Description
The guard checker in Rocq Prover does not recheck the recursive tree representation of an inductive type parameter after that parameter has been changed by transport. A fixpoint may apply a rewrite along an equality between types to its recursive argument, which the guard checker accepts because the inductive type is preserved, while the recursive tree recorded for the parameter is altered. A second fixpoint that calls the first inherits the altered recursive tree without verification, so a call that is not structurally decreasing is accepted as terminating. The resulting non-terminating definition proves that a natural number equals its own successor and therefore False, from which any proposition follows. The demonstration uses two axioms that follow from univalence and are consistent with the calculus of inductive constructions, so the contradiction comes from the guard check rather than from the assumptions. A fix is proposed but not merged.
Published: 2026-08-24
Score: 6.8 Medium
EPSS: < 1% Very Low
KEV: No
Impact: n/a
Action: n/a
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History

Tue, 25 Aug 2026 18:30:00 +0000

Type Values Removed Values Added
Metrics ssvc

{'options': {'Automatable': 'no', 'Exploitation': 'poc', 'Technical Impact': 'partial'}, 'version': '2.0.3'}

Mon, 24 Aug 2026 20:15:00 +0000

Type Values Removed Values Added
Description The guard checker in Rocq Prover does not recheck the recursive tree representation of an inductive type parameter after that parameter has been changed by transport. A fixpoint may apply a rewrite along an equality between types to its recursive argument, which the guard checker accepts because the inductive type is preserved, while the recursive tree recorded for the parameter is altered. A second fixpoint that calls the first inherits the altered recursive tree without verification, so a call that is not structurally decreasing is accepted as terminating. The resulting non-terminating definition proves that a natural number equals its own successor and therefore False, from which any proposition follows. The demonstration uses two axioms that follow from univalence and are consistent with the calculus of inductive constructions, so the contradiction comes from the guard check rather than from the assumptions. A fix is proposed but not merged.
Title Rocq Prover through 9.2.0 Guard Checker Trusts Corrupted Recursive Tree After Transport
Weaknesses CWE-670
References
Metrics cvssV3_1

{'score': 6.3, 'vector': 'CVSS:3.1/AV:L/AC:L/PR:N/UI:R/S:C/C:N/I:H/A:N'}

cvssV4_0

{'score': 6.8, 'vector': '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'}

Subscriptions

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cve-icon MITRE

Status: PUBLISHED

Assigner: VulnCheck

Published:

Updated: 2026-08-25T16:38:12.987Z

Reserved: 2026-08-10T13:02:20.829Z

Link: CVE-2026-72704

cve-icon Vulnrichment

Updated: 2026-08-25T16:37:41.491Z

cve-icon NVD

Status : Received

Published: 2026-08-24T20:17:18.390

Modified: 2026-08-25T17:18:15.777

Link: CVE-2026-72704

cve-icon Redhat

No data.

cve-icon OpenCVE Enrichment

Updated: 2026-08-24T22:00:04Z

Weaknesses
  • CWE-670

    Always-Incorrect Control Flow Implementation