The dl module in Ruby 1.8.5 and earlier, 1.8.6 through 1.8.6-p286, 1.8.7 through 1.8.7-p71, and 1.9 through r18423 does not check "taintness" of inputs, which allows context-dependent attackers to bypass safe levels and execute dangerous functions by accessing a library using DL.dlopen.
The product receives input or data, but it does not validate or incorrectly validates that the input has the properties that are required to process the data safely and correctly.