high CVSS 7.8 EPSS <0.1%

CVE-2021-47061·Kernel vulnerability

In the Linux kernel, the following vulnerability has been resolved: KVM: Destroy I/O bus devices on unregister failure _after_ sync'ing SRCU If allocating a new instance of an I/O bus fails when unregistering a device, wait to destroy the device until after all readers are guaranteed to see the new null bus. Destroying devices before the bus is nullified could lead to use-after-free since readers expect the devices on their reference of the bus to remain valid.

Severity
high
Software
Kernel
Fixed in
5.12.4
Published

How likely is exploitation

FIRST's Exploit Prediction Scoring System puts the probability of this CVE being exploited in the next 30 days at <0.1% — higher than 4% of all published CVEs. EPSS is an independent forecast of exploitation, not a severity rating: it says nothing about how much damage a successful attack does, which is what a CVSS score measures.

Read the two together. CVE-2021-47061 scores 7.8 on CVSS, so a successful attack would be serious — but at <0.1%, almost nobody is currently attempting it. That is a different situation from a flaw under mass exploitation, and it is usually the difference between patching on your next maintenance window and patching tonight.

Affected versions

From: 5.12

Until: 5.12.4

Fixed in: 5.12.4

How to fix this CVE

Update your Linux kernel to version 5.12.4 or later to resolve a critical memory safety issue in KVM's I/O bus device handling. This vulnerability can lead to use-after-free conditions when device unregistration fails, potentially allowing local privilege escalation. Apply the kernel update immediately if you are running an affected 5.12.x kernel on a system with KVM virtualization enabled.

sudo dnf update kernel kernel-devel

Defensia detects this vulnerability

How to check if you are affected

  1. Run 'uname -r' to check your current kernel version. If it shows 5.12.0 through 5.12.3, your system is vulnerable.
  2. Verify KVM is enabled by checking if '/sys/module/kvm' exists and 'lsmod | grep kvm' returns output.
  3. Check dmesg and /var/log/kern.log for error messages containing 'io_bus' or 'kvm' combined with 'allocation' or 'unregister' failures.
  4. After patching, reboot and run 'uname -r' again to confirm the kernel version is 5.12.4 or higher, and verify '/sys/module/kvm' still functions normally.

FAQ

What is CVE-2021-47061?

This is a use-after-free vulnerability in KVM's I/O bus device management where improper cleanup during device unregistration can leave freed device references accessible to active readers, potentially enabling local privilege escalation on systems running the affected kernel versions.

Is CVE-2021-47061 being actively exploited?

No, this vulnerability is not listed in CISA's Known Exploited Vulnerabilities catalog and no public exploits are currently available.

What versions of Kernel are affected by CVE-2021-47061?

Linux kernel versions 5.12 through 5.12.3 are vulnerable. The issue is resolved in kernel 5.12.4 and all subsequent releases.

How do I check if my server is vulnerable to CVE-2021-47061?

Run 'uname -r' and check if the output shows kernel version 5.12.0, 5.12.1, 5.12.2, or 5.12.3. If it does and KVM is enabled (verify with 'lsmod | grep kvm'), your system requires an immediate kernel update.

Does Defensia detect CVE-2021-47061?

Yes — Defensia's CVE advisory scanner compares installed package versions against the NVD database. If Kernel is installed on a monitored server, CVE-2021-47061 will appear in your dashboard with remediation steps.

Related Kernel CVEs

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CVE-2026-53228CVSS 9.8In the Linux kernel, the following vulnerability has been resolved: ipv6: sit: reload inner IPv6 header after GSO offloads ipip6_tunnel_xmit() caches the inner IPv6 header pointer at function entry and continues using it after iptunnel_handle_offloads(). For GSO skbs, iptunnel_handle_offloads() calls skb_header_unclone(). When the skb header is cloned, skb_header_unclone() can call pskb_expand_head(), which may move the skb head. The pskb_expand_head() contract requires pointers into the skb header to be reloaded after the call. If the later skb_realloc_headroom() branch is not taken, SIT uses the stale iph6 pointer to read the inner hop limit and DS field. That can read from a freed skb head after the old head's remaining clone is released. Reload iph6 after the offload helper succeeds and before subsequent reads from the inner IPv6 header. Keep the existing reload after skb_realloc_headroom(), since that branch can also replace the skb.

References

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