CyberArk Credential Provider Race Condition / Authorization Bypass

2021.09.04
Risk: Medium
Local: No
Remote: Yes

KL-001-2021-009: CyberArk Credential Provider Race Condition And Authorization Bypass Title: CyberArk Credential Provider Race Condition And Authorization Bypass Advisory ID: KL-001-2021-009 Publication Date: 2021.09.01 Publication URL: https://korelogic.com/Resources/Advisories/KL-001-2021-009.txt 1. Vulnerability Details Affected Vendor: CyberArk Affected Product: Application Access Manager/Credential Provider Affected Version: Prior to 12.1 Platform: Linux/Windows/zOS CWE Classification: CWE-326: Inadequate Encryption Strength, CWE-362: Concurrent Execution using Shared Resource with Improper Synchronization ('Race Condition'), CWE-923: Improper Restriction of Communication Channel to Intended Endpoints CVE ID: CVE-2021-31797 2. Vulnerability Description CyberArk's Credential Provider loopback communications on TCP port 18923 are encrypted with key material that has extremely low entropy. In all currently-known use cases, the effective key space is less than 2^16. For an attacker who understands the key derivation scheme and encryption mechanics, knowledge of the source port and access to the payloads of a given client-server exchange are sufficient to reduce effective key space to one. In cases where the source port is not known, the encrypted payloads will be unable to withstand a brute force attack. Additionally, the user identification mechanism used by CyberArk's Credential Provider is vulnerable to a race condition where an unauthorized/unprivileged user can submit one or more encrypted query requests. If the race is won, the attacker will be able to retrieve sensitive information including passwords and password metadata. 3. Technical Description Based on analysis and observations, the key derivation process for CyberArk's Credential Provider loopback communications on TCP port 18923 can be summarized as follows: - start a SHA1 hash (Hash1) - update Hash1 with the decimal representation of the source port - update Hash1 with an undocumented, hard-coded byte sequence - finalize Hash1 - construct encryption key using Hash1[0:16] Capturing loopback communications (e.g., with a sniffer) requires elevated privilege. Thus, the risk associated with this attack vector can be partially mitigated through basic system hardening. However, access to loopback communications is not the only method by which the Credential Provider can be attacked. An unprivileged user can simply open a connection, submit an encrypted request, and if the conditions are right, the Credential Provider may be "tricked" into generating a valid response. Below, a summary of two distinct races is given. Each race consists of two query requests: one made by an authorized user (u_auth) and the other made by an unauthorized/unprivileged user (u_unauth). Observe (see details provided below) that only the authorized query is satisfied in the first race, while both queries are satisfied in the second race. By piggybacking on a lock file created during u_auth's request, the user controlling u_unauth's account is able to retrieve information s/he is not authorized to view/possess. Clearly, this is a security breach. Factors that make this attack possible include: - The key material generated for these communications is weak. The effective key space is less than 2^16 because the key is derived from the TCP source port used to make the request and an undocumented, hard-coded byte sequence embedded in the key derivation code (henceforth referred to as Suffix1). - The user identification mechanism used by the Credential Provider is vulnerable to a race condition where a shared, ephemeral lock file exists in a common folder from the time that the client makes its request to the time that the Credential Provider's response is received/processed. The window of time in which this file persists affords an unauthorized user the opportunity to submit one or more distinct requests that subsequently induce the provider to reuse the yet-to-be-removed lock file. At that point, pending requests (piggyback requests) will be deemed to have originated from the same user as the first request (original request), and if that user is authorized to make each request, then each will be satisfied by the Credential Provider so long as the lock file persists. An attacker who understands how this mechanism works, can simply wait for a lock file to come into existence (or predict its existence based on process table monitoring and TCP port allocation). Once detected (or predicted), a piggyback request can be submitted provided that it originates from the "same" source port as the original request. - Requests originating from loopback addresses other than 127.0.0.1 (e.g., 127.0.0.