CVE-2017-13876

Score / Severity

7.8

High

CVE Descriptions

An issue was discovered in certain Apple products. iOS before 11.2 is affected. macOS before 10.13.2 is affected. tvOS before 11.2 is affected. watchOS before 4.2 is affected. The issue involves the "Kernel" component. It allows attackers to execute arbitrary code in a privileged context or cause a denial of service (memory corruption) via a crafted app.

CVE Informations

Related Weaknesses

CWE-ID	Weakness Name	Source
CWE-119	Improper Restriction of Operations within the Bounds of a Memory Buffer The product performs operations on a memory buffer, but it reads from or writes to a memory location outside the buffer's intended boundary. This may result in read or write operations on unexpected memory locations that could be linked to other variables, data structures, or internal program data.

Metrics

Metrics	Score	Severity	CVSS Vector	Source
V3.0	7.8	HIGH	CVSS:3.0/AV:L/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. Local A vulnerability exploitable with Local access means that the vulnerable component is not bound to the network stack, and the attacker's path is via read/write/execute capabilities. In some cases, the attacker may be logged in locally in order to exploit the vulnerability, otherwise, she may rely on User Interaction to execute a malicious file. 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	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.

Date	EPSS V0	EPSS V1	EPSS V2 (> 2022-02-04)	EPSS V3 (> 2025-03-07)	EPSS V4 (> 2025-03-17)
2021-04-18	2.42%	–	–	–	–
2021-09-05	–	2.42%	–	–	–
2021-10-17	–	2.42%	–	–	–
2022-01-09	–	2.42%	–	–	–
2022-02-06	–	–	7.49%	–	–
2022-03-06	–	–	6.55%	–	–
2022-04-03	–	–	6.55%	–	–
2023-03-12	–	–	–	0.24%	–
2023-04-02	–	–	–	0.24%	–
2023-05-21	–	–	–	0.27%	–
2023-09-10	–	–	–	0.27%	–
2024-02-11	–	–	–	0.27%	–
2024-06-02	–	–	–	0.27%	–
2024-12-15	–	–	–	0.27%	–
2024-12-22	–	–	–	0.91%	–
2025-01-19	–	–	–	0.91%	–
2025-03-18	–	–	–	–	1.24%
2025-03-30	–	–	–	–	2.26%
2025-03-30	–	–	–	–	2.26,%

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	78%
2021-10-17	79%
2022-01-09	54%
2022-02-06	81%
2022-03-06	79%
2022-04-03	91%
2023-03-12	6%
2023-04-02	61%
2023-05-21	63%
2023-09-10	64%
2024-02-11	67%
2024-06-02	68%
2024-12-15	69%
2024-12-22	83%
2025-01-19	83%
2025-03-18	78%
2025-03-30	83%
2025-03-30	83%

Exploit information

Exploit Database EDB-ID : 43325

Publication date : 2017-12-11 23h00 +00:00
Author : Google Security Research
EDB Verified : Yes

