The modern Gold Mine Rush – Office 365 as a crypto miner’s paradise

Discovery

Earlier this month, Netskope Threat Protection detected a file named coinhive.min.js in Microsoft Office 365 OneDrive for Business. We examined the functionality and found that it is a browser-based plugin coin miner. Using Netskope Discovery and additional details from the Netskope Active Introspection alert, we found the presence of the Coinhive miner in a website named https://www.sky-future[.]net.

An excerpt of the Coinhive code seen in the website is shown in Figure 2.

Figure 2: Coinhive miner in the website, https://www.sky-future[.]net

Overview of Coinhive

Coinhive is a JavaScript library launched in 2017. It allows a website to use the client computer to mine Monero cryptocurrency offering site owners an alternative to online advertising. The money mined by the browser-based hosted scripts is credited from Coin Hive to the website Owners/Administrators. Using Coinhive, one can get the number of hashes solved for a user account, withdraw hashes, verify tokens and programmatically create short links.

Unlike popular miners, Coinhive does not provide any specific information about the account owner as per the privacy terms.

The mining options provided by the Coinhive constructor is shown in Figure 3.

Figure 3: Coinhive Constructor options

Authedmine

Coinhive miner browser plugin is simple and easy to install. This plugin has been lately adopted by several websites, thus raising concerns about the usage of CPU power and resources without the user’s consent. Approximately, 2.2 percent of the top 100,000 websites on the Alexa list have been reported to be mining through user PCs. To address this, Coinhive launched Authedmine, which enforces an explicit opt-in consent from the end user to run the miner. If the user approves the permission, an opt-in token is stored in a cookie to perform the mining operation. However, this is not a mandatory implementation, thus making all previous solutions to continue to work without the explicit opt-in consent as shown in Figure 4.

Figure 4: Loading the Coinhive miner

Analysis of the Coinhive miner

On visiting the website, https://www.sky-future[.]net, a GET request was called to the Coinhive miner plugin from the browser as shown in Figure 5.

Figure 5: GET request called to the coin hive miner plugin

Since the website owner did not implement the Authedmine option, no explicit opt-in consent is displayed while visiting the website. Hence, any user from the organization is unaware that the corporate machine is used for mining without their knowledge or consent.

The Coinhive config present in the script “coinhive.min.js” contained a list of websocket shards in to perform the mining Figure 6.

Figure 6: Coinhive websocket shards

Using the user key mentioned in Figure 2, Coinhive begins the worker job as shown in Figure 7.

Figure 7 : Worker job of Coinhive

Since the mining is performed without users’ consent without displaying the explicit opt-in consent, we can give a verdict that the Coinhive miner plugin is carrying out a cryptojacking operation, causing the user’s browser to consume high CPU usage.

Conclusion

Browser-based mining is an innovation and also an alternative revenue generating model. Since the mining operation takes place while visiting the website, we expect an increasing potential of browser miners. This leads to an interesting arena of choosing the option of displaying ads or giving away CPU cycles every time the website is visited. On the same note, this can be a serious concern for organizations who are unaware that the corporate machines are being used for mining without their knowledge or consent. The browser miners can also be abused by malware authors to exploit the victims computing power and resources. As enterprises are rapidly adopting the use of cloud and cloud services, the need for a threat-aware solution and defensive mechanism for SSL inspection is required. After the initial analysis and reporting, the website, https://www.sky-future[.]net removed the Coinhive plugin from the website. We will continue to monitor the trends of the coin miners and update accordingly.

General Recommendations

Netskope recommends the following to combat cloud malware and threats:

Detect and remediate cloud threats using a threat-aware CASB solution like Netskope and enforce policy on usage of unsanctioned services as well as unsanctioned instances of sanctioned cloud services
Sample policies to enforce:
- Scan all uploads from unmanaged devices to sanctioned cloud applications for malware
- Scan all uploads from remote devices to sanctioned cloud applications for malware
- Scan all downloads from unsanctioned cloud applications for malware
- Scan all downloads from unsanctioned instances of sanctioned cloud applications for malware
- Enforce quarantine/block actions on malware detection to reduce user impact
- Block unsanctioned instances of sanctioned/well known cloud apps, to prevent attackers from exploiting user trust in cloud. While this seems a little restrictive, it significantly reduces the risk of malware infiltration attempts via cloud
Enforce DLP policies to control files and data en route to or from your corporate environment
Regularly back up and turn on versioning for critical content in cloud services
Enable the “View known file extensions” option on Windows machines
Warn users to avoid executing unsigned macros and macros from an untrusted source, unless they are very sure that they are benign
Administrators can create firewall rules to block bitcoin pools documented in the Wikipedia article
Warn users to avoid executing any file unless they are very sure that they are benign
Warn users against opening untrusted attachments, regardless of their extensions or filenames
Keep systems and antivirus updated with the latest releases and patches