{2,3,4}, etc.) are honored by the Credential Provider. This makes it possible for an attacker to make a piggyback request from the "same" source port as the original request. Without the SO_REUSEPORT socket option and cooperating processes, each source address/port tuple must be unique on a given system. In other words, two non-cooperating processes can't simultaneously bind to the same source address/port (e.g., 127.0.0.1:10000). One or the other will be rejected by the operating system. However, since 127.0.0.1:10000 and 127.0.0.2:10000 are distinct tuples, they are viewed as distinct endpoints (by the operating system) and are therefore allowed to coexist. The Credential Provider, upon receiving a request from either endpoint, ignores (or discards) the source address, and incorrectly associates that communication as having originated from the "same" port (i.e., port 10000 per the example at hand). - Note that if the Credential Provider is unable to communicate with the Vault, it will continue to answer cached queries. If log records are world readable, they can reveal past queries that were both legitimate and successful. If the provider is configured to maintain a cache, those query results are likely still resident in the cache. Thus, an attacker may use knowledge of cache behavior and available log records to construct piggyback requests that are likely to be satisfied when the race is won. Below, a sanitized excerpt of the authorization policy recovered from the Credential Provider's local cache (appprovider_cache.dat) on [NAME REDACTED] is shown. Observe that u_auth is listed as an authorized user. The scope of this authorization is not fully known. Through testing, it was determined that u_auth is authorized to make password queries from at least two distinct safes (AIM_SAFE1 and AIM_SAFE2). --- policy --- <?xml version="1.0" encoding="UTF-8"?> <AppMetaData> <AIM> <AuthMechanism Type="AppID" /> <AuthReq LastID="142"> <IPAuth> <IPReqData ReqVal="10.10.1.11" ID="11" /> <IPReqData ReqVal="10.10.1.12" ID="12" /> ... <IPReqData ReqVal="10.10.1.140" ID="140" /> </IPAuth> <OSUserAuth> ... <OSUserReqData ReqVal="u_auth" ID="6" /> ... </OSUserAuth> </AuthReq> <ExtendedApplicationRestrictions isEnable="True" /> </AIM> </AppMetaData> --- policy --- Below, the result of an authorized query made by u_auth to retrieve app_123's password located in the AIM_SAFE1 safe is shown. Note that the CyberArk "clipasswordsdk" utility transparently encrypts the request and decrypts the response. $ clipasswordsdk GetPassword -p AppDescs.AppID=APP_ID -p Query="Address=enco;TokenID=APP123Pass;Env=env1" -o Password --- output --- [PASSWORD REDACTED] --- output --- Below, the result of two unauthorized queries made by u_unauth are shown. These queries attempt to retrieve app_456's password located in the AIM_SAFE2 safe. Note that these responses are essentially what an observer could capture if sniffing the loopback interface. Also note that the Elements field contains encrypted query results. Once decrypted (as shown farther below), it is evident that the first response contains an error message indicating that the query failed. Hence, that race was lost. However, the second response clearly contains valid data. Hence, that race was won. This implies that no single race is a sure bet. Rather, factors such as current system load, number of processors, processor speed, etc. will influence the outcome of any given race. Note that an unconstrained user may be able to influence system load sufficiently to consistently win the race. --- RESPONSE_1_BEGIN [TCP_PORT=44222] --- <?xml version="1.0" encoding="UTF-8" standalone="no" ?