Daniel Bleichenbacher recently described an attack on PKCS #1 v1.5
signatures. If an RSA key with exponent 3 is used it may be possible
to forge a PKCS #1 v1.5 signature signed by that key. Implementations
may incorrectly verify the certificate if they are not checking for
excess data in the RSA exponentiation result of the signature.
Since there are CAs using exponent 3 in wide use, and PKCS #1 v1.5 is
used in X.509 certificates, all software that uses OpenSSL to verify
X.509 certificates is potentially vulnerable, as well as any other use
of PKCS #1 v1.5. This includes software that uses OpenSSL for SSL or
TLS.
