There are a growing number of Western technology companies, including Cisco, IBM, and SAP, that are caving to demands by Moscow for access to closely guarded product security secrets.

What the Russians are demanding is that Western tech companies to allow them to review source code for security products such as firewalls, anti-virus applications and software containing encryption. This must be done they say before they’ll allow the products to be imported and sold in the country. These “requests”, which have increased since 2014, are done, according to the Russians to ensure foreign spy agencies have not hidden any “backdoors” that would allow them to burrow into Russian systems.

The timing of these demands by the Russians comes at a particularly sticky time when Russia has been accused of a growing number of cyber-attacks on the West, including the recent US Presidential election.

But those inspections also provide the Russians an opportunity to find vulnerabilities in the products’ source code – instructions that control the basic operations of computer equipment – current and former U.S. officials and security experts said.

While a number of U.S. firms say they are playing ball to preserve their entree to Russia’s huge tech market, at least one U.S. firm, Symantec, told Reuters it has stopped cooperating with the source code reviews over security concerns. That halt has not been previously reported.

Symantec said one of the labs inspecting its products was not independent enough from the Russian government.

U.S. officials say they have warned firms about the risks of allowing the Russians to review their products’ source code, because of fears it could be used in cyber attacks. But they say they have no legal authority to stop the practice unless the technology has restricted military applications or violates U.S. sanctions.

From their side, companies say they are under pressure to acquiesce to the demands from Russian regulators or risk being shut out of a lucrative market. The companies say they only allow Russia to review their source code in secure facilities that prevent code from being copied or altered.