Perl-Compatible Regular Expression (PCRE) library before 6.7 does not properly calculate the compiled memory allocation for regular expressions that involve a quantified "subpattern containing a named recursion or subroutine reference," which allows context-dependent attackers to cause a denial of service (error or crash).

More... (http://nvd.nist.gov/nvd.cfm?cvename=CVE-2006-7226)