CVE-2023-3446: Excessive time spent checking DH keys and parameters

Excessive time spent checking DH keys and parameters

— Microsoft

Issue summary: Checking excessively long DH keys or parameters may be very slow.

Impact summary: Applications that use the functions DHcheck(), DHcheckex() or EVPPKEYparamcheck() to check a DH key or DH parameters may experience long delays. Where the key or parameters that are being checked have been obtained from an untrusted source this may lead to a Denial of Service.

The function DHcheck() performs various checks on DH parameters. One of those checks confirms that the modulus ('p' parameter) is not too large. Trying to use a very large modulus is slow and OpenSSL will not normally use a modulus which is over 10,000 bits in length.

However the DHcheck() function checks numerous aspects of the key or parameters that have been supplied. Some of those checks use the supplied modulus value even if it has already been found to be too large.

An application that calls DHcheck() and supplies a key or parameters obtained from an untrusted source could be vulernable to a Denial of Service attack.

The function DHcheck() is itself called by a number of other OpenSSL functions. An application calling any of those other functions may similarly be affected. The other functions affected by this are DHcheckex() and EVPPKEYparamcheck().

Also vulnerable are the OpenSSL dhparam and pkeyparam command line applications when using the '-check' option.

The OpenSSL SSL/TLS implementation is not affected by this issue. The OpenSSL 3.0 and 3.1 FIPS providers are not affected by this issue.

OpenSSL is vulnerable to a denial of service, caused by a flaw when using the DHcheck(), DHcheckex() or EVPPKEYparamcheck() functions to check a DH key or DH parameters. By sending a specially crafted request using long DH keys or parameters, a remote attacker could exploit this vulnerability to cause long delays, and results in a denial of service condition.

— IBM