high CVSS 7.8 EPSS <0.1%

CVE-2021-47355·Kernel vulnerability

In the Linux kernel, the following vulnerability has been resolved: atm: nicstar: Fix possible use-after-free in nicstar_cleanup() This module's remove path calls del_timer(). However, that function does not wait until the timer handler finishes. This means that the timer handler may still be running after the driver's remove function has finished, which would result in a use-after-free. Fix by calling del_timer_sync(), which makes sure the timer handler has finished, and unable to re-schedule itself.

Severity
high
Software
Kernel
Fixed in
5.13.3
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-2021-47355 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: 5.13

Until: 5.13.3

Fixed in: 5.13.3

How to fix this CVE

Update your Linux kernel to version 5.13.3 or later to resolve this use-after-free vulnerability in the ATM nicstar driver. The vulnerability stems from improper timer cleanup during module removal, allowing the timer handler to execute after driver resources are freed. Apply security patches immediately to all systems running affected kernel versions between 5.13 and 5.13.2.

sudo dnf update kernel kernel-devel && sudo reboot

Defensia detects this vulnerability

How to check if you are affected

  1. Step 1: Check current kernel version with: uname -r | grep -E '5\.13\.[0-2]'
  2. Step 2: Verify if ATM nicstar driver is loaded: lsmod | grep nicstar
  3. Step 3: Search kernel logs for timer-related crashes or use-after-free warnings: dmesg | grep -i 'use-after-free\|timer\|nicstar'
  4. Step 4: After patching, confirm kernel version: uname -r and verify it shows 5.13.3 or higher

FAQ

What is CVE-2021-47355?

CVE-2021-47355 is a use-after-free vulnerability in the Linux kernel's ATM nicstar driver caused by improper timer cleanup during module removal. The driver's remove function uses del_timer() instead of del_timer_sync(), allowing the timer handler to access freed memory.

Is CVE-2021-47355 being actively exploited?

No, CVE-2021-47355 is not actively exploited in the wild according to CISA KEV data, and no public exploits are currently available. However, it remains a high-severity vulnerability requiring immediate patching.

What versions of Kernel are affected by CVE-2021-47355?

Linux kernel versions 5.13 through 5.13.2 are affected. The vulnerability was patched in kernel 5.13.3 and later releases.

How do I check if my server is vulnerable to CVE-2021-47355?

Run: uname -r and check if output matches 5.13.[0-2]. Additionally, verify ATM nicstar driver presence with: lsmod | grep nicstar. If both conditions are true, your system is vulnerable.

Does Defensia detect CVE-2021-47355?

Yes — Defensia's CVE advisory scanner compares installed package versions against the NVD database. If Kernel is installed on a monitored server, CVE-2021-47355 will appear in your dashboard with remediation steps.

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References

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