CVE-2021-23839Network attack vector

SSLv2 rollback acceptance via inverted RSA padding check

Published Feb 16, 2021 · Updated Sep 17, 2024

Incorrect rollback protection in OpenSSL 1.0.2s through 1.0.2x allows remote attackers to bypass SSLv2 downgrade rejection via crafted RSA padding. The RSA_padding_check_SSLv23() function in OpenSSL inverts the delimiter-padding test, accepting padding that signals a rollback and rejecting normal SSLv2 padding. Exploitation requires a server built with SSLv2 support, runtime SSLv2 enablement, and configured SSLv2 ciphersuites, all disabled or absent by default; the consequence is protocol-downgrade acceptance and failed normal SSLv2 handshakes.

CVSS severity7.5
High
EPSS probability2.96%
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

Incorrect rollback protection in OpenSSL 1.0.2s through 1.0.2x allows remote attackers to bypass SSLv2 downgrade rejection via crafted RSA padding. The RSA_padding_check_SSLv23() function in OpenSSL inverts the delimiter-padding test, accepting padding that signals a rollback and rejecting normal SSLv2 padding. Exploitation requires a server built with SSLv2 support, runtime SSLv2 enablement, and configured SSLv2 ciphersuites, all disabled or absent by default; the consequence is protocol-downgrade acceptance and failed normal SSLv2 handshakes.

The record

CVE
CVE-2021-23839
Published
Feb 16, 2021
Updated
Sep 17, 2024
Vendor
OpenSSL Project
Product
OpenSSL
Classifications
CWE-327, T1600
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. Feb 16, 2021CVE publishedPublication date reported by the CVE source.
  2. Sep 17, 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?

Compare these published version ranges with your installed build and any vendor patches.

  1. Affected versionversion=Fixed in OpenSSL 1.0.2y (Affected 1.0.2s-1.0.2x)

What conditions does exploitation require?

Attack vectornetwork
Required privilegesunauthenticated

What is affected?

OpenSSL Project · OpenSSLversion=Fixed in OpenSSL 1.0.2y (Affected 1.0.2s-1.0.2x)

Published CVSS scores

7.5NVDCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:N
5.0NVDAV:N/AC:L/Au:N/C:N/I:P/A:N

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)
Vectornetwork
Privilegesunauthenticated
Known exploitationUnconfirmed
Public exploitUnconfirmed

Weakness, pattern, technique

CWE-327Use of a Broken or Risky Cryptographic Algorithm
T1600Weaken Encryption

Public exploit references

No public exploit references are available in this record.

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