Have the security vulnerabilities found in Bitcoin been fixed in other altcoins? This page maintains a list of vulnerabilities found in Bitcoin, and when they were fixed:

https://en.bitcoin.it/wiki/Common_Vulnerabilities_and_Exposures

Are major altcoins, being based off of the same code, vulnerable to the same attacks? If so, have they been patched?
Generally not. It's very difficult to audit the state of most altcoins just due to the ridiculous number of them in existence, and the fact that many of them intentionally sever the history by rebuilding the Bitcoin repo from the files rather than properly forking the Git repository. 

A large number of Proof of Stake altcoins were forked from 0.6 source code and remain using BerkeleyDB for block chain state storage rather than the LevelDB UTXO used in modern Bitcoin Core. Most Litecoin forks seem to have been made around Bitcoin 0.7 and maintain some of its eccentricities as well. Some more popular forks have back ported fixes or brought their forks to be somewhat level to Bitcoin, but these are the underwhelming majority. It's unclear if they have applied all the security patches they should have, or if they did them correctly given the differing source and histories.  

From a quick glance at a small sample of some of the most popular forks listed on coinkmarketcap:


Litecoin - mostly in line with Bitcoin Core, with some patches cherry picked from upstream 
DarkCoin - forked from Litecoin but does not appear to backport any patches. 
DogeCoin - forked from Litecoin, with some patches cherry picked from upstream.
PeerCoin - appears to be a 0.6 or 0.7 vintage fork of Bitcoin, limited backported patches. 
NameCoin - ancient fork of Bitcoin 0.3, very limited back porting of patches. 


Most other altcoins will be the same or worse in terms of security updates of known bugs. It's a very real risk forking Bitcoin if you are unable to maintain it, and many of the people creating forks are completely ill equipped for the task.