How severe is CVE-2026-40477?

CVE-2026-40477 has a CVSS score of 9/10, rated CRITICAL. Immediate patching is strongly recommended.

Which products are affected by CVE-2026-40477?

Affected package information for CVE-2026-40477 is not yet fully mapped. Check the official advisory links for details.

How do I patch CVE-2026-40477?

Apply the thymeleaf thymeleaf security patch or hotfix for CVE-2026-40477 on the affected version, following the vendor advisory for your exact build.

How do I detect CVE-2026-40477 across my infrastructure?

Inventory every thymeleaf thymeleaf deployment and check each version against the affected-products list above. Because the exploit targets the running system rather than your application code, also watch for exploitation at the network and runtime layer — O3 flags the exploit behaviour from runtime telemetry and egress traffic even before a vulnerable build is confirmed.

How do I mitigate CVE-2026-40477 if there is no patch (or I can't update yet)?

Cut exposure now: restrict the management/admin interface to trusted networks, segment the device, and apply the vendor's recommended configuration mitigations and any WAF/IPS signature. O3's runtime protection blocks the exploit chain at execution, holding the line on unpatched or end-of-life systems until you can patch.

How does O3 Security protect against CVE-2026-40477?

O3 detects and blocks CVE-2026-40477 exploitation at runtime: eBPF exploit-chain detection, plus L7 egress monitoring that catches the post-exploitation callback and severs the attacker's outbound channel.

Is CVE-2026-40477 actively exploited in the wild?

No public exploit code has been indexed for CVE-2026-40477 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-2026-40477?

CVE-2026-40477 has an EPSS (Exploit Prediction Scoring System) score of 0.6%, placing it in the 46th 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-2026-40477?

CVE-2026-40477 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.

Which products and versions are affected by CVE-2026-40477?

CVE-2026-40477 affects thymeleaf thymeleaf. Check the affected-products list on this page against your deployed version to confirm exposure.

When was CVE-2026-40477 published, and has it been updated?

CVE-2026-40477 was published on April 17, 2026 and was last updated on June 17, 2026. Advisory data evolves as severity scores, affected ranges, and exploit intelligence are revised — always check the latest version of the advisory before acting.

CVE-2026-40477: Thymeleaf Expression Language Injection (Critic…

EPSS Exploitation Probability

via FIRST.org ↗

0.6%probability of exploitation in next 30 days

Lower Risk46th percentile+0.59%

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.

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 Products

1 product · 1 configurations

Application

thymeleafthymeleaf

< 3.1.4

range

Detection & mitigation playbook

Vendor / appliance

Detect
Inventory every thymeleaf thymeleaf deployment and check each version against the affected-products list above. Because the exploit targets the running system rather than your application code, also watch for exploitation at the network and runtime layer — O3 flags the exploit behaviour from runtime telemetry and egress traffic even before a vulnerable build is confirmed.
Fix
Apply the thymeleaf thymeleaf security patch or hotfix for CVE-2026-40477 on the affected version, following the vendor advisory for your exact build.
Workarounds
Cut exposure now: restrict the management/admin interface to trusted networks, segment the device, and apply the vendor's recommended configuration mitigations and any WAF/IPS signature. O3's runtime protection blocks the exploit chain at execution, holding the line on unpatched or end-of-life systems until you can patch.
How O3 protects you
O3 detects and blocks CVE-2026-40477 exploitation at runtime: eBPF exploit-chain detection, plus L7 egress monitoring that catches the post-exploitation callback and severs the attacker's outbound channel.

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.

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 · Runtime Protection

Is CVE-2026-40477 being exploited in your environment?

O3's eBPF runtime sensors and L7 egress monitoring detect and block the CVE-2026-40477 exploit chain at execution — protecting unpatched and end-of-life systems until the vendor patch is applied.

Protect my workloads How runtime protection works

CVE-2026-40477

EPSS Exploitation Probability

Description

Affected Products

Detection & mitigation playbook

Detect

Fix

Workarounds

How O3 protects you

Frequently Asked Questions

Is CVE-2026-40477 being exploited in your environment?