat OpenShift Dev Spaces 3.28devspaces/openvsx-rhel9:1779528224RHSA-2026:21772

Frequently Asked Questions

Thymeleaf is a server-side Java template engine for web and standalone environments. Versions 3.1.3.RELEASE and prior contain a security bypass vulnerability in the expression execution mechanisms. Although the library provides mechanisms to prevent expression injection, it fails to properly restrict the scope of accessible objects, allowing specific potentially sensitive objects to be reached from within a template. 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 Serve
O3 Security · Impact-Aware SCA

Is CVE-2026-40477 in your dependencies?

O3 Security finds CVE-2026-40477 across Maven dependencies, including transitive ones, and its impact-aware SCA ranks findings by whether your code actually calls the vulnerable path.

Scan my dependencies How O3 SCA works
CVE-2026-40477: thymeleaf (Critical 9) | O3 Security