HIGH

CVE-2021-33909

Q: What is CVE-2021-33909?

fs/seq_file.c in the Linux kernel 3.16 through 5.13.x before 5.13.4 does not properly restrict seq buffer allocations, leading to an integer overflow, an Out-of-bounds Write, and escalation to root by an unprivileged user, aka CID-8cae8cd89f05.

Q: Is CVE-2021-33909 actively exploited?

Active exploitation of CVE-2021-33909 has not been confirmed. Its EPSS score was 9.7% on 2026-10-19, the estimated probability of exploitation in the next 30 days.

Q: What is the CVSS score for CVE-2021-33909?

CVE-2021-33909 has a CVSS v3 base score of 7.8 (HIGH severity).

CVSS v3

7.8

HIGH

EPSS Score

9.7 %

exploit probability, as of 2026-10-19

CISA KEV

known exploited

Exploitation

—

SSVC status

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Affected and fixed versions

Packages from OSV.dev and products from the vendors' CSAF advisories.

version not verifiedVersions not verified: read as OSV.dev and the vendors publish them, not checked against each vendor's release list.

Packages

kernel-tools-libs-develAlmaLinux:8all versions · fixed in 4.18.0-305.10.2.el8_4

Vendor advisories

CVE-2021-33909 Red Hat Product Security1,348 products fixed · 16 known affected
kernel: size_t-to-int conversion vulnerability in the filesystem layer
kernel as a component of Red Hat Enterprise Linux 7 · kernel-abi-whitelists as a component of Red Hat Enterprise Linux 7 · kernel-alt.src as a component of Red Hat Enterprise Linux 7
ICSA-24-074-07 CISA1 known affected
Siemens SIMATIC
Siemens SIMATIC RF160B (6GT2003-0FA00): <V2.2
SSA-770721 Siemens ProductCERT1 known affected
SSA-770721: Multiple Vulnerabilities in SIMATIC RF160B before V2.2
SIMATIC RF160B (6GT2003-0FA00)
RHSA-2021:2736 Red Hat Product Security2 products fixed
Red Hat Security Advisory: Red Hat Virtualization Host security and bug fix update [ovirt-4.4.7]
RHSA-2021:2737 Red Hat Product Security4 products fixed
Red Hat Security Advisory: RHV-H security update (redhat-virtualization-host) 4.3.17
RHSA-2021:2728 Red Hat Product Security120 products fixed
Red Hat Security Advisory: kernel security update

18 more advisories

Full record on NVD

Description

fs/seq_file.c in the Linux kernel 3.16 through 5.13.x before 5.13.4 does not properly restrict seq buffer allocations, leading to an integer overflow, an Out-of-bounds Write, and escalation to root by an unprivileged user, aka CID-8cae8cd89f05.

Technical details

Published: 2021-07-20

Frequently asked questions

What is CVE-2021-33909?

Is CVE-2021-33909 actively exploited?

Active exploitation of CVE-2021-33909 has not been confirmed. Its EPSS score was 9.7% on 2026-10-19, the estimated probability of exploitation in the next 30 days.

What is the CVSS score for CVE-2021-33909?

CVE-2021-33909 has a CVSS v3 base score of 7.8 (HIGH severity).

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