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
|
[email protected] |
| V2 |
9.3 |
|
AV:N/AC:M/Au:N/C:C/I:C/A:C |
[email protected] |
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 : 47099
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 handleBlend() function in afdko/c/public/lib/source/cffread/cffread.c is called when a cff_blend operator is encountered while parsing a CFF DICT object in readDICT():
--- cut ---
1466 case cff_blend:
1467 if (h->stack.numRegions == 0) {
1468 /* priv->vsindex is set to 0 by default; it is otherwise only if the vsindex operator is used */
1469 setNumMasters(h, priv->vsindex);
1470 }
1471 handleBlend(h);
1472 continue;
--- cut ---
The prologue of handleBlend() is as follows:
--- cut ---
757 static void handleBlend(cfrCtx h) {
[...]
776
777 int numBlends = INDEX_INT(h->stack.cnt - 1);
778 stack_elem *firstItem;
779 int i = 0;
780 int numDeltaBlends = numBlends * h->stack.numRegions;
781 int firstItemIndex;
782 h->flags |= CFR_SEEN_BLEND;
783
784 h->stack.cnt--;
785
786 if (numBlends < 0 || numDeltaBlends < 0)
787 fatal(h, cfrErrStackUnderflow);
788 CHKUFLOW(numBlends + numDeltaBlends);
789 firstItemIndex = (h->stack.cnt - (numBlends + numDeltaBlends));
790 firstItem = &(h->stack.array[firstItemIndex]);
791
--- cut ---
Here is what happens in the code: the 32-bit numBlends variable is initialized with a fully controlled value from the top of the interpreter stack. The numDeltaBlends variable is calculated using numBlends and h->stack.numRegions, which is a typically small (theoretically up to 65535) but also controlled value. The code then makes sure that neither numBlends or numDeltaBlends are negative (line 786), and that there are at least numBlends+numDeltaBlends values on the stack (line 788). If these conditions are met, the function proceeds to writing to h->stack.array[h->stack.cnt - (numBlends + numDeltaBlends)] and further elements assuming the access is safe.
However, the sanity checks in lines 786-788 are not sufficient, as they miss one corner case - when both numBlends and numDeltaBlends are positive, but the numBlends + numDeltaBlends sum is negative due to signed 32-bit integer arithmetic. For example, if:
- numBlends is 0x31313131
- h->stack.numRegions is 2
then:
- numDeltaBlends is 0x62626262
- numBlends + numDeltaBlends is 0x93939393
The above values can be set by a specially crafted font and they meet the conditions verified by handleBlend(), yet the index of -0x93939393 (which translates to 1819044973) is largely out of bounds. This may be used to overwrite memory both inside of the stack-based cfrCtx object, and outside of it.
-----=====[ Proof of Concept ]=====-----
The proof of concept file contains a Private DICT beginning with the following two operators:
1. cff_longint(0x31313131)
2. cff_blend
This causes the above signedness issue to occur, leading to an attempt to write to a stack element at h->stack.array[1819044973].
The font is also specially crafted to parse correctly with DirectWrite but trigger the bug in AFDKO. The original CFF2 table was left untouched, and a second copy of it with the modified DICT was inserted at the end of the font with the tag "CFF ". This way, DirectWrite successfully loads the legitimate variable font, and AFDKO processes the modified version as the CFF table takes precedence over CFF2 due to the logic implemented in srcOpen() in afdko/c/public/lib/source/cffread/cffread.c.
-----=====[ Crash logs ]=====-----
A 64-bit build of "tx" started with ./tx -cff poc.otf crashes with a SIGSEGV while trying to write to an unmapped memory address:
--- cut ---
Program received signal SIGSEGV, Segmentation fault.
