CVE-2021-43399Public exploit

Process crash via unchecked SSH certificate length

Published Dec 8, 2021 · Updated Aug 4, 2024

Out-of-bounds write in Yubico YubiHSM 2 SDK 2021.08 and earlier allows attackers to crash yubihsm-shell via crafted SSH certificate input. The yh_com_sign_ssh_certificate() function copies a caller-supplied length into a 3072-byte stack buffer without validating the length, so inputs from 3073 to 8192 bytes overwrite the stack. Exploitation requires control of the certificate file processed by yubihsm-shell; hardened builds terminate the process, producing denial of service rather than code execution.

CVSS severity7.5
High
EPSS probability1.37%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

Out-of-bounds write in Yubico YubiHSM 2 SDK 2021.08 and earlier allows attackers to crash yubihsm-shell via crafted SSH certificate input. The yh_com_sign_ssh_certificate() function copies a caller-supplied length into a 3072-byte stack buffer without validating the length, so inputs from 3073 to 8192 bytes overwrite the stack. Exploitation requires control of the certificate file processed by yubihsm-shell; hardened builds terminate the process, producing denial of service rather than code execution.

The record

CVE
CVE-2021-43399
Published
Dec 8, 2021
Updated
Aug 4, 2024
Vendor
Unknown vendor
Product
Unknown product
Classifications
CWE-787, T1203
Attack vector
local
Privileges
authenticated

Timeline

How it unfolded

  1. Dec 8, 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

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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

7.8NVDAV:N/AC:L/Au:N/C:N/I:N/A:C
7.5NVDCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H

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.

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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-787Out-of-bounds Write
T1203Exploitation for Client Execution

Public exploit references

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

Technologies

Your stack

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Your stack

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