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Fencer Vulns
low 3.7

CVE-2021-23839 json

Incorrect SSLv2 rollback protection

published 2021-02-16 5 y ago modified 2026-06-17 4 mo ago also EUVD-2021-10767

cvss
3.7
version 3.1, NVD
cisa kev
Not listed
not in CISA's catalog
epss
3.0%
EPSS 3.0% chance of exploitation in 30 days, percentile 87%
public exploits
0
none known

Summary

OpenSSL 1.0.2 supports SSLv2. If a client attempts to negotiate SSLv2 with a server that is configured to support both SSLv2 and more recent SSL and TLS versions then a check is made for a version rollback attack when unpadding an RSA signature. Clients that support SSL or TLS versions greater than SSLv2 are supposed to use a special form of padding. A server that supports greater than SSLv2 is supposed to reject connection attempts from a client where this special form of padding is present, because this indicates that a version rollback has occurred (i.e. both client and server support greater than SSLv2, and yet this is the version that is being requested). The implementation of this padding check inverted the logic so that the connection attempt is accepted if the padding is present, and rejected if it is absent. This means that such as server will accept a connection if a version rollback attack has occurred. Further the server will erroneously reject a connection if a normal SSLv2 connection attempt is made. Only OpenSSL 1.0.2 servers from version 1.0.2s to 1.0.2x are affected by this issue. In order to be vulnerable a 1.0.2 server must: 1) have configured SSLv2 support at compile time (this is off by default), 2) have configured SSLv2 support at runtime (this is off by default), 3) have configured SSLv2 ciphersuites (these are not in the default ciphersuite list) OpenSSL 1.1.1 does not have SSLv2 support and therefore is not vulnerable to this issue. The underlying error is in the implementation of the RSA_padding_check_SSLv23() function. This also affects the RSA_SSLV23_PADDING padding mode used by various other functions. Although 1.1.1 does not support SSLv2 the RSA_padding_check_SSLv23() function still exists, as does the RSA_SSLV23_PADDING padding mode. Applications that directly call that function or use that padding mode will encounter this issue. However since there is no support for the SSLv2 protocol in 1.1.1 this is considered a bug and not a security issue in that version. OpenSSL 1.0.2 is out of support and no longer receiving public updates. Premium support customers of OpenSSL 1.0.2 should upgrade to 1.0.2y. Other users should upgrade to 1.1.1j. Fixed in OpenSSL 1.0.2y (Affected 1.0.2s-1.0.2x).

Patch / fix

  • openssl openssl 1.0.2y
  • oracle graalvm 19.3.6, 20.3.2, 21.1.0
  • oracle graalvm_enterprise 19.3.6, 20.3.2, 21.1.0
  • openssl Debian 14, Debian 13, Debian 12 1.0.0d-1

debian advisories DEBIAN-CVE-2021-23839

  • openssl Alpine 3.24, Alpine 3.23, Alpine 3.22 +13 more 1.1.1j-r0
  • openssl3 Alpine 3.16, Alpine 3.15 1.1.1j-r0
  • openssl1.1-compat Alpine 3.18, Alpine 3.17 1.1.1j-r0

alpine advisories ALPINE-CVE-2021-23839

Alpine secdb © Alpine Linux, from https://secdb.alpinelinux.org via OSV; changed: normalised to fixed package versions per release. CC BY-SA 4.0

Vendor advisories

What the vendors themselves say about their products: affected, fixed or not affected, and what to do.

red hat product security vex CVE-2021-23839 updated cvss 3.7

openssl: incorrect SSLv2 rollback protection

  • Red Hat Advanced Cluster Management for Kubernetes 2 management-ingress-rhel7
    affected: no fix available
  • Red Hat Enterprise Linux 6 openssl, openssl-perl Red Hat JBoss Enterprise Application Platform 6 openssl
    affected: no fix planned “Out of support scope”
  • Red Hat Enterprise Linux 6 openssl098e Red Hat Enterprise Linux 7 openssl, openssl098e, openssl-libs, openssl-perl Red Hat Enterprise Linux 8 openssl, openssl-libs, openssl-perl, compat-openssl10 Red Hat Enterprise Linux 9 openssl, openssl-libs, openssl-perl Red Hat JBoss Core Services jbcs-httpd24-openssl
    not affected: vulnerable code not present

Red Hat CSAF VEX © Red Hat, Inc., from https://security.access.redhat.com/data/csaf/v2/vex/; changed: reduced to product statuses and remediations. CC BY 4.0 · csaf

CISA SSVC

Exploitation
none
Automatable
no
Technical impact
partial
Assessed
2 y ago

Exploit prediction

2.96% chance of exploitation in the next 30 days, higher than 87% of all scored CVEs (FIRST EPSS, 2026-11-08).

CVSS scores

assigned by version score vector
NVD 3.1 3.7 CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:L/A:N
NVD 2.0 4.3 AV:N/AC:M/Au:N/C:N/I:P/A:N

Affected products

  • OpenSSL OpenSSL

Classification

Weakness
  • CWE-327 Use of a Broken or Risky Cryptographic Algorithm
Technology feeds

References

Sources

Fencer Vulns keeps one entry per vulnerability: the CVE id wins, then GHSA, and records that alias each other are merged, except that two CVEs always keep separate entries. The same data is available as JSON.

CPEs and fixed versions use Anchore's correction of NVD's data: Improve version ranges to indicate fix.