CVE-2021-29631
📋 TL;DR
This vulnerability in FreeBSD's bhyve hypervisor allows malicious virtual machine guests to cause memory corruption in the host's bhyve process. This can lead to process crashes or potentially arbitrary code execution with bhyve process privileges. Affected systems are FreeBSD installations running vulnerable versions with bhyve virtualization enabled.
💻 Affected Systems
- FreeBSD bhyve hypervisor
📦 What is this software?
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Freebsd by Freebsd
Freebsd by Freebsd
Freebsd by Freebsd
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Freebsd by Freebsd
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⚠️ Risk & Real-World Impact
Worst Case
A malicious guest VM achieves arbitrary code execution in the bhyve process context, potentially compromising the host system and other VMs.
Likely Case
Malicious guests cause bhyve process crashes leading to denial of service for affected virtual machines.
If Mitigated
With proper network segmentation and guest isolation, impact is limited to individual VM availability.
🎯 Exploit Status
Requires guest VM access and ability to interact with VirtIO device models
🛠️ Fix & Mitigation
✅ Official Fix
Patch Version: FreeBSD 13.0-STABLE n246941-20f96f215562+, 12.2-STABLE r370400+, 11.4-STABLE r370399+, 13.0-RELEASE p4+, 12.2-RELEASE p10+, 11.4-RELEASE p13+
Vendor Advisory: https://security.FreeBSD.org/advisories/FreeBSD-SA-21:13.bhyve.asc
Restart Required: Yes
Instructions:
1. Update FreeBSD using freebsd-update fetch && freebsd-update install
2. Rebuild world/kernel if using source-based updates
3. Reboot the system
4. Restart all bhyve VMs
🔧 Temporary Workarounds
Disable VirtIO devices
FreeBSDReplace VirtIO-based device models with alternative device types in bhyve VM configurations
Edit bhyve VM configuration to use non-VirtIO devices (e.g., e1000 instead of virtio-net)
Temporary bhyve disable
FreeBSDStop running bhyve VMs until patching can be completed
bhyvectl --destroy --vm=vmname
service vm stop
🧯 If You Can't Patch
- Isolate bhyve hosts on separate network segments from untrusted networks
- Implement strict access controls for VM creation and limit bhyve usage to trusted administrators only
🔍 How to Verify
Check if Vulnerable:
Check FreeBSD version with 'uname -a' and compare against affected versions. Verify bhyve is enabled with 'kldstat | grep vmm'Check Version:
uname -aVerify Fix Applied:
Verify updated version with 'uname -a' shows patched version. Check FreeBSD security advisory for specific commit hashes.📡 Detection & Monitoring
Log Indicators:
- bhyve process crashes in /var/log/messages
- Kernel panic messages related to vmm or VirtIO
Network Indicators:
- Sudden loss of connectivity to VMs
- Unexpected VM reboots
SIEM Query:
source="freebsd_messages" AND ("bhyve" OR "vmm") AND ("panic" OR "crash" OR "segmentation fault")