A new version of the TDL rootkit-type malware program downloads and abuses an open-source library called the CEF (Chromium Embedded Framework) that allows developers to embed the Chromium Web rendering engine inside their own applications, according to security researchers from antivirus vendor Symantec.
In an effort to temporarily block the abuse, CEF project administrators suspended the framework's primary download location on Google Code.
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The TDL malware generates profit for its authors by redirecting the victims' search results to websites and services of a dubious nature, by displaying pop-up advertisements for various products and services or by infecting computers with other threats as part of a pay-per-install malware distribution scheme.
The latest TDL variant no longer uses custom code to implement its Web browser functionality, and instead relies on CEF, which it downloads from the project's site on Google Code. This allows the malware to have smaller components that are easier to update, the Symantec researchers said Friday in a blog post.
After learning that the CEF library is being abused by malware, the CEF creators decided to block automated downloads of their software by implementing CAPTCHA and session verification. However, since this couldn't be done on Google Code, they had to move their file downloads to a third-party site.
"It has come to our attention that a CEF binary release file (zip archive) hosted on our project page was being directly downloaded by a distributed malware product for illegal purposes," they wrote in a notice posted on the project's official site on Google Code. "The CEF project and its authors do not condone or promote the use of the CEF framework for illegal or illicit purposes. We will take all actions reasonably within our power to frustrate this use case. For that reason current and future downloads will be hosted externally at http://www.magpcss.net/cef_downloads/. This new download location offers improved features and protections over those offered by Google Code hosting."
This news prompted one CEF user to ask on the project forum if there's any risk of antivirus products flagging CEF as malware because of the abuse.
"Given the large number of companies currently using libcef for legitimate purposes I think it's unlikely that we'll end up on any anti-virus black lists," said Marshall Greenblatt, the CEF lead developer, in response to the question. However, companies that bundle CEF with their applications are encouraged to digitally sign the binary files before distribution, he said.
Malware authors have increasingly included open-source libraries and other legitimate tools in their malicious creations for the past few years, said Bogdan Botezatu, a senior e-threat analyst at antivirus vendor Bitdefender, via email Monday. This offers several benefits to them including lower development costs and reducing the amount of code that antivirus vendors can sign as malicious, he said.
There have been some cases where the level of misuse of a legitimate tool has been so great that the tool was eventually flagged as malware or riskware by security vendors, because the malicious use cases far exceeded the legitimate ones. However, chances that a popular library like OpenSSL would be blacklisted because of malware abuse are extremely low, Botezatu said. "Common practices in the AV industry demand that malware researchers or automated systems do not sign [as malware] open-source code or freeware libraries."
Botezatu agreed with Greenblatt's advice of signing the third-party libraries distributed with an application. "This minimizes the risk of these libraries getting picked by antivirus solutions and is also a good way to minimize operating system or browser warnings upon download," he said. "However, there is the legal aspect of whether the developer is allowed to distribute the library along with their products or not. Always get written consent before distributing or digitally signing a file."