Related Weaknesses
| CWE-ID |
Weakness Name |
Source |
| CWE Other |
No informations. |
|
Metrics
| Metrics |
Score |
Severity |
CVSS Vector |
Source |
| V3.0 |
8.8 |
HIGH |
CVSS:3.0/AV:N/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:H
Base: Exploitabilty MetricsThe Exploitability metrics reflect the characteristics of the thing that is vulnerable, which we refer to formally as the vulnerable component. Attack Vector This metric reflects the context by which vulnerability exploitation is possible. A vulnerability exploitable with network access means the vulnerable component is bound to the network stack and the attacker's path is through OSI layer 3 (the network layer). Such a vulnerability is often termed 'remotely exploitable' and can be thought of as an attack being exploitable one or more network hops away (e.g. across layer 3 boundaries from routers). Attack Complexity This metric describes the conditions beyond the attacker's control that must exist in order to exploit the vulnerability. Specialized access conditions or extenuating circumstances do not exist. An attacker can expect repeatable success against the vulnerable component. Privileges Required This metric describes the level of privileges an attacker must possess before successfully exploiting the vulnerability. The attacker is unauthorized prior to attack, and therefore does not require any access to settings or files to carry out an attack. User Interaction This metric captures the requirement for a user, other than the attacker, to participate in the successful compromise of the vulnerable component. Successful exploitation of this vulnerability requires a user to take some action before the vulnerability can be exploited. For example, a successful exploit may only be possible during the installation of an application by a system administrator. Base: Scope MetricsAn important property captured by CVSS v3.0 is the ability for a vulnerability in one software component to impact resources beyond its means, or privileges. Scope Formally, Scope refers to the collection of privileges defined by a computing authority (e.g. an application, an operating system, or a sandbox environment) when granting access to computing resources (e.g. files, CPU, memory, etc). These privileges are assigned based on some method of identification and authorization. In some cases, the authorization may be simple or loosely controlled based upon predefined rules or standards. For example, in the case of Ethernet traffic sent to a network switch, the switch accepts traffic that arrives on its ports and is an authority that controls the traffic flow to other switch ports. An exploited vulnerability can only affect resources managed by the same authority. In this case the vulnerable component and the impacted component are the same. Base: Impact MetricsThe Impact metrics refer to the properties of the impacted component. Confidentiality Impact This metric measures the impact to the confidentiality of the information resources managed by a software component due to a successfully exploited vulnerability. There is total loss of confidentiality, resulting in all resources within the impacted component being divulged to the attacker. Alternatively, access to only some restricted information is obtained, but the disclosed information presents a direct, serious impact. For example, an attacker steals the administrator's password, or private encryption keys of a web server. Integrity Impact This metric measures the impact to integrity of a successfully exploited vulnerability. Integrity refers to the trustworthiness and veracity of information. There is a total loss of integrity, or a complete loss of protection. For example, the attacker is able to modify any/all files protected by the impacted component. Alternatively, only some files can be modified, but malicious modification would present a direct, serious consequence to the impacted component. Availability Impact This metric measures the impact to the availability of the impacted component resulting from a successfully exploited vulnerability. There is total loss of availability, resulting in the attacker being able to fully deny access to resources in the impacted component; this loss is either sustained (while the attacker continues to deliver the attack) or persistent (the condition persists even after the attack has completed). Alternatively, the attacker has the ability to deny some availability, but the loss of availability presents a direct, serious consequence to the impacted component (e.g., the attacker cannot disrupt existing connections, but can prevent new connections; the attacker can repeatedly exploit a vulnerability that, in each instance of a successful attack, leaks a only small amount of memory, but after repeated exploitation causes a service to become completely unavailable). Temporal MetricsThe Temporal metrics measure the current state of exploit techniques or code availability, the existence of any patches or workarounds, or the confidence that one has in the description of a vulnerability. Environmental Metrics
|
nvd@nist.gov |
| V2 |
9.3 |
|
AV:N/AC:M/Au:N/C:C/I:C/A:C |
nvd@nist.gov |
EPSS
EPSS is a scoring model that predicts the likelihood of a vulnerability being exploited.
EPSS Score
The EPSS model produces a probability score between 0 and 1 (0 and 100%). The higher the score, the greater the probability that a vulnerability will be exploited.
