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CVE Record

CVE-2021-23839: Incorrect SSLv2 rollback protection

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).

UnknownCVSS not scoredNot KEV-listedUpdated
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Glexia's TakeAutomated analysismoderate

Security readout for executives and security teams

Plain-English summary

This is a rollback-protection flaw in old OpenSSL 1.0.2 servers that still allow SSLv2. A vulnerable server could accept a downgraded SSLv2 connection when it should reject it. Exposure is narrow because SSLv2 compile-time support, runtime support, and SSLv2 ciphers all must be enabled, none of which are defaults.

Executive priority

Prioritize this for legacy internet-facing systems and vendor appliances. The vulnerability is not broadly exposed by default, but any remaining SSLv2 support is a material security hygiene issue and should be removed or upgraded during legacy-risk remediation.

Technical view

OpenSSL 1.0.2s through 1.0.2x inverted the SSLv2 rollback padding check in RSA_padding_check_SSLv23(). Servers configured for SSLv2 could accept padding that indicates rollback, and reject normal SSLv2 attempts. OpenSSL 1.1.1 lacks SSLv2 protocol support; direct RSA_SSLV23_PADDING use has the bug but is not treated as a security issue there.

Likely exposure

Most modern environments should have low exposure unless they run OpenSSL 1.0.2s-1.0.2x with explicitly enabled SSLv2 support and SSLv2 ciphers. Legacy appliances or vendor-maintained products may need vendor-specific review because downstream advisories exist.

Exploitation context

The source bundle does not show CISA KEV listing or active exploitation. Practical exploitation depends on a reachable service using the rare vulnerable SSLv2 configuration. Treat this as a legacy cryptographic exposure, not a broad modern OpenSSL issue.

Researcher notes

The security-relevant path is OpenSSL 1.0.2 server-side SSLv2 negotiation with RSA_padding_check_SSLv23() rollback padding logic inverted. The bundle provides no CVSS, CWE, or active-exploitation evidence. Be careful separating protocol-level OpenSSL 1.0.2 exposure from the non-security 1.1.1 padding-mode bug.

Mitigation direction

  • Upgrade affected OpenSSL 1.0.2 premium support deployments to 1.0.2y.
  • For other users, upgrade to OpenSSL 1.1.1j or later vendor-supported versions.
  • Disable SSLv2 support and SSLv2 ciphers wherever present.
  • Review vendor advisories for bundled OpenSSL exposure and fixed product releases.

Validation and detection

  • Inventory systems using OpenSSL 1.0.2s through 1.0.2x.
  • Confirm whether OpenSSL was compiled with SSLv2 support.
  • Check runtime TLS configuration for enabled SSLv2 protocol support.
  • Check cipher configuration for explicit SSLv2 ciphersuites.
  • Review Oracle, NetApp, Pulse Secure, and Siemens advisories for affected products.
Prepared
Confidence
high
Sources
8

Public sources used

Generated from the cited source records. This long-tail analysis has not been individually reviewed by a named human.

Potential ATT&CK relevance

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Vulnerability profileCVE Program record
Severity
Unknown
CVSS
Not scored
Known Exploited
No
Published
Official CVE source material

CNA and ADP enrichment extracted from CVE v5

These fields come from the CVE record and ADP containers, not from Glexia's Take. They preserve time-varying source decisions such as CISA SSVC, KEV status, CVSS metrics, and provider references.

0CVSS vectors
0Timeline events
0ADP providers
11Source links

CVSS and timeline data

No CVSS vectors or timeline events were available in the normalized CVE source material.

Source materials

Affected products

Products and packages named in the record

VendorProductVersion / packageStatus
OpenSSLOpenSSLFixed in OpenSSL 1.0.2y (Affected 1.0.2s-1.0.2x)Listed
Weakness

CWE details

No CWE listed

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