posix_spawn is a complex syscall which takes a lot of arguments from userspace. The third argument is a pointer to a further arguments descriptor in userspace with the following structure (on 32-bit): struct user32__posix_spawn_args_desc { uint32_t attr_size; /* size of attributes block */ uint32_t attrp; /* pointer to block */ uint32_t file_actions_size; /* size of file actions block */ uint32_t file_actions; /* pointer to block */ uint32_t port_actions_size; /* size of port actions block */ uint32_t port_actions; /* pointer to block */ uint32_t mac_extensions_size; uint32_t mac_extensions; uint32_t coal_info_size; uint32_t coal_info; uint32_t persona_info_size; uint32_t persona_info; } port_actions then points to another structure in userspace of this type: struct _posix_spawn_port_actions { int pspa_alloc; int pspa_count; _ps_port_action_t pspa_actions[]; } and finally _ps_port_action_t looks like this: struct _ps_port_action { pspa_t port_type; exception_mask_t mask; mach_port_name_t new_port; exception_behavior_t behavior; thread_state_flavor_t flavor; int which; } Note that pspa_actions is a zero-sized array. pspa_count is supposed to be the number of entries in this array. The following constraints are checked in posix_spawn in kern_exec.c: if (px_args.port_actions_size != 0) { /* Limit port_actions to one page of data */ if (px_args.port_actions_size < PS_PORT_ACTIONS_SIZE(1) || px_args.port_actions_size > PAGE_SIZE) { error = EINVAL; goto bad; PS_PORT_ACTIONS_SIZE is defined like this: #define PS_PORT_ACTIONS_SIZE(x) \ __offsetof(struct _posix_spawn_port_actions, pspa_actions[(x)]) if port_actions_size passes this then we reach the following code: MALLOC(px_spap, _posix_spawn_port_actions_t, px_args.port_actions_size, M_TEMP, M_WAITOK); if (px_spap == NULL) { error = ENOMEM; goto bad; } imgp->ip_px_spa = px_spap; if ((error = copyin(px_args.port_actions, px_spap, px_args.port_actions_size)) != 0) goto bad; This allocates a kernel heap buffer to hold the port_actions buffer and copies from userspace into it. The code then attempts to check whether the pspa_count valid is correct: /* Verify that the action count matches the struct size */ if (PS_PORT_ACTIONS_SIZE(px_spap->pspa_count) != px_args.port_actions_size) { error = EINVAL; goto bad; } There is an integer overflow here because offsetof is just simple arithmetic. With a carefully chosen value for pspa_count we can make it very large but when it's passed to the PS_PORT_ACTIONS_SIZE macro the result is equal to port_actions_size. Nothing bad has happened yet but we can now get pspa_count to be much larger than it should be. Later on we reach the following code: if (px_spap->pspa_count != 0 && is_adaptive) { portwatch_count = px_spap->pspa_count; MALLOC(portwatch_ports, ipc_port_t *, (sizeof(ipc_port_t) * portwatch_count), M_TEMP, M_WAITOK | M_ZERO); } else { portwatch_ports = NULL; } if ((error = exec_handle_port_actions(imgp, &portwatch_present, portwatch_ports)) != 0) We can cause another integer overflow here, sizeof(ipc_port_t) is 4 (on 32-bit) so with a carefully chosen value of pspa_count we can cause the integer overflow here and earlier too whilst still passing the checks. exec_handle_port_actions then uses portwatch ports like this: for (i = 0; i < pacts->pspa_count; i++) { act = &pacts->pspa_actions[i]; if (MACH_PORT_VALID(act->new_port)) { kr = ipc_object_copyin(get_task_ipcspace(current_task()), act->new_port, MACH_MSG_TYPE_COPY_SEND, (ipc_object_t *) &port); ... switch (act->port_type) { ... case PSPA_IMP_WATCHPORTS: if (portwatch_ports != NULL && IPC_PORT_VALID(port)) { *portwatch_present = TRUE; /* hold on to this till end of spawn */ portwatch_ports[i] = port; note that pspa_actions was allocated earlier also based on the result of an integer overflow. This means we can cause an OOB write to portwatch_ports only if we can successfully read suitable valid values OOB of pspa_actions. That's why this PoC first fills a kalloc.1024 buffer with suitable values before freeing it and then hoping that it will get reallocated as pspa_actions (but less thatn 1024 bytes will be written) such that we control what's read OOB and the ipc_object_copyin will succeed. This seems to be pretty reliable. You can use this to build a nice primitive of a heap overflow with pointers to ipc_port structures. I don't believe there are any iOS 11 32-bit iPod/iPhone/iPad/AppleTV devices but the new Apple Watch Series 3 is running essentially the same kernel but has a 32-bit CPU. This PoC is provided as an Apple watch app and has been tested on Apple Watch Series 3 (Watch3,2) running WatchOS 4.0.1. I also tested on an older 32-bit iOS 9 device. Apple Watch Series 3 now has its own LTE modem and can be used without an iPhone making it a suitably interesting target for exploitation by itself. Note that all the uses of offsetof in those posix_spawn macros are quite wrong, I think you might be able to get a kernel memory disclosure with one of them also on 64-bit platforms. The fix is to add correct bounds checking. Please also note that this really shouldn't be attack surface reachable from an app sandbox. The MAC hook in posix_spawn is very late and there's a *lot* of code which you can hit before it. Proof of Concept: https://gitlab.com/exploit-database/exploitdb-bin-sploits/-/raw/main/bin-sploits/43325.zip