# CVE-2021-23839: Incorrect SSLv2 rollback protection > OpenSSL servers with non-default SSLv2 support may incorrectly accept version rollback attacks due to inverted padding logic. - **Severity:** Low 3.7 (`CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:L/A:N`) - **Exploited in the wild:** Not known - **Login needed:** Not needed - **Attackable:** Over the internet - **Victim action needed:** Not needed - **Fix available:** Yes - **Published:** 16 Feb 2021 - **Updated:** 17 Sep 2024 - **Also known as:** ALPINE-CVE-2021-23839, CLEANSTART-2026-DU95001, DEBIAN-CVE-2021-23839, GHSA-rh82-fhxg-rxx8 ## What is CVE-2021-23839? CVE-2021-23839 is a low-severity vulnerability in Openssl Openssl, published on 16 Feb 2021. An attacker can exploit it over the internet without logging in, with no action from the victim. ### Official description 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). Weakness: CWE-327 (Use of a Broken or Risky Cryptographic Algorithm) ## Which versions are affected by CVE-2021-23839? It affects 8 products. Products: Openssl Openssl, Oracle Business Intelligence, Oracle Enterprise Manager for Storage Management, Oracle Enterprise Manager Ops Center, Oracle Graalvm, Oracle Jd Edwards World Security, Oracle Zfs Storage Appliance Kit, Siemens Sinec Ins. ## How do I fix CVE-2021-23839? Upgrade to a fixed version. ## Is CVE-2021-23839 being exploited? There is no known exploitation of CVE-2021-23839 in the wild and no public exploit code. ## Other vulnerabilities in Openssl Openssl - [CVE-2009-3245](https://insights.sbomify.com/CVE-2009-3245/): Null dereference in n/a (critical 10.0) - [CVE-2006-3738](https://insights.sbomify.com/CVE-2006-3738/): Memory corruption in n/a (critical 10.0) - [CVE-2026-63073](https://insights.sbomify.com/CVE-2026-63073/): Untrusted Sender DN Used as Format String in CMP Response Validation (critical 9.8) - [CVE-2016-6309](https://insights.sbomify.com/CVE-2016-6309/): Use after free in n/a (critical 9.8) - [CVE-2016-0799](https://insights.sbomify.com/CVE-2016-0799/): Integer overflow in n/a (critical 9.8) ## Fix prompt for AI coding agents ```text Check this codebase for CVE-2021-23839 and fix it if it is affected. What it is: CVE-2021-23839 is a low-severity vulnerability in Openssl Openssl, published on 16 Feb 2021. An attacker can exploit it over the internet without logging in, with no action from the victim. Exploitation: There is no known exploitation of CVE-2021-23839 in the wild and no public exploit code. Affected product: Openssl Openssl. No package-level version data is available, so check the vendor advisory linked from the reference page. Task: 1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies. 2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it. 3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead. 4. Run the tests and summarise what changed and why. 5. If nothing in the project is affected, say so and change nothing. Reference: https://insights.sbomify.com/CVE-2021-23839/ (Markdown: https://insights.sbomify.com/CVE-2021-23839/index.md) ``` ## Sources - CVE record: https://www.cve.org/CVERecord?id=CVE-2021-23839 - NVD: https://nvd.nist.gov/vuln/detail/CVE-2021-23839 - OSV: https://osv.dev/vulnerability/CVE-2021-23839 --- From sbomify Insights: https://insights.sbomify.com/CVE-2021-23839/ Summaries are generated and may contain mistakes; the official description above is the record.