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check_chain_extensions: Do not override error return value by check_c…
…urve 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 with explicitly encoded elliptic curve parameters in the chain was added to the strict checks. 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 a subsequent check that the certificate is consistent with that purpose also checks that it is a valid CA. 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 overriden by an application. Affected applications explicitly set the X509_V_FLAG_X509_STRICT verification flag and either do not set a purpose for the certificate verification or, in the case of TLS client or server applications, override the default purpose to make it not set. CVE-2021-3450 Reviewed-by: Matt Caswell <firstname.lastname@example.org> Reviewed-by: Paul Dale <email@example.com>
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