0x000000000041670e in handleBlend (h=0x7103a0) at ../../../../../source/cffread/cffread.c:811
811 firstItem->numBlends = (unsigned short)numBlends;
(gdb) print numBlends
$1 = 825307441
(gdb) print numDeltaBlends
$2 = 1650614882
(gdb) print numBlends+numDeltaBlends
$3 = -1819044973
(gdb) print firstItemIndex
$5 = 1819044973
(gdb) x/10i $rip
=> 0x41670e <handleBlend+878>: mov %cx,0x8(%rdx)
0x416712 <handleBlend+882>: mov -0x8(%rbp),%rdi
0x416716 <handleBlend+886>: movslq -0x20(%rbp),%rdx
0x41671a <handleBlend+890>: shl $0x2,%rdx
0x41671e <handleBlend+894>: mov %rdx,%rsi
0x416721 <handleBlend+897>: callq 0x416880 <memNew>
0x416726 <handleBlend+902>: mov -0x18(%rbp),%rdx
0x41672a <handleBlend+906>: mov %rax,0x10(%rdx)
0x41672e <handleBlend+910>: mov -0x1c(%rbp),%eax
0x416731 <handleBlend+913>: cmp -0x20(%rbp),%eax
(gdb) info reg $rdx
rdx 0xa2a9b3280 43664487040
(gdb) x/10gx $rdx
0xa2a9b3280: Cannot access memory at address 0xa2a9b3280
(gdb) bt
#0 0x000000000041670e in handleBlend (h=0x7103a0) at ../../../../../source/cffread/cffread.c:811
#1 0x0000000000411318 in readDICT (h=0x7103a0, region=0x7156d8, topdict=0) at ../../../../../source/cffread/cffread.c:1471
#2 0x000000000041241f in readPrivate (h=0x7103a0, iFD=0) at ../../../../../source/cffread/cffread.c:1637
#3 0x0000000000411a17 in readFDArray (h=0x7103a0) at ../../../../../source/cffread/cffread.c:1711
#4 0x000000000040dc5c in cfrBegFont (h=0x7103a0, flags=4, origin=0, ttcIndex=0, top=0x6f6048, UDV=0x0) at ../../../../../source/cffread/cffread.c:2761
#5 0x0000000000405e4e in cfrReadFont (h=0x6f6010, origin=0, ttcIndex=0) at ../../../../source/tx.c:137
#6 0x0000000000405c9e in doFile (h=0x6f6010, srcname=0x7fffffffdf1b "poc.otf") at ../../../../source/tx.c:429
#7 0x000000000040532e in doSingleFileSet (h=0x6f6010, srcname=0x7fffffffdf1b "poc.otf")
at ../../../../source/tx.c:488
#8 0x0000000000402f59 in parseArgs (h=0x6f6010, argc=2, argv=0x7fffffffdc20) at ../../../../source/tx.c:558
#9 0x0000000000401df2 in main (argc=2, argv=0x7fffffffdc20) at ../../../../source/tx.c:1631
(gdb)
--- cut ---
A similar Microsoft Edge renderer process crash (but in a slightly different code path) is also shown below:
--- cut ---
(50d4.f24): Access violation - code c0000005 (first chance)
First chance exceptions are reported before any exception handling.
This exception may be expected and handled.
DWrite!handleBlend+0xc1:
00007ffb`29e68f5d 4439a4cb28030000 cmp dword ptr [rbx+rcx*8+328h],r12d ds:0000016e`de978c30=????????
0:039> ? rbx
Evaluate expression: 1532035423952 = 00000164`b46d5ed0
0:039> ? rcx
Evaluate expression: 5457134919 = 00000001`45454547
0:039> k
# Child-SP RetAddr Call Site
00 00000021`5eeea990 00007ffb`29e6b2e3 DWrite!handleBlend+0xc1
01 00000021`5eeea9d0 00007ffb`29e6c41e DWrite!readDICT+0xf67
02 00000021`5eeeaa30 00007ffb`29e6bc33 DWrite!readPrivate+0x3a
03 00000021`5eeeaa60 00007ffb`29e6ddf4 DWrite!readFDArray+0xcb
04 00000021`5eeeaa90 00007ffb`29e621e7 DWrite!cfrBegFont+0x548
05 00000021`5eeeb320 00007ffb`29df157a DWrite!AdobeCFF2Snapshot+0x10f
06 00000021`5eeeb820 00007ffb`29df0729 DWrite!FontInstancer::InstanceCffTable+0x212
07 00000021`5eeeba00 00007ffb`29df039a DWrite!FontInstancer::CreateInstanceInternal+0x249
08 00000021`5eeebc20 00007ffb`29dd5a4e DWrite!FontInstancer::CreateInstance+0x192
09 00000021`5eeebf80 00007ffb`34eb61ab DWrite!DWriteFontFace::CreateInstancedStream+0x9e
0a 00000021`5eeec010 00007ffb`34ea9148 d2d1!dxc::TextConvertor::InstanceFontResources+0x19f
0b 00000021`5eeec130 00007ffb`0f8b50f4 d2d1!dxc::CXpsPrintControl::Close+0xc8
0c 00000021`5eeec180 00007ffb`0f88fcb0 edgehtml!CDXPrintControl::Close+0x44
0d 00000021`5eeec1d0 00007ffb`0f8947ad edgehtml!CTemplatePrinter::EndPrintD2D+0x5c
0e 00000021`5eeec200 00007ffb`0f76b515 edgehtml!CPrintManagerTemplatePrinter::endPrint+0x2d
0f 00000021`5eeec230 00007ffb`0f3c9175 edgehtml!CFastDOM::CMSPrintManagerTemplatePrinter::Trampoline_endPrint+0x45
10 00000021`5eeec270 00007ffa`f02e68f1 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/47099.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
