The MessageDigest.isEqual function in Java Runtime Environment (JRE) in Sun Java SE in JDK and JRE 5.0 before Update 22, JDK and JRE 6 before Update 17, SDK and JRE 1.3.x before 1.3.1_27, and SDK and JRE 1.4.x before 1.4.2_24 allows remote attackers to spoof HMAC-based digital signatures, and possibly bypass authentication, via unspecified vectors related to "timing attack vulnerabilities," aka Bug Id 6863503.