Well, there may be no known issues in this regard, but you must be cautious when using a large open source product like this. I'm talking generally about huge open source project's not specifically ubuntu. Such projects will have thousands of volunteers. The project may be divided into hundreds of modules and each module scrutinized to ensure no unwanted/malicious code is present.
But this fact cannot be guaranteed, one cannot guarantee that each line of code will be reviewed or the person doing the development/reviews has good intentions. Out of the thousands of contributers one could have malicious intentions and could cleverly hide some code which could pass unchecked.
Coming back to ubuntu, it's creative common's license clause 6 states :
"Limitation on Liability. EXCEPT TO THE EXTENT REQUIRED BY APPLICABLE LAW, IN NO EVENT WILL LICENSOR BE LIABLE TO YOU ON ANY LEGAL THEORY FOR ANY SPECIAL, INCIDENTAL, CONSEQUENTIAL, PUNITIVE OR EXEMPLARY DAMAGES ARISING OUT OF THIS LICENSE OR THE USE OF THE WORK, EVEN IF LICENSOR HAS BEEN ADVISED OF THE POSSIBILITY OF SUCH DAMAGES."
Such an incident hasn't occured, but there is still a possibility.
On the otherhand, in closed source products, for example microsoft windows , if people find some intrinsic malicious code in the product, you can sue the proprietor for damages or possible damages, as it would be a violation of their guarantee of a clean product.
All said, you have a better chance of getting malicious code in windows through viruses than the above scenario in open source.
