1.2.5.3 adds a new version of the netty library (3.4.2 -> 3.5.11).
Recently, 1.2.5.1 was released with just a security fix and 1.2.6 with all fixes sinces 1.2.5. Same for 1.2.5.2 and 1.2.7.
Since then, the 1.2.5.3 was released with only the netty upgrade. There wasn't a 1.2.8 releases with that containing all other fixes. You should check the issue Carsten mentioned to check the commits on both versions to make sure you've got what you need with 1.2.7 because as far as I can see, the netty upgrade is not yet in 1.2.7