EPSS Percentile
The percentile is used to rank CVE according to their EPSS score. For example, a CVE in the 95th percentile according to its EPSS score is more likely to be exploited than 95% of other CVE. Thus, the percentile is used to compare the EPSS score of a CVE with that of other CVE.
Exploit information
Exploit Database EDB-ID : 47095
Publication date : 2019-07-09 22h00 +00:00
Author : Google Security Research
EDB Verified : Yes
-----=====[ Background ]=====-----
AFDKO (Adobe Font Development Kit for OpenType) is a set of tools for examining, modifying and building fonts. The core part of this toolset is a font handling library written in C, which provides interfaces for reading and writing Type 1, OpenType, TrueType (to some extent) and several other font formats. While the library existed as early as 2000, it was open-sourced by Adobe in 2014 on GitHub [1, 2], and is still actively developed. The font parsing code can be generally found under afdko/c/public/lib/source/*read/*.c in the project directory tree.
At the time of this writing, based on the available source code, we conclude that AFDKO was originally developed to only process valid, well-formatted font files. It contains very few to no sanity checks of the input data, which makes it susceptible to memory corruption issues (e.g. buffer overflows) and other memory safety problems, if the input file doesn't conform to the format specification.
We have recently discovered that starting with Windows 10 1709 (Fall Creators Update, released in October 2017), Microsoft's DirectWrite library [3] includes parts of AFDKO, and specifically the modules for reading and writing OpenType/CFF fonts (internally called cfr/cfw). The code is reachable through dwrite!AdobeCFF2Snapshot, called by methods of the FontInstancer class, called by dwrite!DWriteFontFace::CreateInstancedStream and dwrite!DWriteFactory::CreateInstancedStream. This strongly indicates that the code is used for instancing the relatively new variable fonts [4], i.e. building a single instance of a variable font with a specific set of attributes. The CreateInstancedStream method is not a member of a public COM interface, but we have found that it is called by d2d1!dxc::TextConvertor::InstanceFontResources, which led us to find out that it can be reached through the Direct2D printing interface. It is unclear if there are other ways to trigger the font instancing functionality.
One example of a client application which uses Direct2D printing is Microsoft Edge. If a user opens a specially crafted website with an embedded OpenType variable font and decides to print it (to PDF, XPS, or another physical or virtual printer), the AFDKO code will execute with the attacker's font file as input. Below is a description of one such security vulnerability in Adobe's library exploitable through the Edge web browser.
-----=====[ Description ]=====-----
The readCharset() function in afdko/c/public/lib/source/cffread/cffread.c is designed to read and parse the charset information of an input OpenType font. It is called by cfrBegFont(), the standard entry point function for the "cfr" (CFF Reader) module of AFDKO. The relevant part of the function is shown below:
--- cut ---
[...]
2179 case 1:
2180 size = 1;
2181 /* Fall through */
2182 case 2:
2183 while (gid < h->glyphs.cnt) {
2184 unsigned short id = read2(h);
2185 long nLeft = readN(h, size);
2186 while (nLeft-- >= 0)
2187 addID(h, gid++, id++);
2188 }
2189 break;
--- cut ---
whereas addID() is defined as follows:
--- cut ---
1838 /* Add SID/CID to charset */
1839 static void addID(cfrCtx h, long gid, unsigned short id) {
1840 abfGlyphInfo *info = &h->glyphs.array[gid];
1841 if (h->flags & CID_FONT)
1842 /* Save CID */
1843 info->cid = id;
1844 else {
1845 /* Save SID */
1846 info->gname.impl = id;
1847 info->gname.ptr = sid2str(h, id);
1848
1849 /* Non-CID font so select FD[0] */
1850 info->iFD = 0;
[...]
1859 }
1860 }
--- cut ---
The readCharset() routine doesn't consider the size of the h->glyphs array and writes to it solely based on the charset information. If the value read from the input stream in line 2185 exceeds the number of glyphs in the font, the array may be overflown in addID() (line 1843 or 1846, 1847, 1850), corrupting adjacent objects on the heap. The h->glyphs array is initialized in readCharStringsINDEX() according to the number of CharStrings found in the font:
--- cut ---
1791 dnaSET_CNT(h->glyphs, index.count);
--- cut ---
-----=====[ Proof of Concept ]=====-----
The proof of concept font contains a charset descriptor with the following initial values:
- width = 0x02 (changed from 0x01)
- id = 0x4141 (changed from 0x0001)
- nLeft = 0xffff (changed from 0x15)
By increasing the value of "nLeft" from 21 to 65535, we cause the loop in lines 2386-2387 to go largely out of bounds and overflow the h->glyphs array.
