high CVSS 7.8 EPSS <0.1%

CVE-2021-47293·Kernel vulnerability

In the Linux kernel, the following vulnerability has been resolved: net/sched: act_skbmod: Skip non-Ethernet packets Currently tcf_skbmod_act() assumes that packets use Ethernet as their L2 protocol, which is not always the case. As an example, for CAN devices: $ ip link add dev vcan0 type vcan $ ip link set up vcan0 $ tc qdisc add dev vcan0 root handle 1: htb $ tc filter add dev vcan0 parent 1: protocol ip prio 10 \ matchall action skbmod swap mac Doing the above silently corrupts all the packets. Do not perform skbmod actions for non-Ethernet packets.

Severity
high
Software
Kernel
Fixed in
5.13.6
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 8% 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-2021-47293 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: 5.11

Until: 5.13.6

Fixed in: 5.13.6

How to fix this CVE

Update your Linux kernel to version 5.13.6 or later to resolve this traffic classification vulnerability. The flaw caused the kernel's traffic shaping module (act_skbmod) to incorrectly process non-Ethernet packets (such as CAN frames), leading to silent packet corruption. Apply the kernel security update and reboot your system to complete remediation.

sudo dnf update kernel kernel-devel

Defensia detects this vulnerability

How to check if you are affected

  1. Step 1: Check the running kernel version with `uname -r` and compare against 5.13.6 or later
  2. Step 2: Verify if traffic control (tc) rules with skbmod actions are configured by running `tc filter show dev <interface>` on all network interfaces
  3. Step 3: Inspect system logs for packet corruption symptoms using `journalctl -u kernel --since='1 hour ago' | grep -i 'skb\|corruption\|mac'`
  4. Step 4: After updating, confirm the new kernel version with `uname -r` and test skbmod filters on non-Ethernet interfaces (CAN, etc.) to verify normal packet flow

FAQ

What is CVE-2021-47293?

This vulnerability is a logic error in the Linux kernel's traffic control skbmod action module, which blindly assumed all packets used Ethernet framing. When applied to non-Ethernet devices like CAN interfaces, it silently corrupted packet headers instead of skipping the operation.

Is CVE-2021-47293 being actively exploited?

No, this vulnerability is not listed in CISA's Known Exploited Vulnerabilities (KEV) catalog and has no public exploit code available. However, misconfigured traffic control rules could inadvertently trigger the flaw.

What versions of Kernel are affected by CVE-2021-47293?

Kernel versions 5.11 through 5.13.5 are vulnerable. The fix was released in 5.13.6 and has been backported to stable branches.

How do I check if my server is vulnerable to CVE-2021-47293?

Run `uname -r` to check your kernel version. If it reports 5.11.x through 5.13.5, you are vulnerable. Additionally, check if traffic control rules with skbmod actions target non-Ethernet interfaces using `tc filter show`.

Does Defensia detect CVE-2021-47293?

Yes — Defensia's CVE advisory scanner compares installed package versions against the NVD database. If Kernel is installed on a monitored server, CVE-2021-47293 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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