> <Message> <Header> <ProtocolVersion>995000</ProtocolVersion> <ProtocolInfo>3</ProtocolInfo> </Header> <Elements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lements> </Message> --- RESPONSE_1_END [TCP_PORT=44222] --- --- RESPONSE_2_BEGIN [TCP_PORT=44228] --- <?xml version="1.0" encoding="UTF-8" standalone="no" ?> <Message> <Header> <ProtocolVersion>995000</ProtocolVersion> <ProtocolInfo>3</ProtocolInfo> </Header> <Elements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lements> </Message> --- RESPONSE_2_END [TCP_PORT=44228] --- --- DECRYPTED_ELEMENTS_RESPONSE_1_BEGIN [TCP_PORT=44222] --- <Elements> <Element> <ElementType>ErrorResponse</ElementType> <ResponseBase> <ResponseId>1</ResponseId> <ErrorResponse> <ErrorCode>-1</ErrorCode> <ErrorMsg>APPAP087E Application authentication failure</ErrorMsg> </ErrorResponse> </ResponseBase> </Element> </Elements> --- DECRYPTED_ELEMENTS_RESPONSE_1_END [TCP_PORT=44222] --- --- DECRYPTED_ELEMENTS_RESPONSE_2_BEGIN [TCP_PORT=44228] --- <Elements> <Element> <ElementType>PasswordResponse</ElementType> <ResponseBase> <ResponseId>1</ResponseId> <PasswordResponse> <Password>REDACTED</Password> <Flags>0</Flags> <AlterPassword></AlterPassword> <PassProps> <Address>appprodapplication</Address> <ApplicationName>APP</ApplicationName> <ClassType>Class H</ClassType> <CreationMethod>PVWA</CreationMethod> <Description>SR0118365</Description> <DeviceType>Operating System</DeviceType> <Env>all-prod</Env> <PolicyID>H_SRV_GENERIC_Q_XXX_APC</PolicyID> <RegSCI>ISCI</RegSCI> <TokenID>APPPassword</TokenID> <UserName>app_456</UserName> </PassProps> <PasswordChangeInProcess>false</PasswordChangeInProcess> </PasswordResponse> </ResponseBase> </Element> </Elements> --- DECRYPTED_ELEMENTS_RESPONSE_2_END [TCP_PORT=44228] --- Below, the relevant log entries generated for queries made during the first race are shown. Note how all timestamps are identical. Note also that the reason the race was lost is revealed by the first record in APPConsole.log. Essentially, the authorized query (first entry in APPAudit.log) was satisfied and its associated lock file (/tmp/AIM44222) was removed before the unauthorized query (second entry in APPAudit.log) could be processed by the Credential Provider. And since the lock file was removed, the provider had no means to look up the process ID of the requesting process. --- APPAudit.log --- [DATE | 18:09:08] | :: | APPAU001I Provider Prov_[REDACTED] has successfully fetched password [safe=AIM_SAFE1,folder=Root,name=[REDACTED]] with query [Address=enco;TokenID=APP123Pass;Env=env1] for application [APP_ID]. Fetch reason: [] [DATE | 18:09:08] | :: | APPAU002E Provider Prov_[REDACTED] has failed to fetch password with query [Address=appprodapplication;Env=all-prod;TokenID=APPPassword] for application [APP_ID]. Fetch reason: []. Failure reason: [APPAP087E Application authentication failure] --- APPAudit.log --- --- APPConsole.log --- [DATE | 18:09:08] | :: | APPAP087E Application authentication failure for Application APP_ID (CASCU086E Failed to find application process id. (Error: The lock file (/tmp/AIM44222) couldn't be opened, Error code: 2).) [DATE | 18:09:08] | :: | APPAP002E Provider Prov_[REDACTED] has failed to fetch password with query [Address=appprodapplication;Env=all-prod;TokenID=APPPassword] for application [APP_ID]. Fetch reason: []. Failure reason: [APPAP087E Application authentication failure] --- APPConsole.log --- Below, the relevant log entries generated for queries made during the second race are shown. Note how all timestamps are identical. The authorized query corresponds to the first entry in APPAudit.log, and the unauthorized query corresponds to the second entry. Note how two distinct safes were queried. This suggests that safes not normally accessed/queried by a given Credential Provider may be targeted by an attacker from a different system. Clients should be cautioned that the contents of any given safe should be confined to a single security domain. --- APPAudit.log --- [DATE | 18:09:13] | :: | APPAU001I Provider Prov_[REDACTED] has successfully fetched password [safe=AIM_SAFE1,folder=Root,name=[REDACTED]] with query [Address=enco;TokenID=APP123Pass;Env=env1] for application [APP_ID]. Fetch reason: [] [DATE | 18:09:13] | :: | APPAU001I Provider Prov_[REDACTED] has successfully fetched password [safe=AIM_SAFE2,folder=Root,name=[REDACTED]] with query [Address=appprodapplication;Env=all-prod;TokenID=APPPassword] for application [APP_ID]. Fetch reason: [] --- APPAudit.log --- 4. Mitigation and Remediation Recommendation The vendor has released an updated version (v12.1) which remediates the described vulnerability. Release notes are available at: https://docs.cyberark.com/Product-Doc/OnlineHelp/PAS/Latest/en/Content/Release%20Notes/RN-WhatsNew12-1-CPs.htm?tocpath=Get%20Started%7CWhat%E2%80%99s%20New%7CRelease%20Notes%7C_____4 5. Credit This vulnerability was discovered by Klayton Monroe of KoreLogic, Inc. 6. Disclosure Timeline 2020.11.04 - KoreLogic submits vulnerability details to CyberArk. 2020.11.05 - CyberArk acknowledges receipt and the intention to investigate. 2020.11.16 - KoreLogic and CyberArk meet to discuss the details of this and other reported vulnerabilities. Both parties agree that the remediation timeline will extend significantly longer than the standard 45 business days specified in the KoreLogic Public Disclosure Policy. 2021.01.14 - 45 business days have elapsed since the vulnerability was reported to CyberArk. 2021.01.21 - KoreLogic and CyberArk meet to discuss proposed remediation efforts for this and other reported vulnerabilities. 2021.03.24 - 90 business days have elapsed since the vulnerability was reported to CyberArk. 2021.04.22 - CyberArk notifies KoreLogic that the reported vulnerability will be mitigated in a version scheduled for release in late May, 2021. 2021.05.10 - 120 business days have elapsed since the vulnerability was reported to CyberArk. 2021.05.10 - CyberArk provides KoreLogic with the CVE for this vulnerability. Vendor requests KoreLogic delay public disclosure until the end of June, 2021. 2021.06.08 - KoreLogic and CyberArk meet to discuss the details of the product release and revisit timeline for public disclosure. CyberArk informs KoreLogic that the Linux/Windows version of the Credential Provider will be released at the end of June, 2021. A Credential Provider for the zOS platform will be released at the end of July, 2021. KoreLogic agrees to delay public disclosure of this and other reported vulnerabilities until 2021.08.15. 2021.06.23 - CyberArk releases Credential Provider v12.1 for Linux/Windows platforms. 2021.08.05 - 180 business days have elapsed since the vulnerability was reported to CyberArk. 2021.08.10 - CyberArk informs KoreLogic that the zOS Credential Provider update has been released to their customers. Requests that KoreLogic forgo publication of the Proof of Concept code as an unforseen issue prevents some customers from updating in the near term. 2021.08.27 - KoreLogic suggests delaying the release of the Proof of Concept until a to-be-determined future date. 2021.08.30 - CyberArk tenders 2022.01.01 release date for the Proof of Concept. 2021.09.01 - KoreLogic public disclosure. 7. Proof of Concept At the vendor's request, KoreLogic has agreed to delay publication of the Proof of Concept while customers continue to deploy the updated versions of the product. The contents of this advisory are copyright(c) 2021 KoreLogic, Inc. and are licensed under a Creative Commons Attribution Share-Alike 4.0 (United States) License: http://creativecommons.org/licenses/by-sa/4.0/ KoreLogic, Inc. is a founder-owned and operated company with a proven track record of providing security services to entities ranging from Fortune 500 to small and mid-sized companies. We are a highly skilled team of senior security consultants doing by-hand security assessments for the most important networks in the U.S. and around the world. We are also developers of various tools and resources aimed at helping the security community. https://www.korelogic.com/about-korelogic.html Our public vulnerability disclosure policy is available at: https://korelogic.com/KoreLogic-Public-Vulnerability-Disclosure-Policy.v2.3.txt


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