CVE-2021-33909

Exp

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.

Published: 2021-07-20 Last update: 2026-06-16 Assigner: [email protected] Source: [email protected]
NVD Status：Modified，CVE State: published
View at NVD, CVE.org, EUVD

CVE-2021-33909 is rated High Exploit Risk (84.4/100): CVSS High severity, with high exploitation likelihood (EPSS 9.81%, 95th percentile). 2 public exploit reference(s) are indexed (Exploit-DB). EPSS rose +8.03% over the last day, indicating growing attacker interest. Public exploits are available—assess exposure, apply mitigations, and prioritize patching.

Risk is dynamic; we continuously reassess and refresh what is shown on this page as upstream context changes.

Public exploit references (Exploit-DB) for CVE-2021-33909

EDB-ID Source Kind Published Link
— nvd_ref exploit_tag Exploit-DB ↗
— nvd_ref exploit_tag Exploit-DB ↗

Exploit prediction scoring system (EPSS) score for CVE-2021-33909

EPSS lead: Daily EPSS estimates relative likelihood of exploitation; percentile ranks this CVE among scored vulnerabilities (higher = more severe relative rank).

# Date Old EPSS score New EPSS score Delta (New - Old)
1 2026-06-15 1.78% 9.81% +8.03%
2 2026-05-13 2.22% 1.78% -0.44%
3 2026-04-01 — 2.22% —

Full EPSS history (61 records total)

Common vulnerability scoring system (CVSS) metrics for CVE-2021-33909

CVSS metrics for this CVE.

Base score Version Severity Vector Exploitability Impact Score source
7.8 3.1 HIGH
CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H Click to expand
Attack vector (AV:L)
They already need access on the box, or another person has to do something wrong; it’s not a remote drive-by.
Attack complexity (AC:L)
Once they can reach the bug, pulling it off is straightforward—no weird race conditions or rare setup.
Privileges required (PR:L)
A normal user session is enough; they don’t have to be admin.
User interaction (UI:N)
Nobody has to click “OK” or open a trap file; it can work without a victim helping.
Scope (S:U)
Damage stays in the same “trust bubble” as the broken component—no big spill into unrelated systems.
Confidentiality (C:H)
Serious risk that confidential data gets exposed in a big way.
Integrity (I:H)
They could widely tamper with or forge data—trust in the data is badly hurt.
Availability (A:H)
Could take the service down hard or make it unusable for people who depend on it.
 1.8 5.9 [email protected]
7.2 2.0 HIGH
AV:L/AC:L/Au:N/C:C/I:C/A:C Click to expand
Access vector (AV:L)
Requires local access to the target system.
Access complexity (AC:L)
Exploitation conditions are straightforward and predictable.
Authentication (AU:N)
No authentication is required.
Confidentiality impact (C:C)
Complete confidentiality impact.
Integrity impact (I:C)
Complete integrity impact.
Availability impact (A:C)
Complete availability impact.
 3.9 10.0 [email protected]

Weakness enumeration for CVE-2021-33909

OS Trackers for CVE-2021-33909

vendor priority summary link
debian not yet assigned CVE-2021-33909 not yet assigned priority: Debian including 1 source packages (linux), 5 status rows across 5 suites (bookworm, bullseye, forky, sid, trixie): resolved 5. https://security-tracker.debian.org/tracker/CVE-2021-33909
redhat high — https://access.redhat.com/security/cve/CVE-2021-33909
suse high — https://www.suse.com/security/cve/CVE-2021-33909/
ubuntu high CVE-2021-33909 high priority: Ubuntu including 168 source packages (linux, linux-allwinner, …), 2165 status rows across 16 suites (bionic, focal, groovy, hirsute, impish, jammy, kinetic, lunar, mantic, noble, oracular, plucky, questing, trusty, upstream, xenial): DNE 1681, released 230, not-affected 213, ignored 41. https://ubuntu.com/security/CVE-2021-33909