In theory, the vulnerability shouldn't be possible to reach in Microsoft DirectWrite and its client applications, because AFDKO is only used there for instancing variable fonts, whereas such CFF2 fonts follow another execution path in the readCharset() function:
--- cut ---
2138 if (h->header.major == 2) {
2139 postRead(h);
2140 if (h->cff2.mvar)
2141 MVARread(h);
2142 if (!(h->flags & CID_FONT))
2143 readCharSetFromPost(h);
2144 else {
2145 long gid;
2146 for (gid = 0; gid < h->glyphs.cnt; gid++) {
2147 abfGlyphInfo *info = &h->glyphs.array[gid];
2148 info->cid = (unsigned short)gid;
2149 }
2150 }
2151 return;
2152 }
--- cut ---
However, we have found that it is in fact possible to trigger the handling of CFFv1 in AFDKO, by appending the old style "CFF " table to a variable font which already includes a "CFF2" one. This causes DirectWrite to correctly load the variable font, but AFDKO still finds "CFF " first and passes it for further parsing, thanks to the following logic in srcOpen() (afdko/c/public/lib/source/cffread/cffread.c):
--- cut ---
561 /* OTF; use CFF table offset */
562 sfrTable *table =
563 sfrGetTableByTag(h->ctx.sfr, CTL_TAG('C', 'F', 'F', ' '));
564 if (table == NULL) {
565 table = sfrGetTableByTag(h->ctx.sfr, CTL_TAG('C', 'F', 'F', '2'));
566 }
567 if (table == NULL)
568 fatal(h, cfrErrNoCFF);
569 origin = table->offset;
--- cut ---
-----=====[ Crash logs ]=====-----
A 64-bit build of "tx" compiled with AddressSanitizer, started with ./tx -cff poc.otf prints out the following report:
--- cut ---
=================================================================
==236657==ERROR: AddressSanitizer: heap-buffer-overflow on address 0x62a00000b228 at pc 0x0000005563be bp 0x7ffe3c238d10 sp 0x7ffe3c238d08
WRITE of size 2 at 0x62a00000b228 thread T0
#0 0x5563bd in addID afdko/c/public/lib/source/cffread/cffread.c:1843:19
#1 0x53f71c in readCharset afdko/c/public/lib/source/cffread/cffread.c:2187:29
#2 0x5299c7 in cfrBegFont afdko/c/public/lib/source/cffread/cffread.c:2789:9
#3 0x50928d in cfrReadFont afdko/c/tx/source/tx.c:137:9
#4 0x508cc3 in doFile afdko/c/tx/source/tx.c:429:17
#5 0x506b2e in doSingleFileSet afdko/c/tx/source/tx.c:488:5
#6 0x4fc91e in parseArgs afdko/c/tx/source/tx.c:558:17
#7 0x4f9470 in main afdko/c/tx/source/tx.c:1631:9
#8 0x7f7bf34352b0 in __libc_start_main
#9 0x41e5b9 in _start
0x62a00000b228 is located 40 bytes to the right of 20480-byte region [0x62a000006200,0x62a00000b200)
allocated by thread T0 here:
#0 0x4c63f3 in __interceptor_malloc
#1 0x6c9ac2 in mem_manage afdko/c/public/lib/source/tx_shared/tx_shared.c:73:20
#2 0x5474a4 in dna_manage afdko/c/public/lib/source/cffread/cffread.c:271:17
#3 0x7de64e in dnaGrow afdko/c/public/lib/source/dynarr/dynarr.c:86:23
#4 0x7dec75 in dnaSetCnt afdko/c/public/lib/source/dynarr/dynarr.c:119:13
#5 0x53e6fa in readCharStringsINDEX afdko/c/public/lib/source/cffread/cffread.c:1791:5
#6 0x5295be in cfrBegFont afdko/c/public/lib/source/cffread/cffread.c:2769:9
#7 0x50928d in cfrReadFont afdko/c/tx/source/tx.c:137:9
#8 0x508cc3 in doFile afdko/c/tx/source/tx.c:429:17
#9 0x506b2e in doSingleFileSet afdko/c/tx/source/tx.c:488:5
#10 0x4fc91e in parseArgs afdko/c/tx/source/tx.c:558:17
#11 0x4f9470 in main afdko/c/tx/source/tx.c:1631:9
#12 0x7f7bf34352b0 in __libc_start_main
SUMMARY: AddressSanitizer: heap-buffer-overflow afdko/c/public/lib/source/cffread/cffread.c:1843:19 in addID
