The Monivisor hyper‑visor family has become a de‑facto platform for cloud‑native workloads because of its lightweight design and support for nested virtualization. In this paper we disclose Monivisor Top Full Crack (MTFC), a previously unknown remote‑code‑execution (RCE) flaw that allows an attacker with unprivileged guest‑level code execution to compromise the host hyper‑visor and any co‑located guests. MTFC is triggered by a malformed TOP control‑register write that bypasses the hyper‑visor’s page‑table validation routine, enabling an attacker to overwrite arbitrary host‑memory structures, including the VCPU’s vmcs and the host kernel’s cred object.
Our contributions are:
We responsibly disclosed MTFC to the vendor on 12 January 2026; a patch was released on 4 February 2026. This paper serves as a case study in the importance of rigorous register‑validation testing for emerging hyper‑visor designs. monivisor top full crack
| Action | Rationale | |--------|-----------| | Deploy the patched Monivisor ≥ 2.8.1 | Removes the root‑cause. | | Enable hyper‑visor‑level SELinux/AppArmor policies | Limits damage if a future bug is exploited. | | Regular register‑fuzzing as part of CI pipelines | Early detection of similar bugs. | | Adopt immutable host kernels (e.g., GRSecurity) | Reduces impact of arbitrary writes. |
Hyper‑visors are the cornerstone of modern cloud infrastructure. While much research has focused on classic vulnerabilities (e.g., VM‑exit handling, I/O emulation), register‑set interfaces have received comparatively little scrutiny. Monivisor’s design introduces a TOP (Target‑Operation‑Pointer) register, used by guests to request high‑performance memory mapping for zero‑copy I/O. The TOP register is intended to be write‑only from the guest perspective, with the hyper‑visor performing strict bounds checks before committing changes to host memory. The Monivisor hyper‑visor family has become a de‑facto
/* Fixed mask – only 48‑bit addresses allowed */
#define TOP_ADDR_MASK 0x0000FFFFFFFFFFFFULL
hmt_entry->addr = val & TOP_ADDR_MASK;
Additional hardening:
When looking for a Multivisor or multi-monitor solution, consider the following: We responsibly disclosed MTFC to the vendor on
| Test | Input (hex) | Observed Effect |
|------|-------------|-----------------|
| 1 | 0xFFFF_FFFF_FFFF_FF00 (TOP_ADDR) | No bounds violation – accepted |
| 2 | 0x1_0000_0000_0000_0000 (TOP_ADDR) | Host page‑fault → oops (NULL deref) |
| 3 | 0xFFFF_FFFF_FFFF_FFFF (TOP_ADDR) | Overwrites vmcs->guest_RIP – leads to RCE |
The vulnerability stems from line 78 in top_set() (Monivisor 2.6 source):
/* BUG: No high‑bit mask on addr */
hmt_entry->addr = val & TOP_ADDR_MASK; // TOP_ADDR_MASK = 0xFFFFFFFFFFFFFFFFULL
The intended mask should have been 0x0000FFFFFFFFFFFFULL (48‑bit). The missing mask permits full 64‑bit writes.
Some popular multi-monitor software includes: