CallBackery versions before 0.62.0 for Perl allow denial of service via the request path interpolated as a regular expression under CGI, FastCGI and PSGI. The before_dispatch hook installed by startup() removes PATH_INFO from the end of the request URI with a substitution that interpolates PATH_INFO unquoted. Under CGI, FastCGI and PSGI both values come from the request, so the client chooses both the pattern and the string it is matched against. A short unauthenticated request with a backreference in the path backtracks exponentially and can keep a worker process busy for minutes.
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