CVE-2021-33909Public exploit

Root privilege escalation via seq_file integer conversion

Published Jul 20, 2021 · Updated Aug 4, 2024

Integer overflow in the Linux kernel 3.16 through 5.13.x before 5.13.4 allows local users to gain root via filesystem operations. The seq_file buffer-growth path passes a 2 GB size_t length into dentry_path's signed int parameter, causing prepend to write "//deleted" 2 GB before the allocation. A local unprivileged account can trigger the overwrite by mounting and deleting a directory tree longer than 1 GB and reading /proc/self/mountinfo; Qualys demonstrated full root access on default installations.

CVSS severity7.8
High
EPSS probability9.73%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
Coverage for this vulnerability

See if you're affected

Explore vulnerability checks for your environment with Hinoki.

Book a demo

Summary

What happened

Integer overflow in the Linux kernel 3.16 through 5.13.x before 5.13.4 allows local users to gain root via filesystem operations. The seq_file buffer-growth path passes a 2 GB size_t length into dentry_path's signed int parameter, causing prepend to write "//deleted" 2 GB before the allocation. A local unprivileged account can trigger the overwrite by mounting and deleting a directory tree longer than 1 GB and reading /proc/self/mountinfo; Qualys demonstrated full root access on default installations.

The record

CVE
CVE-2021-33909
Published
Jul 20, 2021
Updated
Aug 4, 2024
Vendor
Unknown vendor
Product
Unknown product
Classifications
CWE-190, T1068
Attack vector
local
Privileges
authenticated

Timeline

How it unfolded

  1. Jul 20, 2021CVE publishedPublication date reported by the CVE source.
  2. Aug 4, 2024Record updatedLatest update available in the CVE record.

Exploitability

Present is not the same as exploitable

Compare your product and version with the public record. A matching version still requires validation against your environment.

Is a vulnerable build present?

Affected buildAffected product and version details are unavailable.

What conditions does exploitation require?

Attack vectorlocal
Required privilegesauthenticated

What is affected?

Affected products and versions are unavailable in this record.

Published CVSS scores

7.8NIST NVDCVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H
7.2NIST NVDAV:L/AC:L/Au:N/C:C/I:C/A:C

CVSS describes severity. EPSS estimates exploitation probability.

Attacks

What attackers are doing with it

Daily unique IPs observed by Shadowserver honeypots for known exploited vulnerabilities (KEVs). Missing observations do not establish an absence of attacks.

Daily unique IPsNo honeypot observations are available for this CVE in the selected window.

No observations available

Sep 10, 2026Sep 16, 2026
Latest reporting daySep 16, 2026
Latest daily unique IPsUnavailable
Prior 30-day averageUnavailable
SourceShadowserver honeypots (KEV)
Vectorlocal
Privilegesauthenticated
Known exploitationUnconfirmed
Public exploitPublished

Weakness, pattern, technique

CWE-190Integer Overflow or Wraparound
T1068Exploitation for Privilege Escalation

Public exploit references

Labels summarize the accepted research assessment. They do not indicate a test against your environment.

Technologies

Your stack

See the directory against your own environment.

Your stack

Check the software in your environment

Book a demo to see how Hinoki identifies affected software and validates exploitability in your environment.

Book a demo