CVE-2019-8672 : Detail

CVE-2019-8672

8.8
/
High
Overflow
41.73%V4
Network
2019-12-18
16h33 +00:00
2019-12-18
16h33 +00:00
CVE.org
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CVE Descriptions

Multiple memory corruption issues were addressed with improved memory handling. This issue is fixed in iOS 12.4, macOS Mojave 10.14.6, tvOS 12.4, watchOS 5.3, Safari 12.1.2, iTunes for Windows 12.9.6, iCloud for Windows 7.13, iCloud for Windows 10.6. 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

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.

 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.
EPSS First.org

Exploit information

Exploit Database EDB-ID : 47191

Publication date : 2019-07-29 22h00 +00:00
Author : Google Security Research
EDB Verified : Yes

While fuzzing JSC, I encountered the following JS program which crashes JSC from current HEAD and release (/System/Library/Frameworks/JavaScriptCore.framework/Resources/jsc): // Run with --useConcurrentJIT=false --thresholdForJITAfterWarmUp=10 function fullGC() { for (var i = 0; i < 10; i++) { new Float64Array(0x1000000); } } function v62() { function v141() { try { const v146 = v141(); } catch(v147) { const v154 = Object(); function v155(v156,v157,v158) { try { // This typed array gets collected // but is still referenced from the // value profile of TypedArray.values const v167 = new Uint32Array(); const v171 = v167.values(); } catch(v177) { } } const v181 = v155(); } } v141(); function edenGC() { for (let v194 = 0; v194 < 100; v194++) { const v204 = new Float64Array(0x10000); } } const v205 = edenGC(); } for (let i = 0; i < 6; i++) { const v209 = v62(); } fullGC(); If the loop that calls v62 is run 100 instead of 6 times it will also crash without --thresholdForJITAfterWarmUp=10, albeit a bit less reliable. Running this sample will crash JSC in debug builds with an assertion like this: ASSERTION FAILED: structureIndex < m_capacity Source/JavaScriptCore/runtime/StructureIDTable.h(175) : JSC::Structure *JSC::StructureIDTable::get(JSC::StructureID) 1 0x101aadcf9 WTFCrash 2 0x101aadd19 WTFCrashWithSecurityImplication 3 0x10000cb18 JSC::StructureIDTable::get(unsigned int) 4 0x10000ca23 JSC::VM::getStructure(unsigned int) 5 0x10000c7cf JSC::JSCell::structure(JSC::VM&) const 6 0x10001887b JSC::JSCell::structure() const 7 0x10072fc05 JSC::speculationFromCell(JSC::JSCell*) 8 0x10072fd9f JSC::speculationFromValue(JSC::JSValue) 9 0x1006963dc JSC::ValueProfileBase<1u>::computeUpdatedPrediction(JSC::ConcurrentJSLocker const&) ... The crash is due to a JSValue pointing to a previously freed chunk which will have its JSCell header overwritten. As such, it then crashes when accessing the structure table out-of-bounds with the clobbered structure ID. The JSValue that is being accessed is part of a ValueProfile: a data structure attached to bytecode operations which keeps track of input types that have been observed for its operation. During execution in the interpreter or baseline JIT, input types for operations will be stored in their associated ValueProfile as can e.g. be seen in the implementation of the low-level interpreter (LLInt) [1]. This is a fundamental mechanism of current JS engines allowing optimizing JIT compilers (like the DFG and FTL) to speculate about types of variables in the compiled program by inspecting previously observed types collected in these ValueProfiles. A ValueProfile is implemented by the ValueProfileBase C++ struct: struct ValueProfileBase { ... int m_bytecodeOffset; // -1 for prologue unsigned m_numberOfSamplesInPrediction { 0 }; SpeculatedType m_prediction { SpecNone }; EncodedJSValue m_buckets[totalNumberOfBuckets]; }; Here, m_buckets will store the raw JSValues that have been observed during execution. m_prediction in turn will contain the current type prediction [2] for the associated value, which is what the JIT compilers ultimately rely on. The type prediction is regularly computed from the observed values in computeUpdatedPrediction [3]. This raises the question how the JSValues in m_buckets are kept alive during GC, as they are not stored in a MarkedArgumentBuffer [4] or similar (which automatically inform the GC of the objects