Security readout for executives and security teams
Plain-English summary
Affected Thymeleaf versions can mistake attacker-controlled input for executable template logic when an application passes that input directly into the template engine. This can bypass Thymeleaf’s safeguards and expose sensitive server-side objects. The issue is critical but conditional: applications that never evaluate unvalidated user input as templates may not be directly exploitable.
Executive priority
Triage immediately and remediate promptly where the vulnerable data flow exists, especially on internet-facing services. The possible impact is critical, but organizational risk depends on whether unvalidated user input reaches template evaluation. Prioritize confirming both deployed versions and application data flows rather than relying on dependency scanning alone.
Technical view
CVE-2026-40477 is an object-scope restriction bypass in Thymeleaf expression execution, classified under CWE-1336 and CWE-917. Versions before 3.1.4.RELEASE may permit unauthenticated remote SSTI when unvalidated user input reaches template evaluation. CVSS 3.1 is 9.1 with high confidentiality, integrity, and availability impacts; attack complexity is rated high.
Likely exposure
Potentially exposed systems use thymeleaf, thymeleaf-spring5, or thymeleaf-spring6 below 3.1.4.RELEASE and pass unvalidated user-controlled content directly to template evaluation. Internet-facing template customization, dynamic template creation, or similar flows deserve immediate review. Dependency presence alone does not prove exploitability.
Exploitation context
The supplied record is not listed in CISA KEV and provides no evidence of active exploitation. Exploitation requires an application-specific path that sends unvalidated attacker input into the template engine. The high attack-complexity rating reflects this prerequisite, while no authentication or user interaction is required once such a path exists.
Researcher notes
The source bundle establishes a protection bypass enabling SSTI but does not identify the specific accessible objects, demonstrate downstream execution effects, or report exploitation in the wild. No CPEs are supplied. Assessment should distinguish ordinary variable interpolation from application-controlled template or expression evaluation and should avoid assuming every installation is reachable.
Mitigation direction
- Upgrade affected Thymeleaf components to 3.1.4.RELEASE or later.
- Stop passing unvalidated user input directly into template evaluation.
- Treat user-controlled content as data rather than executable template source.
- Apply applicable Red Hat remediation documented in RHSA-2026:21772.
- Check current vendor guidance for deployment-specific remediation details.
Validation and detection
- Inventory direct and transitive Thymeleaf dependencies and record resolved runtime versions.
- Trace user-controlled input flows into template parsing and expression evaluation.
- Confirm deployed artifacts resolve all affected components to 3.1.4.RELEASE or later.
- Verify untrusted content is rendered as data and never evaluated as template logic.
- For Red Hat deployments, confirm installed package status against the cited errata.
Public sources used
Generated from the cited source records. This long-tail analysis has not been individually reviewed by a named human.
Conservative CVE-to-ATT&CK context
These mappings and lookup hints may be relevant to the vulnerability behavior, CWE, affected product, or exposure path. Glexia-inferred context is not an official MITRE, ATT&CK, CWE, or CVE Program mapping.
ATT&CK lookup starting points
Use these exact CWE pages and searches to review the Glexia ATT&CK library from this CVE's weakness and description context.
CWE-1336: Exact CWE lookup
Use the exact CWE identifier as the starting point before reviewing related ATT&CK behavior. Open the exact CWE lookup page first, then review the ATT&CK searches from that MITRE weakness context. This is a Glexia lookup hint, not an official ATT&CK mapping.
Open ATT&CK lookupCWE-917: Exact CWE lookup
Use the exact CWE identifier as the starting point before reviewing related ATT&CK behavior. Open the exact CWE lookup page first, then review the ATT&CK searches from that MITRE weakness context. This is a Glexia lookup hint, not an official ATT&CK mapping.
Open ATT&CK lookupCVE-2026-40477 mapping review
Open the CVE-to-ATT&CK bridge for reviewed, inferred, or future official mappings tied to this CVE.
Open ATT&CK lookup- Severity
- Critical
- CVSS
- 9.1 (3.1)
- Known Exploited
- No
- Published
Vector: CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:C/C:H/I:H/A:H
CNA and ADP enrichment extracted from CVE v5
These fields come from the CVE record and ADP containers, not from Glexia's Take. They preserve time-varying source decisions such as CISA SSVC, KEV status, CVSS metrics, and provider references.
SSVC decision data
CVSS vector scores
2 official scoresWe collect every scored CVSS vector available in the official CNA and ADP containers. When more than one version is present, the table keeps the source vectors side by side instead of collapsing them into the highest score.
CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:C/C:H/I:H/A:H2.26GitHub_MCVSS:3.1/AV:N/AC:H/PR:L/UI:N/S:C/C:H/I:H/A:H1.86redhat-SADPVulnerability scoring details
Base CVSS 3.1 score
9.1CriticalVector: CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:C/C:H/I:H/A:H
Base CVSS 3.1 score
8.5HighVector: CVSS:3.1/AV:N/AC:H/PR:L/UI:N/S:C/C:H/I:H/A:H
Vulnerability timeline
Timeline events are normalized from CVE metadata, CNA source timelines, ADP timelines, and KEV metadata when present.
- CVE reservedCVE Program
The CVE ID was reserved by the assigning CNA.
- ADP timelineredhat-SADP
Made public.
- CVE publishedCVE Program
The CVE record was published.
- ADP timelineredhat-SADP
Reported to Red Hat.
- CVE updatedCVE Program
The CVE record metadata indicates this as the latest update time.
ADP provider summaries
- 2026-04-17T23:00:52.793Z: Reported to Red Hat.
- 2026-04-17T21:53:47.271Z: Made public.
Source materials
- CVE List V5 sourceCVE List V5
- https://github.com/thymeleaf/thymeleaf/security/advisories/GHSA-r4v4-5mwr-2fwrCVE reference, GitHub_M · x_refsource_CONFIRM
- https://access.redhat.com/security/cve/CVE-2026-40477CVE reference, redhat-SADP · vdb-entry, x_refsource_REDHAT
- RHBZ#2459344CVE reference, redhat-SADP · issue-tracking, x_refsource_REDHAT
- https://security.access.redhat.com/data/csaf/v2/vex/2026/cve-2026-40477.jsonCVE reference, redhat-SADP · x_sadp-csaf-vex
- https://access.redhat.com/errata/RHSA-2026:21772CVE reference, redhat-SADP · vendor-advisory, x_refsource_REDHAT
Products and packages named in the record
CWE details
CWE links open Glexia weakness intelligence pages with official CWE context, developer remediation guidance, and related CVE mappings.
Improper Neutralization of Special Elements Used in a Template Engine
Improper Neutralization of Special Elements Used in a Template Engine represents a recurring weakness pattern that can create exploitable paths when design, validation, or implementation controls are missing.
Improper Neutralization of Special Elements used in an Expression Language Statement ('Expression Language Injection')
Improper Neutralization of Special Elements used in an Expression Language Statement ('Expression Language Injection') represents a recurring weakness pattern that can create exploitable paths when design, validation, or implementation controls are missing.