# CVE-2026-72844: Lean 4 Kernel Type Checking Bypass via Mismatched Structure Projections

> Attackers can execute arbitrary code or bypass type safety by importing malicious Lean 4 metaprograms that exploit kernel type checking flaws.

- **Severity:** Medium 6.8 (`CVSS:4.0/AV:L/AC:L/AT:N/PR:N/UI:P/VC:N/VI:H/VA:N/SC:N/SI:N/SA:N`)
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Local or same network only
- **Victim action needed:** A user has to click or open something
- **Fix available:** No
- **Published:** 20 Aug 2026
- **Updated:** 26 Oct 2026
- **Also known as:** GHSA-4fhc-8rq3-cjvg

## What is CVE-2026-72844?

CVE-2026-72844 is a medium-severity code injection vulnerability in leanprover lean4, published on 20 Aug 2026. It can only be exploited from the same machine or local network, and only if a user opens a file or follows a link.

### Official description

The Lean 4 kernel does not verify that the structure named in a projection expression matches the type of the value being projected, and environment::add_inductive in src/kernel/inductive.cpp did not type check the nested inductive applications that are replaced by auxiliary types, so their parametric arguments escaped checking. A metaprogram running in the Lean process can register an ill-typed nested inductive whose constructor applies a .proj C 0 projection to a value of the unrelated type W, and the kernel admits the declaration through the ordinary checked addDecl path at maximum kernel checking, without sorry, unsafeCast, debug.skipKernelTC, addDeclWithoutChecking, FFI, or a modified .olean file. The result is a type confusion yielding a proof of False that carries no axioms, from which any proposition can be derived. The published proof of concept additionally pads two expressions until their hashes and approximate depths collide, which defeats kernel caching; that is the technique used to reach the flaw, not its cause. Exploitation requires running a metaprogram in-process, for example by building a project or importing a malicious Lake dependency.

Weakness: CWE-843 (Access of Resource Using Incompatible Type)

## Which versions are affected by CVE-2026-72844?


## How do I fix CVE-2026-72844?

No fix has been published for CVE-2026-72844 yet. Limit exposure of the affected software until one is available.

## Is CVE-2026-72844 being exploited?

It is not confirmed as exploited in the wild, but public exploit code for CVE-2026-72844 exists.

Public exploit code:

- CVE-2026-72844 : Example of proving "0 = 1" using a vulnerability in the Lean 4 kernel. (github_poc): https://github.com/endrazine/lean-cve-poc

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-72844 and fix it if it is affected.

What it is: CVE-2026-72844 is a medium-severity code injection vulnerability in leanprover lean4, published on 20 Aug 2026. It can only be exploited from the same machine or local network, and only if a user opens a file or follows a link.
Exploitation: It is not confirmed as exploited in the wild, but public exploit code for CVE-2026-72844 exists.

Affected product: leanprover lean4. 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-2026-72844/ (Markdown: https://insights.sbomify.com/CVE-2026-72844/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-72844
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-72844
- OSV: https://osv.dev/vulnerability/CVE-2026-72844

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-72844/
Summaries are generated and may contain mistakes; the official description above is the record.