SILC Toolkit 1.1.8 fixed following issue:

  http://silcnet.org/docs/changelog/SILC%20Toolkit%201.1.8

    ASN1: Fix stack variable overwrite when encoding OID.
    
    The call to sscanf specifies a format string of "%lu", a long unsigned
    int.  The pointer argument was cast to unsigned long *, but this is
    wrong for 64 bit systems.  On 64 bit systems, unsigned long is 64 bits,
    but the oid value is a SilcUInt32 on all systems.  As a result, sscanf
    will overwrite a neighboring variable on the stack.  Fix this by
    changing the format string to "%u" and removing the cast.

Upstream fix:
http://git.silcnet.org/gitweb/?p=silc.git;a=commitdiff;h=ebfe5dc8641467efea3506a4797a2b1260b2da55

This problem is already fixed in Fedora libsilc packages, which are based on fixed 1.1.8 version.

Version of libsilc shipped in Red Hat Enterprise Linux 4 and 5 do not contain affected code and hence are not affected by this problem.