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High severityNVD Advisory· Published Mar 25, 2021· Updated Sep 17, 2024

CA certificate check bypass with X509_V_FLAG_X509_STRICT

CVE-2021-3450

Description

The X509_V_FLAG_X509_STRICT flag enables additional security checks of the certificates present in a certificate chain. It is not set by default. Starting from OpenSSL version 1.1.1h a check to disallow certificates in the chain that have explicitly encoded elliptic curve parameters was added as an additional strict check. An error in the implementation of this check meant that the result of a previous check to confirm that certificates in the chain are valid CA certificates was overwritten. This effectively bypasses the check that non-CA certificates must not be able to issue other certificates. If a "purpose" has been configured then there is a subsequent opportunity for checks that the certificate is a valid CA. All of the named "purpose" values implemented in libcrypto perform this check. Therefore, where a purpose is set the certificate chain will still be rejected even when the strict flag has been used. A purpose is set by default in libssl client and server certificate verification routines, but it can be overridden or removed by an application. In order to be affected, an application must explicitly set the X509_V_FLAG_X509_STRICT verification flag and either not set a purpose for the certificate verification or, in the case of TLS client or server applications, override the default purpose. OpenSSL versions 1.1.1h and newer are affected by this issue. Users of these versions should upgrade to OpenSSL 1.1.1k. OpenSSL 1.0.2 is not impacted by this issue. Fixed in OpenSSL 1.1.1k (Affected 1.1.1h-1.1.1j).

AI Insight

LLM-synthesized narrative grounded in this CVE's description and references.

An error in OpenSSL's X509_V_FLAG_X509_STRICT implementation bypasses CA certificate validation, allowing non-CA certificates to issue certificates under specific conditions.

Vulnerability

CVE-2021-3450 is an error in the implementation of a security check introduced in OpenSSL 1.1.1h. The X509_V_FLAG_X509_STRICT flag enables additional certificate chain checks, including a new check that disallows certificates with explicitly encoded elliptic curve parameters. However, the code mistakenly overwrites the result of a prior check that verifies certificates in the chain are valid CA certificates [1]. This flaw allows non-CA certificates to be incorrectly accepted as issuers of other certificates when the strict flag is used.

Exploitation

Exploitation requires an application to explicitly set the X509_V_FLAG_X509_STRICT verification flag and either not set a certificate verification purpose or, for TLS client/server applications, override the default purpose [1]. By default, libssl sets a purpose, which includes a subsequent CA check that still rejects non-CA certificates. Thus, the vulnerability only affects custom applications or those that disable the default purpose [2]. An attacker would need a valid non-CA certificate (e.g., an end-entity certificate) and craft a chain that includes that certificate in an issuing role.

Impact

A successful attack permits a non-CA certificate to act as a certificate authority, meaning it could sign certificates for arbitrary entities [2]. This could undermine trust in TLS connections or other PKI-dependent operations, potentially enabling man-in-the-middle attacks or issuance of fraudulent certificates.

Mitigation

The vulnerability affects OpenSSL versions 1.1.1h through 1.1.1j. It was fixed in OpenSSL 1.1.1k [1]. OpenSSL 1.0.2 is not impacted. Users should upgrade to 1.1.1k or apply a vendor-specific patch. FreeBSD released an advisory with patches [4], and Cisco provided guidance for affected products [2]. No workaround exists other than avoiding the use of the strict flag without a purpose.

References
  1. NVD - CVE-2021-3450
  2. Cisco Security Advisory: Multiple Vulnerabilities in OpenSSL Affecting Cisco Products: March 2021
  3. https://security.FreeBSD.org/advisories/FreeBSD-SA-21:07.openssl.asc

AI Insight generated on May 21, 2026. Synthesized from this CVE's description and the cited reference URLs; citations are validated against the source bundle.

Affected packages

Versions sourced from the GitHub Security Advisory.

PackageAffected versionsPatched versions
openssl-srccrates.io
>= 111.11.0, < 111.15.0111.15.0

Affected products

27

Patches

0

No patches discovered yet.

Vulnerability mechanics

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References

30

News mentions

0

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