Impersonation Fraud Still Effective in Obtaining Code Signatures

From IRS tax scams to business email compromises, fraudsters impersonating authority figures are a well-known hazard for online users. However, impersonation is also becoming an increasingly used tactic to steal a valuable commodity for the underground market: digital code-signing certificates.

One such scam, analyzed by security firm ReversingLabs — and illuminating the small, yet significant, component of the cybercriminal supply chain — resulted in a certificate authority issuing a valid code-signing certificate. Eventually, the certificate was used to sign an adware program known as OpenSUpdater, as well as 21 other malicious programs.  

The attack is not unique but increasingly used by fraudsters to gain a valuable commodity that is then resold on the Dark Web, says Tomislav Pericin, co-founder and chief software architect of ReversingLabs.

"We have seen the exact same pattern we described in our blog around a dozen times just this year," he says. "We have seen this attacker use the described method to acquire extended validation certificates as well."

As antimalware defenses have evolved, attackers have likewise improved their malware, including extensive obfuscation, anti-analysis techniques, and the abiity to steal legitimate digital certificates to make malware appear as legitimate code to antivirus software. 

For example, in its recent analysis of the Chinese cyber-espionage group APT41, security firm FireEye found that the group targeted video game companies and stole valid code-signing certificates from developers so it could sign future malware. The tactic, along with APT41's penchant for injecting malicious code into software in development, showed a focus on using the software supply chain against targets, FireEye stated.

"These supply chain compromise tactics have also been characteristic of APT41's best known and most recent espionage campaigns," the company said.

In the case analyzed by ReversingLabs, the attacker followed a six-phase process to first identify a likely target of impersonation, next identify a potential domain for which that person is the founder or technical contact to spoof, and then create the infrastructure to mimic the selected company. Typically, a fraudster will set up the infrastructure to redirect Web requests to the actual corporate domain but send email traffic to the server they own. 

Once the digital groundwork was done, the attacker completed the last three phases of the operation: buy a certificate, verify that it works, and put it up for sale on one or more underground forums. The success of the scam requires the person reviewing the application not notice, for example, that the domain registered as a .co.uk does not belong to the same company as the legitimate .com domain. 

"The gamble is that the person verifying the certificate issuance request will assume that the same company owns both the global .com and the regional .co. uk domains for their business," Pericin stated in the blog post.

He elaborated in a separate interview with Dark Reading. "The certificate authorities are under constant pressure to issue certificates as fast as possible," he says. "And the usual three to five business-day turnaround for code signing certificate purchase can be insufficient to do thorough checks."

The process highlights weaknesses in the security check that underpins the entire Web of trust model. Certificate authorities, tasked with validating the information behind the certificates, are responsible for the security of the entire system. 

"They represent a mechanism that guarantees the identities behind the signatures are verified," Pericin stated in the blog. "For this reason, it is crucial that the processes they implement to check the identities are resilient to potential abuse — something much easier said than done."

In particular, the passing of the EU's General Data Protection Regulation (GDPR) means domains registered in Europe are protected from casual queries, so it is difficult to find out who actually owns a domain, the blog pointed out.

The analysis is not the first time a company has shed light on the market for code-signing signatures. In 2018, open source intelligence firm Recorded Future reported that an increasing number of code-signing certificated available in underground markets were the result of impersonation. The company found that fraudsters were selling the certificates for $300 to $1,600 online. 

Those prices are similar to those advertised by the subject of ReversingLabs' investigation. The attacker likely made $500 to $1,000 on the fraudulent certificate, Pericin tells Dark Reading. 

Companies that could be a target for such attacks are at a disadvantage in defending against impersonation. Most defenses amount to monitoring and preventative registration, such as buying up as many related domains as possible to minimize confusion and to monitor for mentions of their credentials' misuse, he says.

"Since these threats are external to the organization, the best advice we can give out at this time is to invest in brand protection, to register domains that are easily mistaken with the brand owner ahead of time, [and] if possible, to monitor the Web for mentions of their brand in the context of malware misuse," Pericin says. 

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