Vulnerability Note VU#412478

OpenSSL implements the Secure Sockets Layer (SSL) and Transport Layer Security (TLS) protocols and includes a general purpose cryptographic library. SSL and TLS are commonly used to provide authentication, encryption, integrity, and non-repudiation services to network applications such as HTTP, IMAP, POP3, LDAP, and others. Clients and servers exchange authentication information in X.509 certificates. While the SSL and TLS protocols are not directly based on ASN.1, they do rely on ASN.1 objects used in X.509 certificates and other cryptographic elements (e.g. PKCS#1 encoded RSA values).

OpenSSL 0.9.6k does not properly handle certain ASN.1 sequences. As a result, OpenSSL performs a recursive function call that could exhaust system resources and crash the process using the OpenSSL library. This denial-of-service condition has only been observed on Microsoft Windows platforms. OpenSSL 0.9.7 is not affected.

From the OpenSSL advisory:

A bug in OpenSSL 0.9.6 would cause certain ASN.1 sequences to trigger
a large recursion.  On platforms such as Windows this large recursion
cannot be handled correctly and so the bug causes OpenSSL to crash.  A
remote attacker could exploit this flaw if they can send arbitrary
ASN.1 sequences which would cause OpenSSL to crash.  This could be
performed for example by sending a client certificate to a SSL/TLS
enabled server which is configured to accept them.

By providing specially crafted ASN.1 encoded data to a vulnerable system, a remote attacker could cause a denial of service by consuming system resources. This has only been observed on Microsoft Windows systems. One potential attack vector is a client certificate message containing specially crafted X.509 certificates. Note that OpenSSL versions 0.9.7c or 0.9.6k and above do not accept unsolicited client certificates (VU#732952).

Upgrade or Patch
Upgrade to OpenSSL 0.9.7c or 0.9.6l. Alternatively, upgrade or apply a patch as specified by your vendor. Further information is available in an advisory from OpenSSL. Note that it is necessary to recompile any applications that are statically linked to OpenSSL libraries.