This is CVE-2013-0977. This vulnerability was first mentioned by i0n1c: "So iOS 6.1.3 beta 2 also fixes the Overlapping Segment Attack against dyld used in evasi0n." and "The Accuvant Labs analysis is incomplete and does not mention this. They do not realize / mention that it is not possible to have a codeless dylib without exploiting dyld."
So a deeper investigation is necessary here.
Apple's description in the iOS 6.1.3 security fixes:
Impact: A local user may be able to execute unsigned code
Description: A state management issue existed in the handling of Mach-O executable files with overlapping segments. This issue was addressed by refusing to load an executable with overlapping segments.