Browser-maker Mozilla has disabled a new Firefox opportunistic encryption capability it introduced only last week in Firefox 37 after being informed of a major security flaw in the browser’s implementation of the feature.

In an advisory, Mozilla described the flaw as “critical” and said it caused the browser to not display warnings of invalid SSL certificates. The flaw allows attackers to impersonate a legitimate website via a man-in-the-middle (MitM) attack and replace a site’s valid digital certificate with their own.

Mozilla has mitigated the issue for the moment with a new version of its browser, Firefox 37.0.1, in which the encryption capability has been turned off. It has not disclosed whether it would reintroduce opportunistic encryption on Firefox, if at all.

Unauthenticated Encryption

Firefox opportunistic encryption is a feature designed to encrypt communications between a client and Web server, even if the server does not support the HTTPS protocol. Mozilla describes it as a feature that provides unauthenticated encryption over the Transport Layer Security protocol for traffic that would otherwise have been transmitted in clear text. The feature encrypts HTTP traffic on servers that support the HTTP/2 AltSvc capability.

Though such opportunistic encryption does not offer the same level of protection offered by HTTPS, it does offer a certain measure of confidentiality against passive eavesdropping, Mozilla said. The Internet Engineering Task Force describes opportunistic encryption as a useful alternative to all-or-nothing protection, which is the only option that is otherwise currently available through authentication-based encryption.

Implementation Issue

The flaw that caused Mozilla to disable the Firefox opportunistic encryption capability exists in the browser’s implementation of HTTP Alternative Services.

Reported by security researcher Muneaki Nishimura, the flaw basically allows SSL certification verification to be bypassed if an Alt-Svc header is specified in the HTTP/2 header, Mozilla said in its advisory. As a result, warnings of invalid SSL certificates would not be displayed, creating a potential for MitM attacks.

Mozilla’s response to the flaw continues to be a trend among the major browser-makers to quickly address reported security vulnerabilities. In a recent review of software vulnerabilities in the five most popular browsers, security vendor Secunia discovered a faster-than-average response time to security threats among browser makers.

Secunia’s research showed that most browser vulnerabilities over the past two years were patched in 30 days or less, suggesting browser-makers are serious about security, the company noted.

Browser Vulnerabilities Increase

However, at the same time, the research also revealed a sharp increase in browser vulnerabilities recently. Between 2009 and 2014, the number of flaws reported in Google Chrome, Mozilla Firefox, Microsoft Internet Explorer, Apple Safari and Opera jumped from 207 to 1,035. Between 2013 and 2014 alone, browser vulnerabilities increased by 42 percent, the report showed.

Despite vendor efforts to bolster security, browsers remain fairly easy targets for malicious attackers. In the Pwn2Own white hat hacker contest earlier this year, security researchers were able to take down Firefox, Chrome, Internet Explorer and Safari in a matter of minutes. Over a span of just two days, researchers competing in the hackathon unearthed a total of three bugs in Firefox, four in Internet Explorer, one in Google Chrome and two in Safari. All the bugs were remotely exploitable and had not been previously reported.

Hewlett-Packard, which sponsored the event, ended up paying the security researchers upward of $500,000 for their system code-level execution exploits against the browsers. The results were similar to Pwn2Own events in previous years and point to the continuing challenges browser makers face in addressing vulnerabilities in their products despite the heightened attention being paid to the problem.

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