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.
- 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
- None listed
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
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.
Upgrade, and never evaluate templates, expressions or code built from user input. More about Code injection.
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: 1 proofs of concept on GitHub.
Sources
Also known as GHSA-4fhc-8rq3-cjvg.