What is CVE-2021-3491?

CVE-2021-3491 is a high-severity vulnerability in Linux Kernel, classified under Incorrect Calculation of Buffer Size. CVSS score: 7.8/10. Published 2021-06-04.

How severe is CVE-2021-3491?

High severity. CVSS v3 base score is 7.8 out of 10.

Buffer overflow in Linux Kernel

CVE-2021-3491

The io_uring subsystem in the Linux kernel allowed the MAX_RW_COUNT limit to be bypassed in the PROVIDE_BUFFERS operation, which led to negative values being usedin mem_rw when reading /proc/<PID>/mem. This could be used to create a heap overflow leading to arbitrary code execution in the kernel. It was addressed via commit d1f82808877b ("io_uring: truncate lengths larger than MAX_RW_COUNT on provide buffers") (v5.13-rc1) and backported to the stable kernels in v5.12.4, v5.11.21, and v5.10.37. It was introduced in ddf0322db79c ("io_uring: add IORING_OP_PROVIDE_BUFFERS") (v5.7-rc1).

Published 2021-06-04 · last modified 2026-06-17.

CVSS v3 metric

CVSS v3 base score 7.8 (High). Vector: CVSS:3.1/AV:L/AC:H/PR:L/UI:N/S:C/C:H/I:H/A:H.

EPSS exploit prediction

EPSS: 0.006 (49.9th percentile), scored 2026-10-19.

Very low probability of exploitation in the next 30 days; routine patching cadence is appropriate. 50th percentile — 49.9% of CVEs in the catalogue have a lower EPSS than this one. How to read EPSS.

EPSS over last 30 days · oldest: 0.006 · newest: 0.006 · change: 0.000

Affected products

Linux Kernel — versions trunk, linux-5.12.y, linux-5.11.y
Linux Kernel
Canonical Ubuntu Linux — versions 20.04, 20.10, 21.04

Weakness classification (CWE)

References

ubuntu.com/security/notices/USN-4950-1 (vendor-advisory, Third Party Advisory)
ubuntu.com/security/notices/USN-4949-1 (vendor-advisory, Third Party Advisory)
www.openwall.com/lists/oss-security/2021/05/11/13 (mailing-list, Patch, Mailing List, Third Party Advisory)
git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/commit/ (Patch, Vendor Advisory)
www.zerodayinitiative.com/advisories/ZDI-21-589/ (VDB Entry, Third Party Advisory)
security.netapp.com/advisory/ntap-20210716-0004/ (Third Party Advisory)

Frequently asked questions

What is CVE-2021-3491?: CVE-2021-3491 is a high-severity vulnerability in Linux Kernel, classified under Incorrect Calculation of Buffer Size. CVSS score: 7.8/10. Published 2021-06-04.
How severe is CVE-2021-3491?: High severity. CVSS v3 base score is 7.8 out of 10.