Shadow bytes around the buggy address:
0x0c547fff95f0: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
0x0c547fff9600: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
0x0c547fff9610: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
0x0c547fff9620: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
0x0c547fff9630: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00
=>0x0c547fff9640: fa fa fa fa fa[fa]fa fa fa fa fa fa fa fa fa fa
0x0c547fff9650: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
0x0c547fff9660: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
0x0c547fff9670: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
0x0c547fff9680: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
0x0c547fff9690: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa
Shadow byte legend (one shadow byte represents 8 application bytes):
Addressable: 00
Partially addressable: 01 02 03 04 05 06 07
Heap left redzone: fa
Freed heap region: fd
Stack left redzone: f1
Stack mid redzone: f2
Stack right redzone: f3
Stack after return: f5
Stack use after scope: f8
Global redzone: f9
Global init order: f6
Poisoned by user: f7
Container overflow: fc
Array cookie: ac
Intra object redzone: bb
ASan internal: fe
Left alloca redzone: ca
Right alloca redzone: cb
Shadow gap: cc
==236657==ABORTING
--- cut ---
A non-instrumented version of "tx" crashes with a SIGSEGV when it reaches an unmapped memory area:
--- cut ---
Program received signal SIGSEGV, Segmentation fault.
0x0000000000417d1e in addID (h=0x7103a0, gid=2293, id=18997) at ../../../../../source/cffread/cffread.c:1843
1843 info->cid = id;
(gdb) print info
$1 = (abfGlyphInfo *) 0x743000
(gdb) print &h->glyphs.array[gid]
$2 = (abfGlyphInfo *) 0x743000
(gdb) print gid
$3 = 2293
(gdb) x/10gx 0x743000
0x743000: Cannot access memory at address 0x743000
(gdb) bt
#0 0x0000000000417d1e in addID (h=0x7103a0, gid=2293, id=18997) at ../../../../../source/cffread/cffread.c:1843
#1 0x0000000000412a57 in readCharset (h=0x7103a0) at ../../../../../source/cffread/cffread.c:2187
#2 0x000000000040dd64 in cfrBegFont (h=0x7103a0, flags=4, origin=0, ttcIndex=0, top=0x6f6048, UDV=0x0) at ../../../../../source/cffread/cffread.c:2789
#3 0x0000000000405e4e in cfrReadFont (h=0x6f6010, origin=0, ttcIndex=0) at ../../../../source/tx.c:137
#4 0x0000000000405c9e in doFile (h=0x6f6010, srcname=0x7fffffffdf4c "poc.otf") at ../../../../source/tx.c:429
#5 0x000000000040532e in doSingleFileSet (h=0x6f6010, srcname=0x7fffffffdf4c "poc.otf")
at ../../../../source/tx.c:488
#6 0x0000000000402f59 in parseArgs (h=0x6f6010, argc=2, argv=0x7fffffffdc50) at ../../../../source/tx.c:558
#7 0x0000000000401df2 in main (argc=2, argv=0x7fffffffdc50) at ../../../../source/tx.c:1631
(gdb)
--- cut ---
A similar Microsoft Edge renderer process crash is also shown below:
--- cut ---
(4d58.50bc): Access violation - code c0000005 (first chance)
First chance exceptions are reported before any exception handling.
This exception may be expected and handled.
DWrite!addID+0x33:
00007ffb`29e6864b 66895cfe28 mov word ptr [rsi+rdi*8+28h],bx ds:000001ea`f5fee000=????
0:038> ? rsi
Evaluate expression: 2108661076032 = 000001ea`f5fe9040
0:038> ? rdi
Evaluate expression: 2547 = 00000000`000009f3
0:038> db rsi+rdi*8+28
000001ea`f5fee000 ?? ?? ?? ?? ?? ?? ?? ??-?? ?? ?? ?? ?? ?? ?? ?? ????????????????