Affected software / configurations for CVE-2021-33909

Vendor Product Version Raw CPE
linux linux_kernel >= 3.12.43, < 3.13 cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:*
linux linux_kernel >= 3.16, < 4.4.276 cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:*
linux linux_kernel >= 4.5, < 4.9.276 cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:*
linux linux_kernel >= 4.10, < 4.14.240 cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:*
linux linux_kernel >= 4.15, < 4.19.198 cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:*
linux linux_kernel >= 4.20, < 5.4.134 cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:*
linux linux_kernel >= 5.5, < 5.10.52 cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:*
linux linux_kernel >= 5.11, < 5.12.19 cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:*
linux linux_kernel >= 5.13, < 5.13.4 cpe:2.3:o:linux:linux_kernel:*:*:*:*:*:*:*:*
fedoraproject fedora 34 cpe:2.3:o:fedoraproject:fedora:34:*:*:*:*:*:*:*
debian debian_linux 9.0 cpe:2.3:o:debian:debian_linux:9.0:*:*:*:*:*:*:*
debian debian_linux 10.0 cpe:2.3:o:debian:debian_linux:10.0:*:*:*:*:*:*:*
netapp hci_management_node — cpe:2.3:a:netapp:hci_management_node:-:*:*:*:*:*:*:*
netapp solidfire — cpe:2.3:a:netapp:solidfire:-:*:*:*:*:*:*:*
oracle communications_session_border_controller 8.2 cpe:2.3:a:oracle:communications_session_border_controller:8.2:*:*:*:*:*:*:*
oracle communications_session_border_controller 8.3 cpe:2.3:a:oracle:communications_session_border_controller:8.3:*:*:*:*:*:*:*
oracle communications_session_border_controller 8.4 cpe:2.3:a:oracle:communications_session_border_controller:8.4:*:*:*:*:*:*:*
oracle communications_session_border_controller 9.0 cpe:2.3:a:oracle:communications_session_border_controller:9.0:*:*:*:*:*:*:*
sonicwall sma1000_firmware <= 12.4.2-02044 cpe:2.3:o:sonicwall:sma1000_firmware:*:*:*:*:*:*:*:*

References for CVE-2021-33909

URL Tags
http://packetstormsecurity.com/files/163621/Sequoia-A-Deep-Root-In-Linuxs-Filesystem-Layer.html Exploit Third Party Advisory VDB Entry
http://packetstormsecurity.com/files/163671/Kernel-Live-Patch-Security-Notice-LSN-0079-1.html Third Party Advisory VDB Entry
http://packetstormsecurity.com/files/164155/Kernel-Live-Patch-Security-Notice-LSN-0081-1.html Third Party Advisory VDB Entry
http://packetstormsecurity.com/files/165477/Kernel-Live-Patch-Security-Notice-LSN-0083-1.html Third Party Advisory VDB Entry
http://www.openwall.com/lists/oss-security/2021/07/22/7 Mailing List Third Party Advisory
http://www.openwall.com/lists/oss-security/2021/08/25/10 Mailing List Third Party Advisory
http://www.openwall.com/lists/oss-security/2021/09/17/2 Mailing List Third Party Advisory
http://www.openwall.com/lists/oss-security/2021/09/17/4 Mailing List Third Party Advisory
http://www.openwall.com/lists/oss-security/2021/09/21/1 Mailing List Third Party Advisory
https://cdn.kernel.org/pub/linux/kernel/v5.x/ChangeLog-5.13.4 Mailing List Patch Vendor Advisory
https://github.com/torvalds/linux/commit/8cae8cd89f05f6de223d63e6d15e31c8ba9cf53b Patch Third Party Advisory
https://lists.debian.org/debian-lts-announce/2021/07/msg00014.html Mailing List Third Party Advisory
https://lists.debian.org/debian-lts-announce/2021/07/msg00015.html Mailing List Third Party Advisory
https://lists.debian.org/debian-lts-announce/2021/07/msg00016.html Mailing List Third Party Advisory
https://lists.fedoraproject.org/archives/list/package-announce%40lists.fedoraproject.org/message/Z4UHHIGISO3FVRF4CQNJS4IKA25ATSFU/
https://psirt.global.sonicwall.com/vuln-detail/SNWLID-2022-0015 Third Party Advisory
https://security.netapp.com/advisory/ntap-20210819-0004/ Third Party Advisory
https://www.debian.org/security/2021/dsa-4941 Third Party Advisory
https://www.openwall.com/lists/oss-security/2021/07/20/1 Exploit Mailing List Third Party Advisory
https://www.oracle.com/security-alerts/cpujan2022.html Patch Third Party Advisory
cvelogic Threat Intelligence