Microsoft Windows - Uniscribe Font Processing Multiple Heap Out-of-Bounds and Wild Reads (MS17-011)





Become a Certified Penetration Tester

Enroll in Penetration Testing with Kali Linux and pass the exam to become an Offensive Security Certified Professional (OSCP). All new content for 2020.



Through fuzzing, we have discovered a number of different crashes in the Windows Uniscribe user-mode library, while trying to display text using a corrupted font file or calling documented Uniscribe API functions against such malformed fonts. In this bug, we address a variety of crashes manifested through invalid memory READ accesses. Some of them occur at page boundaries, while other at seemingly valid yet non-mapped addresses. The sheer amount of the crashes makes it very difficult for us to assess the root cause, severity and impact of each of them within a reasonable time-frame. Consequently, we have only performed basic deduplication based on the top-level address of the faulting instruction, and are reporting all of such crashes in this single bug tracker entry.

A summary of the crash locations is as follows:

1	USP10!otlMultiSubstLookup::apply+0xa8
2	USP10!otlSingleSubstLookup::applyToSingleGlyph+0x98
3	USP10!otlSingleSubstLookup::apply+0xa9
4	USP10!otlMultiSubstLookup::getCoverageTable+0x2c
5	USP10!otlMark2Array::mark2Anchor+0x18
6	USP10!GetSubstGlyph+0x2e
7	USP10!BuildTableCache+0x1ca
8	USP10!otlMkMkPosLookup::apply+0x1b4
9	USP10!otlLookupTable::markFilteringSet+0x1a
10	USP10!otlSinglePosLookup::getCoverageTable+0x12
11	USP10!BuildTableCache+0x1e7
12	USP10!otlChainingLookup::getCoverageTable+0x15
13	USP10!otlReverseChainingLookup::getCoverageTable+0x15
14	USP10!otlLigCaretListTable::coverage+0x7
15	USP10!otlMultiSubstLookup::apply+0x99
16	USP10!otlTableCacheData::FindLookupList+0x9
17	USP10!ttoGetTableData+0x4b4
18	USP10!GetSubtableCoverage+0x1ab
19	USP10!otlChainingLookup::apply+0x2d
20	USP10!MergeLigRecords+0x132
21	USP10!otlLookupTable::subTable+0x23
22	USP10!GetMaxParameter+0x53
23	USP10!ApplyLookup+0xc3
24	USP10!ApplyLookupToSingleGlyph+0x6f
25	USP10!ttoGetTableData+0x19f6
26	USP10!otlExtensionLookup::extensionSubTable+0x1d
27	USP10!ttoGetTableData+0x1a77

All of the issues reproduce successfully on Windows 7. It is highly encouraged to enable PageHeap for the test program in order to get reliable repros. It is also necessary to use a custom program which displays all of the font's glyphs at various point sizes, and additionally calls some of the Uniscribe-specific API functions.

Attached is an archive with textual crash excerpts and up to 3 samples per each unique crash.

Proof of Concept: