CVE-2026-72844
Vulnérabilité moyenne dans lean4 (leanprover)
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.
Gravité et risque d'exploitation
CVE-2026-72844 obtient un score CVSS de 6.8/10, soit une gravité moyenne. Selon le modèle EPSS, la probabilité qu'elle soit exploitée dans les 30 prochains jours est de 0.18 %, ce qui la place devant 6.7 % des CVE référencées.
Que faire ?
- Vérifiez si lean4 (versions 4.32.2, 4.33.0-rc1, 0, <4.32.2) est présent sur votre parc.
- Appliquez le correctif ou la version corrigée indiqués dans les références de l'éditeur.
- À défaut de correctif, limitez l'exposition (filtrage réseau, désactivation du composant).
- Automatisez la détection : les agents CVE Heimdall (gratuits, open source) corrèlent l'inventaire de vos serveurs avec la base CVE.
Références
- https://github.com/leanprover/lean4/issues/14576
- https://github.com/leanprover/lean4/commit/a39eab69e1eee9ad38f4efe507907b1026a77808
- https://github.com/leanprover/lean4/pull/14577
- https://github.com/endrazine/lean-cve-poc
- https://github.com/xrchz/CollatzLean
- https://www.openwall.com/lists/oss-security/2026/08/02/1
- https://github.com/leanprover/lean4
- https://www.vulncheck.com/advisories/lean-4-kernel-type-checking-bypass-via-mismatched-structure-projections