A possible buffer overflow flaw was found in the RealPlayer's / HelixPlayer's function performing URL unescaping (HTTP %-escapes unescaping, e.g. %20 -> space):
Function always assumed % character was always followed by at least two extra hex characters. If no or only one character followed, unescaping function failed to properly detect the end of the URL string (test is done by comparing character on the current position with '\0') and continued processing content of the memory after the end of URL buffer until first '\0' was found. Depending on the content of that memory area, this could lead to buffer over-read or over-write.
Same function exists in player/hxclientkit/src/CHXClientSink.cpp too.
This issue has been addressed in following products:
Red Hat Enterprise Linux 4
Via RHSA-2010:0094 https://rhn.redhat.com/errata/RHSA-2010-0094.html