With that said - how important do you think security updates are for an OS that is running on a minority architecture on a minority OS? How many attacks would target this particular system? I am not trying to be argumentative - I am just wondering. It looks to me like the malware vulnerabilities in that would be related to an Adobe Flash issue, or perhaps Java.
Architecture has little to do with security. The internet has it's own languages that work with every architecture and OS there is.
The differences are much more at the core of Leopard. Tiger completely lacks modern security implements like socket layer protection and hasn't even had it's already primitive security updated in over 4 years as I noted already. Leopard continues to get security updates because it uses modern security so it's easy for Apple to still make updates for it as they would only slightly vary from Snow Leopard.
These are security features that Leopard added and Tiger lacks:
Library Randomization
Leopard implements library randomization, which randomizes the locations of some libraries in memory. Vulnerabilities that corrupt program memory often rely on known addresses for these library routines, which allow injected code to launch processes or change files. Library randomization is presumably a stepping-stone to a more complete implementation of address space layout randomization at a later date.
Application Layer Firewall
Leopard ships with two firewall engines: the original BSD IPFW, which was present in earlier releases of Mac OS X, and the new Leopard Application Layer Firewall. Unlike IPFW, which intercepts and filters IP datagrams before the kernel performs significant processing, the Application Layer Firewall operates at the socket layer, bound to individual processes. The Application Layer Firewall can therefore make filtering decisions on a per-application basis. Of the two-firewall engines, only the Application Layer Firewall is fully exposed in the Leopard user interface. The new firewall offers less control over individual packet decisions (users can decide to allow or deny connections system wide or to individual applications, but must use IPFW to set fine-grained TCP/IP header level policies). It also makes several policy exceptions for system processes: neither mDNSResponder nor programs running with superuser privileges are filtered.
Sandboxes
Leopard includes kernel-level support for role-based access control (RBAC). RBAC is intended to prevent, for example, an application like Mail from editing the password database.
Application Signing
Leopard provides a framework to use public key signatures for code signing to verify, in some circumstances, that code has not been tampered with. Signatures can also be used to ensure that one program replacing another is truly an "update", and carry any special security privileges across to the new version. This reduces the number of user security prompts, and the likelihood of the user being trained to simply clicking "OK" to everything.
Secure Guest Account
Guests can be given access to a Leopard system with an account that the system erases and resets at logout.
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I notice that you also seem to suffer from at least a little of the shield of invincibility complex many Mac users do. That will hurt you one day even on an Intel machine running Lion. Any OS is vulnerable in some ways and I can't imagine anyone ever making a truly 100% secure OS. What you can do though is use the most current OS you can for the best security solutions.
I certainly don't prefer Leopard because it's worse in any possible way. I prefer it because it's leaps and bounds better than Tiger in many ways that don't even involve security. If people want to use a 6 year old OS to go online then please go ahead but I as a computer tech for 18 years want no part in running Tiger ever again. I would have to give up far to much in terms of security, features and available apps. Leopard may be 4 years old now but it has all the inner workings of a modern OS. Plus on my 1.8GHz Leopard actually runs a bit faster than Tiger.