Security readout for executives and security teams
Plain-English summary
expr-eval can turn attacker-controlled expressions into arbitrary JavaScript when an application uses toJSFunction(). Successful exploitation could let an unauthenticated remote attacker act with the application’s permissions, potentially exposing data, changing information, or disrupting service. Risk depends on whether untrusted input reaches this API.
Executive priority
Treat as urgent where internet-facing or otherwise untrusted input reaches toJSFunction(). Prioritize rapid exposure tracing and containment because reported impact spans confidentiality, integrity, and availability. Systems carrying the dependency without that data flow require investigation, not automatic assumptions of compromise.
Technical view
CVE-2026-12866 is a CWE-94 code-injection vulnerability. Crafted expressions passed through toJSFunction() are compiled with new Function(), allowing escape from the intended expression sandbox and execution within the application context. The supplied record reports all expr-eval versions and the org.webjars.npm packaging as affected, with CVSS 3.1 score 9.8.
Likely exposure
Highest exposure exists where direct or transitive expr-eval usage sends user-controlled expressions to toJSFunction(), especially through remotely reachable functionality. Merely installing the package does not establish exploitability. Both the npm package and its WebJars distribution should be inventoried.
Exploitation context
The supplied bundle does not identify known active exploitation, and the CVE is not marked as CISA KEV. Exploitation is described as remote, low complexity, unauthenticated, and requiring no user interaction, but only when an application exposes the vulnerable data flow.
Researcher notes
The core evidence links risk to toJSFunction() and new Function(). The bundle reports all versions affected but provides no confirmed patched version, formal vendor mitigation, or evidence of exploitation in the wild. Validate reachability and input control before classifying an installed dependency as exploitable.
Mitigation direction
- Identify direct and transitive expr-eval and org.webjars.npm:expr-eval dependencies.
- Stop passing untrusted or externally influenced expressions to toJSFunction().
- Disable affected functionality where the unsafe data flow cannot be removed immediately.
- Check maintainer and package-distributor guidance for an approved fix or replacement.
- Do not assume a routine version upgrade resolves an all-versions advisory.
Validation and detection
- Search application code and dependencies for expr-eval and its WebJars packaging.
- Locate every toJSFunction() call and trace expression inputs to their origin.
- Confirm whether any affected input is reachable without authentication or user interaction.
- Verify mitigations prevent untrusted expressions from reaching native JavaScript compilation.
- Retest affected features after remediation without using offensive payloads.
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-94: Code execution behavior lookup
Code execution and unsafe deserialization weaknesses often justify reviewing execution behavior and process telemetry. 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 lookupExecution behavior lookup
The CVE wording references code or command execution, so execution technique review may help defensive triage. This is a Glexia inferred lookup path, not an official MITRE, ATT&CK, or CVE Program mapping.
Open ATT&CK lookupCVE-2026-12866 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
- 9.8 (3.1)
- Known Exploited
- No
- Published
Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/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:L/PR:N/UI:N/S:U/C:H/I:H/A:H3.95.9snykCVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:H/VI:H/VA:H/SC:N/SI:N/SA:N——snykVulnerability scoring details
Base CVSS 4.0 score
9.2CriticalVector: CVSS:4.0/AV:N/AC:L/AT:P/PR:N/UI:N/VC:H/VI:H/VA:H/SC:N/SI:N/SA:N
Base CVSS 3.1 score
9.8CriticalVector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/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.
- CVE publishedCVE Program
The CVE record was published.
- CVE updatedCVE Program
The CVE record metadata indicates this as the latest update time.
ADP provider summaries
Source materials
- CVE List V5 sourceCVE List V5
- https://security.snyk.io/vuln/SNYK-JS-EXPREVAL-15054690CVE reference, snyk
- https://security.snyk.io/vuln/SNYK-JAVA-ORGWEBJARSNPM-17662018CVE reference, snyk
- https://github.com/silentmatt/expr-eval/blob/master/src/expression.js%23L55CVE reference, snyk
- https://github.com/silentmatt/expr-eval/issues/292CVE reference, snyk
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 Control of Generation of Code ('Code Injection')
Improper Control of Generation of Code ('Code Injection') represents a recurring weakness pattern that can create exploitable paths when design, validation, or implementation controls are missing.