CVE-2021-23839

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).

• CWE-327

• https://cert-portal.siemens.com/productcert/pdf/ssa-637483.pdfPatch, Third Party Advisory
• https://git.openssl.org/gitweb/?p=openssl.git%3Ba=commitdiff%3Bh=30919ab80a478f2d81f2e9acdcca3fa4740cd547
• https://kb.pulsesecure.net/articles/Pulse_Security_Advisories/SA44846Third Party Advisory
• https://security.netapp.com/advisory/ntap-20210219-0009/Third Party Advisory
• https://security.netapp.com/advisory/ntap-20240621-0006/
• https://www.openssl.org/news/secadv/20210216.txtVendor Advisory
• https://www.oracle.com//security-alerts/cpujul2021.htmlPatch, Third Party Advisory
• https://www.oracle.com/security-alerts/cpuApr2021.htmlPatch, Third Party Advisory
• https://www.oracle.com/security-alerts/cpuapr2022.htmlPatch, Third Party Advisory
• https://www.oracle.com/security-alerts/cpuoct2021.htmlPatch, Third Party Advisory
• https://cert-portal.siemens.com/productcert/pdf/ssa-637483.pdfPatch, Third Party Advisory
• https://git.openssl.org/gitweb/?p=openssl.git%3Ba=commitdiff%3Bh=30919ab80a478f2d81f2e9acdcca3fa4740cd547
• https://kb.pulsesecure.net/articles/Pulse_Security_Advisories/SA44846Third Party Advisory
• https://security.netapp.com/advisory/ntap-20210219-0009/Third Party Advisory
• https://security.netapp.com/advisory/ntap-20240621-0006/
• https://www.openssl.org/news/secadv/20210216.txtVendor Advisory
• https://www.oracle.com//security-alerts/cpujul2021.htmlPatch, Third Party Advisory
• https://www.oracle.com/security-alerts/cpuApr2021.htmlPatch, Third Party Advisory
• https://www.oracle.com/security-alerts/cpuapr2022.htmlPatch, Third Party Advisory
• https://www.oracle.com/security-alerts/cpuoct2021.htmlPatch, Third Party Advisory

Published

Feb 16, 2021

Last Modified

Jun 17, 2026

Status

Modified

• How Strix found a critical auth bypass in etcdStrix autonomously discovered a critical authentication bypass in etcd, later designated CVE-2026-33413.
• Autonomous PentestingAI agents that find and validate exploitable vulnerabilities like this one across your applications.
• PR ReviewsPentest every pull request so vulnerable code is caught before it ships to production.
• AI Penetration TestingHow AI-driven penetration testing continuously covers your attack surface.

• CVE-2021-23827Keybase Desktop Client before 5.6.0 on Windows and macOS, an…5.5
• CVE-2021-2383Vulnerability in the MySQL Server product of Oracle MySQL (c…4.9
• CVE-2021-23835An issue was discovered in flatCore before 2.0.0 build 139. …4.9
• CVE-2021-23836An issue was discovered in flatCore before 2.0.0 build 139. …4.8
• CVE-2021-23837An issue was discovered in flatCore before 2.0.0 build 139. …6.5
• CVE-2021-23838An issue was discovered in flatCore before 2.0.0 build 139. …4.8
• CVE-2021-2384Vulnerability in the MySQL Server product of Oracle MySQL (c…4.9
• CVE-2021-23840Calls to EVP_CipherUpdate, EVP_EncryptUpdate and EVP_Decrypt…7.5
• CVE-2021-23841The OpenSSL public API function X509_issuer_and_serial_hash(…5.9
• CVE-2021-23842Communication to the AMC2 uses a state-of-the-art cryptograp…7.1
• CVE-2021-23843The Bosch software tools AccessIPConfig.exe and AmcIpConfig.…7.8
• CVE-2021-23845This vulnerability could allow an attacker to hijack a sessi…8.8