and thus keep them alive). The answer is that they are in fact not kept alive during GC by the ValueProfiles themselves. Instead, computeUpdatedPrediction [3] is invoked from finalizeUnconditionally [5] at the end of the GC marking phase and will clear the m_buckets array before the pointers might become dangling. Basically, it works like this: * Observed JSValues are simply stored into ValueProfiles at runtime by the interpreter or baseline JIT without informing the GC about these references * Eventually, GC kicks in and starts its marking phase in which it visits all reachable objects and marks them as alive * Afterwards, before sweeping, the GC invokes various callbacks (called "unconditionalFinalizers") [6] on certain objects (e.g. CodeBlocks) * The CodeBlock finalizers update all value profiles, which in turn causes their current speculated type to be merged with the runtime values that were observed since the last update * Afterwards, all entries in the m_buckets array of the ValueProfiles are cleared to zero [7]. As such, the ValueProfiles no longer store any pointers to JSObjects * Finally, the sweeping phase runs and frees all JSCells that have not been marked For some time now, JSC has used lightweight GC cycles called "eden" collections. These will keep mark bits from previous eden collections and thus only scan newly allocated objects, not the entire object graph. As such they are quicker than a "full" GC, but might potentially leave unused ("garbage") objects alive which will only be collected during the next full collection. See also [8] for an in depth explanation of JSC's current garbage collector. As described above, the function finalizeMarkedUnconditionalFinalizers [6] is responsible for invoking some callback on objects that have been marked (and thus are alive) after the marking phase. However, during eden collections this function only iterates over JSCells that have been marked in the *current* eden collection, not any of the previous ones *. As such, it is possible that a CodeBlock has been marked in a previous eden collection (and is thus still alive), but hasn't been marked in the current one and will thus not be "unconditionally finalized". In that case, its ValueProfile will not be cleared and will still potentially contain pointers to various JSObjects, which, however, aren't protected from GC and thus might be freed by it. This is what happens in the program above: the TypedArray.values function is a JS builtin [9] and will thus be JIT compiled. At the time of the crash it will be baseline JIT compiled and thus store the newly allocated Uint32Array into one of its ValueProfile [10]. Directly afterwards, the compiled code raises another stack overflow exception [11]. As such, the Uint32Array is not used any further and no more references to it are taken which could protect it from GC. As such, the array will be collected during the next (eden) GC round. However, the CodeBlock for TypedArray.values was already marked in a previous eden collection and will not be finalized, thus leaving the pointer to the freed TypedArray dangling in the ValueProfile. During the next full GC, the CodeBlock is again "unconditionally finalized" and will then inspects its m_buckets, thus crashing when using the freed JSValue. The infinite recursion and following stack overflow exceptions in this sample might be necessary to force a situation in which the newly allocated Uint32Array is only stored into a profiling slot and nowhere else. But maybe they are also simply required to cause the right sequence of GC invocations.

Products Mentioned

Configuraton 0

Apple>>Icloud >> Version To (excluding) 7.13

Apple>>Icloud >> Version From (including) 10.0 To (excluding) 10.6

Apple>>Itunes >> Version To (excluding) 12.9.6

Apple>>Safari >> Version To (excluding) 12.1.2

Apple>>Iphone_os >> Version To (excluding) 12.4

Apple>>Mac_os_x >> Version To (excluding) 10.14.6

Apple>>Tvos >> Version To (excluding) 12.4

Apple>>Watchos >> Version To (excluding) 5.3

Configuraton 0

Redhat>>Enterprise_linux_desktop >> Version 7.0

Redhat>>Enterprise_linux_server >> Version 7.0

Redhat>>Enterprise_linux_workstation >> Version 7.0

References