Lean 4 Kernel Type Checking Bypass via Mismatched Structure Projections

MITRE NVD CVE.ORG JSON API

Summary

CVECVE-2026-72844
StatePUBLISHED
AssignerVulnCheck
Source PriorityCVE Program / NVD first with legacy fallback
Published2026-08-20 18:16:45 UTC
Updated2026-08-21 22:16:44 UTC
DescriptionThe 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.

Risk And Classification

Primary CVSS: v4.0 6.8 MEDIUM from [email protected]

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/E:X/CR:X/IR:X/AR:X/MAV:X/MAC:X/MAT:X/MPR:X/MUI:X/MVC:X/MVI:X/MVA:X/MSC:X/MSI:X/MSA:X/S:X/AU:X/R:X/V:X/RE:X/U:X

EPSS: 0.001200000 probability, percentile 0.021870000 (date 2026-08-21)

Problem Types: CWE-843 | CWE-843 Access of Resource Using Incompatible Type ('Type Confusion')

VersionSourceTypeScoreSeverityVector
4.0[email protected]Secondary6.8MEDIUMCVSS:4.0/AV:L/AC:L/AT:N/PR:N/UI:P/VC:N/VI:H/VA:N/SC:N/SI:N/SA:N/E:X/C...
4.0CNACVSS6.8MEDIUMCVSS:4.0/AV:L/AC:L/AT:N/PR:N/UI:P/VC:N/VI:H/VA:N/SC:N/SI:N/SA:N
3.1[email protected]Secondary6.3MEDIUMCVSS:3.1/AV:L/AC:L/PR:N/UI:R/S:C/C:N/I:H/A:N
3.1CNACVSS6.3MEDIUMCVSS:3.1/AV:L/AC:L/PR:N/UI:R/S:C/C:N/I:H/A:N

CVSS v4.0 Breakdown

Attack Vector
Local
Attack Complexity
Low
Attack Requirements
None
Privileges Required
None
User Interaction
Passive
Confidentiality
None
Integrity
High
Availability
None
Sub Conf.
None
Sub Integrity
None
Sub Availability
None

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/E:X/CR:X/IR:X/AR:X/MAV:X/MAC:X/MAT:X/MPR:X/MUI:X/MVC:X/MVI:X/MVA:X/MSC:X/MSI:X/MSA:X/S:X/AU:X/R:X/V:X/RE:X/U:X

CVSS v3.1 Breakdown

Attack Vector
Local
Attack Complexity
Low
Privileges Required
None
User Interaction
Required
Scope
Changed
Confidentiality
None
Integrity
High
Availability
None

CVSS:3.1/AV:L/AC:L/PR:N/UI:R/S:C/C:N/I:H/A:N

Vendor Declared Affected Products

SourceVendorProductVersionPlatforms
CNA Leanprover Lean4 affected 4.32.2 semver Not specified
CNA Leanprover Lean4 unaffected 4.32.2 semver Not specified
CNA Leanprover Lean4 affected 4.33.0-rc1 semver Not specified

References

ReferenceSourceLinkTags
github.com/leanprover/lean4/pull/14577 [email protected] github.com
github.com/leanprover/lean4/issues/14576 [email protected] github.com
github.com/leanprover/lean4/commit/a39eab69e1eee9ad38f4efe507907b1026a77808 [email protected] github.com
github.com/leanprover/lean4 [email protected] github.com
www.openwall.com/lists/oss-security/2026/08/02/1 [email protected] www.openwall.com
www.vulncheck.com/advisories/lean-4-kernel-type-checking-bypass-via-mismatched-... [email protected] www.vulncheck.com
github.com/xrchz/CollatzLean [email protected] github.com
github.com/endrazine/lean-cve-poc [email protected] github.com
CVE Program record CVE.ORG www.cve.org canonical
NVD vulnerability detail NVD nvd.nist.gov canonical, analysis

Vendor Comments And Credit

Discovery Credit

CNA: kiranandcode (en)

CNA: Ramana Kumar (@xrchz) (en)

CNA: Leonardo de Moura (en)

CNA: Jonathan Brossard (MOABI) (en)

Additional Advisory Data

SourceTimeEvent
CNA2026-07-25T00:00:00.000ZCollatzLean repository published, exploiting the bug
CNA2026-07-28T00:00:00.000ZBug identified and reported in issue #14576
CNA2026-07-28T00:00:00.000ZFix submitted in pull request #14577
CNA2026-07-29T00:00:00.000ZFix available in nightly builds
CNA2026-08-02T00:00:00.000ZPublic advisory on oss-security

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