Related Weaknesses
CWE-ID |
Weakness Name |
Source |
CWE-732 |
Incorrect Permission Assignment for Critical Resource The product specifies permissions for a security-critical resource in a way that allows that resource to be read or modified by unintended actors. |
|
Metrics
Metrics |
Score |
Severity |
CVSS Vector |
Source |
V3.0 |
7.8 |
HIGH |
CVSS:3.0/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H
Base: Exploitabilty MetricsThe 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. 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. 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. The attacker is authorized with (i.e. requires) privileges that provide basic user capabilities that could normally affect only settings and files owned by a user. Alternatively, an attacker with Low privileges may have the ability to cause an impact only to non-sensitive resources. User Interaction This metric captures the requirement for a user, other than the attacker, to participate in the successful compromise of the vulnerable component. The vulnerable system can be exploited without interaction from any user. Base: Scope MetricsAn 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. 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 MetricsThe 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. 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. 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. 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 MetricsThe 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
|
[email protected] |
V2 |
4.6 |
|
AV:L/AC:L/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.
Exploit information
Exploit Database EDB-ID : 43996
Publication date : 2018-02-06 23h00 +00:00
Author : Google Security Research
EDB Verified : Yes
The keystore binder service ("android.security.IKeystoreService") allows users to issue several commands related to key management, including adding, removing, exporting and generating cryptographic keys. The service is accessible to many SELinux contexts, including application contexts, but also unprivileged daemons such as "media.codec".
Binder calls to this service are unpacked by IKeyStoreService (http://androidxref.com/8.0.0_r4/xref/system/security/keystore/IKeystoreService.cpp), and are then passed on to be processed by KeyStoreService. The "generateKey" command is handled by "KeyStoreService::generateKey" (http://androidxref.com/8.0.0_r4/xref/system/security/keystore/key_store_service.cpp#691). Here is a snippet from this function:
1. KeyStoreServiceReturnCode KeyStoreService::generateKey(const String16& name,
2. const hidl_vec<KeyParameter>& params,
3. const hidl_vec<uint8_t>& entropy, int uid,
4. int flags,
5. KeyCharacteristics* outCharacteristics) {
6. uid = getEffectiveUid(uid);
7. KeyStoreServiceReturnCode rc =
8. checkBinderPermissionAndKeystoreState(P_INSERT, uid, flags & KEYSTORE_FLAG_ENCRYPTED);
9. if (!rc.isOk()) {
10. return rc;
11. }
12. if ((flags & KEYSTORE_FLAG_CRITICAL_TO_DEVICE_ENCRYPTION) && get_app_id(uid) != AID_SYSTEM) {
13. ALOGE("Non-system uid %d cannot set FLAG_CRITICAL_TO_DEVICE_ENCRYPTION", uid);
14. return ResponseCode::PERMISSION_DENIED;
15. }
16.
17. if (containsTag(params, Tag::INCLUDE_UNIQUE_ID)) {
18. if (!checkBinderPermission(P_GEN_UNIQUE_ID)) return ResponseCode::PERMISSION_DENIED;
19. }
20. ...
21. }
Like most KeyStore calls, this method uses "KeyStoreService::checkBinderPermission" in order to validate the calling process's permissions. This function uses a twofold approach to verify the caller (http://androidxref.com/8.0.0_r4/xref/system/security/keystore/key_store_service.cpp#checkBinderPermission):
1. The caller's UID is retrieved using IPCThreadState::self()->getCallingUid() and compared against an array of pre-populated UIDs and permissions ("user_perms")
1.1 If the UID matches any in the array, its permission set is retrieved from the array
1.2 If the UID isn't in the array, the default permission set is used ("DEFAULT_PERMS")
2. The caller's SELinux context is retrieved using getpidcon(...) using the PID from the binder transaction (IPCThreadState::self()->getCallingPid())
2.1 An SELinux access check is performed for the given context and operation
Specifically to our case, if a "generateKey" command is called with a "INCLUDE_UNIQUE_ID" tag, the KeyStore will use an attestation certificate for the generated key with an application-scoped and time-bounded device-unique ID. Since creating attestation keys is a privileged operation, it should not be carried out by any user.
This restriction is enforced using the SELinux context enforcement alone -- the "default" permission set ("DEFAULT_PERMS") contains the aforementioned permission:
static const perm_t DEFAULT_PERMS = static_cast<perm_t>(
P_GET_STATE | P_GET | P_INSERT | P_DELETE | P_EXIST | P_LIST | P_SIGN | P_VERIFY |
P_GEN_UNIQUE_ID /* Only privileged apps can do this, but enforcement is done by SELinux */);
As noted in the comment above, this API is restricted to "priv_app" SELinux contexts, which is enforced using validation #2 above.
However, using the calling PID in order to enforce access controls in binder calls is an invalid approach. This is since the calling PID can transition from zombie to dead, allowing other PIDs to take its place. Therefore, the following attack flow is possible:
1. Process A forks and creates process B
2. Process A cycles pids until it reaches the pid before its own
3. Process B issues a binder transaction for the KeyStore service, containing an INCLUDE_UNIQUE_ID tag
4. Process A kills process B, allowing it to transition to dead
5. Process A spawns a new "priv_app" instance, occupying process B's PID
If points 4-5 are completed before the KeyStore service performs the "getpidcon" call, the permission check will use the new app's SELinux context, allowing the access control checks to pass. Otherwise, since no ill effects happen if the race fails, an attacker can continue issuing calls until the race succeeds.
As for spawning a new "priv_app" instance, this can be achieved by issuing a query request to a content provider published by a "priv_app". Many such providers exist (the contacts provider, telephony provider, settings provider, etc.). In this case, I chose to use the "calendar" provider, as it was not running on the device to begin with (and is therefore had to be spawned in order to handle the query request).
In order to expand the timing window for the PoC, I've added a "sleep" call to the KeyStore service's "generateKey" call. You can find the patch under "keystore.diff".
After applying the patch, the attached PoC should be built as part of the Android source tree, by extracting the source files into "frameworks/native/cmds/keystorerace", and running a build (e.g., "mmm keystorerace"). The resulting binary ("keystorerace") contains the PoC code. Running it should result in a new device-unique key being generated, despite not being executed from a "priv_app".
Proof of Concept:
https://gitlab.com/exploit-database/exploitdb-bin-sploits/-/raw/main/bin-sploits/43996.zip
Products Mentioned
Configuraton 0
Google>>Android >> Version 8.0
Google>>Android >> Version 8.1
References