A vulnerability in the way OpenSSL handles ASN.1 elements could allow a remote attacker to cause a denial of service on systems running Microsoft Windows.
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.
A bug in OpenSSL 0.9.6 would cause certain ASN.1 sequences to triggera large recursion. On platforms such as Windows this large recursioncannot be handled correctly and so the bug causes OpenSSL to crash. Aremote attacker could exploit this flaw if they can send arbitraryASN.1 sequences which would cause OpenSSL to crash. This could beperformed for example by sending a client certificate to a SSL/TLSenabled server which is configured to accept them.
The U.K. National Infrastructure Security Co-ordination Centre (NISCC) is tracking this vulnerability as NISCC/006489/openssl2.
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
This vulnerability was discoverd by NISCC and Novell. Thanks to NISCC and OpenSSL for information used in this document.
This document was written by Art Manion.
|Date First Published:||2003-11-04|
|Date Last Updated:||2003-11-05 22:18 UTC|