The wikipedia entry for TIFF does have this to say, which might be the cause of blocking:-
http://en.wikipedia.org/wiki/TIFFExploits
The TIFF file format is unusual in comparison to other image formats, in that it is composed of small descriptor blocks containing offsets into the file which point to the actual pixel image data (composed of bands of pixel rows). This means that incorrect offset values can cause programs to attempt to read erroneous portions of the file or attempt to read past the physical end of file. Like most other image file formats, improperly encoded packet or line lengths within the file can cause poorly written rendering programs to overflow their internal buffers. Properly-written image rendering programs generally avoid such pitfalls.
Multiple buffer overflows have been found in Libtiff.[1][2][3] Some of these have also been used to execute unsigned code on the PlayStation Portable,[4] as well as run third party applications on the iPhone and iPod Touch[citation needed].
