# 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 IndentationAwareTok


- **Severity:** High 8.7 (`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`)
- **Exploited in the wild:** Not known
- **Fix available:** No
- **Published:** 9 Nov 2026
- **Updated:** 9 Nov 2026
- **Also known as:** GHSA-297v-g8mm-qj4h, GHSA-mw8j-wq3m-9899

## What is CVE-2026-124683?

CVE-2026-124683 is a high-severity vulnerability in Eclipse Langium, published on 9 Nov 2026. 

### Official 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.

Weakness: CWE-407 (Inefficient Algorithmic Complexity)

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


## How do I fix CVE-2026-124683?

No fix has been published for CVE-2026-124683 yet. Limit exposure of the affected software until one is available.

## Is CVE-2026-124683 being exploited?

There is no known exploitation of CVE-2026-124683 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-124683 and fix it if it is affected.

What it is: CVE-2026-124683 is a high-severity vulnerability in Eclipse Langium, published on 9 Nov 2026. 
Exploitation: There is no known exploitation of CVE-2026-124683 in the wild and no public exploit code.

Affected product: Eclipse Langium. No package-level version data is available, so check the vendor advisory linked from the reference page.

Task:
1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies.
2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it.
3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead.
4. Run the tests and summarise what changed and why.
5. If nothing in the project is affected, say so and change nothing.

Reference: https://insights.sbomify.com/CVE-2026-124683/ (Markdown: https://insights.sbomify.com/CVE-2026-124683/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-124683
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-124683
- OSV: https://osv.dev/vulnerability/CVE-2026-124683

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From sbomify Insights: https://insights.sbomify.com/CVE-2026-124683/
Summaries are generated and may contain mistakes; the official description above is the record.