Issue summary: Encrypting a CMS message to an EC certificate whose group order is 1 may cause CMS_encrypt() or CMS_encrypt_ex() to loop ind…

Issue summary: Encrypting a CMS message to an EC certificate whose group order
is 1 may cause CMS_encrypt() or CMS_encrypt_ex() to loop indefinitely.

Impact summary: An attacker able to supply the recipient certificate to an
application performing CMS encryption can cause that application to loop
indefinitely, consuming CPU and resulting in Denial of Service.

CWE: CWE-835: Loop with Unreachable Exit Condition ('Infinite Loop')

Description: CMS_encrypt() and CMS_encrypt_ex() encrypt a message to one or
more recipient certificates and, for an EC recipient, generate an ephemeral
key in the recipient's group. If the certificate declares a group order of 1,
random scalar generation can only produce zero. The key generation routine
then retries without limit while waiting for a nonzero private scalar, so the
call never returns.

The OpenSSL TLS and QUIC implementations do not use this path. An application
is exposed only if it calls CMS_encrypt() or CMS_encrypt_ex() with a recipient
certificate whose EC parameters are attacker-controlled. Certificates with
normal EC group orders do not trigger the loop.

OpenSSL 4.0 and later reject unknown explicit EC groups in default builds.
These versions are affected only when configured with the
enable-ec_explicit_curves option.

The result is a hang that consumes a CPU core until the process is
interrupted, a Denial of Service. The severity is Low because the attacker
must supply the recipient certificate and the application must choose to
encrypt to it; ordinary CMS encryption to trusted recipients is unaffected.

The fix makes EC key generation fail when the group has no valid nonzero
private scalar, instead of retrying indefinitely.

FIPS impact: no
The FIPS modules in 4.1, 4.0, 3.5 and 3.0 are not affected. The FIPS provider
does not support arbitrary explicit EC domain parameters, so an order-1 group
is rejected during key import before the vulnerable key generation path is
reached.