CVE-2026-72705 in Rocq
Summary
by MITRE • 08/24/2026
The guard checker in Rocq Prover does not follow recursive calls made through a fixpoint's own arguments. A fixpoint may pass itself as a higher-order argument to a second fixpoint, which then applies it to a value that is not a subterm of the structural argument. Passing the recursive function to a plain definition is rejected because the checker unfolds the definition and observes the call, but passing it to a fixpoint is accepted because higher-order recursive calls through fixpoint arguments are not tracked. This admits a type that is definitionally equal to its own negation, so self-application produces False in purely definitional code, without tactics, axioms, plugins or unsafe flags, and Print Assumptions reports the result as closed under the global context. Fixed in Rocq 9.2.0.
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Analysis
by VulDB Data Team • 08/24/2026
The vulnerability identified within the Rocq Prover involves a critical flaw in its guard checker mechanism, which is responsible for ensuring that recursive functions terminate by verifying that recursive calls are made on structurally smaller arguments. The core issue arises when a fixpoint passes itself as a higher-order argument to another fixpoint function. In this scenario, the second fixpoint applies the passed recursive function to a value that does not constitute a subterm of its own structural argument. While the checker correctly rejects cases where such self-referential recursion is passed through plain definitions by unfolding them and observing the resulting call structure, it fails to track higher-order recursive calls made directly through fixpoint arguments. This oversight allows for the construction of terms that are definitionally equal to their own negation, effectively breaking the logical consistency of the system without requiring any external tactics, axioms, plugins, or unsafe flags.
From a technical perspective, this flaw represents a significant deviation from the expected behavior of dependent type theories like those implemented in Rocq and its predecessor Coq. The guard checker is designed to prevent infinite loops during computation by enforcing well-founded recursion. By failing to account for recursive calls embedded within higher-order function arguments, the system permits the derivation of False through self-application in purely definitional code. This means that a user can construct a proof term that evaluates to a contradiction using only standard language features available in version 9.1 and earlier. The fact that Print Assumptions reports such terms as closed under the global context indicates that the inconsistency is not merely a runtime error but a fundamental breach of type safety, allowing any proposition to be proven true via ex falso quodlibet once False has been derived.
The operational impact of this vulnerability is severe for users relying on Rocq Prover for formal verification and proof assistant tasks. Since the system admits inconsistent types, any proofs constructed using vulnerable versions cannot be trusted as logically sound. This undermines the primary purpose of interactive theorem provers, which is to provide machine-checked guarantees about mathematical statements or software correctness. An attacker or a careless user could exploit this flaw to derive arbitrary conclusions from false premises, rendering formal models built on these systems unreliable. The vulnerability affects the integrity of all developments that depend on higher-order recursion patterns involving self-application through fixpoint arguments, potentially compromising security-critical proofs in areas such as cryptography, operating system verification, and hardware design where Rocq is commonly employed.
This issue aligns with CWE-20 Improper Input Validation, specifically regarding the failure to validate recursive call structures for well-foundedness, and can be mapped to ATT&CK techniques related to exploitation of software vulnerabilities that lead to logical inconsistencies or denial of service through resource exhaustion in proof contexts. The root cause lies in the incomplete tracking of recursion paths when functions are passed as arguments rather than applied directly at the top level of a fixpoint body. To mitigate this risk, users must upgrade to Rocq version 9.2.0 or later, where the guard checker has been patched to correctly identify and reject such higher-order recursive patterns that violate structural subterm constraints. Until an update is applied, developers should avoid passing self-referential functions as arguments to other fixpoints in a manner that could trigger this specific recursion pattern, although relying on manual discipline is not a robust security practice given the complexity of formal proofs.