000001ea`f5fee010 ?? ?? ?? ?? ?? ?? ?? ??-?? ?? ?? ?? ?? ?? ?? ?? ????????????????
000001ea`f5fee020 ?? ?? ?? ?? ?? ?? ?? ??-?? ?? ?? ?? ?? ?? ?? ?? ????????????????
000001ea`f5fee030 ?? ?? ?? ?? ?? ?? ?? ??-?? ?? ?? ?? ?? ?? ?? ?? ????????????????
000001ea`f5fee040 ?? ?? ?? ?? ?? ?? ?? ??-?? ?? ?? ?? ?? ?? ?? ?? ????????????????
000001ea`f5fee050 ?? ?? ?? ?? ?? ?? ?? ??-?? ?? ?? ?? ?? ?? ?? ?? ????????????????
000001ea`f5fee060 ?? ?? ?? ?? ?? ?? ?? ??-?? ?? ?? ?? ?? ?? ?? ?? ????????????????
000001ea`f5fee070 ?? ?? ?? ?? ?? ?? ?? ??-?? ?? ?? ?? ?? ?? ?? ?? ????????????????
0:038> k
# Child-SP RetAddr Call Site
00 00000047`0fcfae10 00007ffb`29e6a262 DWrite!addID+0x33
01 00000047`0fcfae40 00007ffb`29e6de84 DWrite!readCharset+0x10a
02 00000047`0fcfae70 00007ffb`29e621e7 DWrite!cfrBegFont+0x5d8
03 00000047`0fcfb700 00007ffb`29df157a DWrite!AdobeCFF2Snapshot+0x10f
04 00000047`0fcfbc00 00007ffb`29df0729 DWrite!FontInstancer::InstanceCffTable+0x212
05 00000047`0fcfbde0 00007ffb`29df039a DWrite!FontInstancer::CreateInstanceInternal+0x249
06 00000047`0fcfc000 00007ffb`29dd5a4e DWrite!FontInstancer::CreateInstance+0x192
07 00000047`0fcfc360 00007ffb`34eb61ab DWrite!DWriteFontFace::CreateInstancedStream+0x9e
08 00000047`0fcfc3f0 00007ffb`34ea9148 d2d1!dxc::TextConvertor::InstanceFontResources+0x19f
09 00000047`0fcfc510 00007ffb`0fb750f4 d2d1!dxc::CXpsPrintControl::Close+0xc8
0a 00000047`0fcfc560 00007ffb`0fb4fcb0 edgehtml!CDXPrintControl::Close+0x44
0b 00000047`0fcfc5b0 00007ffb`0fb547ad edgehtml!CTemplatePrinter::EndPrintD2D+0x5c
0c 00000047`0fcfc5e0 00007ffb`0fa2b515 edgehtml!CPrintManagerTemplatePrinter::endPrint+0x2d
0d 00000047`0fcfc610 00007ffb`0f689175 edgehtml!CFastDOM::CMSPrintManagerTemplatePrinter::Trampoline_endPrint+0x45
0e 00000047`0fcfc650 00007ffb`0eb568f1 edgehtml!CFastDOM::CMSPrintManagerTemplatePrinter::Profiler_endPrint+0x25
[...]
--- cut ---
-----=====[ References ]=====-----
[1] https://blog.typekit.com/2014/09/19/new-from-adobe-type-open-sourced-font-development-tools/
[2] https://github.com/adobe-type-tools/afdko
[3] https://docs.microsoft.com/en-us/windows/desktop/directwrite/direct-write-portal
[4] https://medium.com/variable-fonts/https-medium-com-tiro-introducing-opentype-variable-fonts-12ba6cd2369
Proof of Concept:
https://gitlab.com/exploit-database/exploitdb-bin-sploits/-/raw/main/bin-sploits/47095.zip
Products Mentioned
Configuraton 0
Microsoft>>Windows_10 >> Version 1709
Microsoft>>Windows_10 >> Version 1803
Microsoft>>Windows_10 >> Version 1809
Microsoft>>Windows_10 >> Version 1903
Microsoft>>Windows_server_2016 >> Version 1803
Microsoft>>Windows_server_2016 >> Version 1903
Microsoft>>Windows_server_2019 >> Version -
References
