Google has made its vulnerability reward program even more lucrative for security researchers who discover bugs in its software and services.

The nearly three-year-old bug bounty program now offers researchers $7,500 per cross-site scripting flaw found on https://accounts.google.com, up from $3,133.70, and $5,000 for XSS and other flaws in Gmail and Google Wallet, up from $1,337. Google will now pay $3,114.70 per bug found in other Google sites -- up from $500 -- and $7,500 for "significant" authentication bypass and information leak flaws, which previously yielded $5,000 per bug.

Why the raise for researchers? Google security team members Adam Mein and Michal Zalewski said in a blog post announcing the changes that the increased payments should better reward the relative difficulty of finding bugs in Google's properties.

"In recognition of the difficulty involved in finding bugs in our most critical applications, we're once again rolling out updated rules and significant reward increases," they said in the post.

Google to date has paid out $828,000 to more than 250 researchers since launching the vulnerability reward program in November 2010. Some of those researchers donated their rewards to charity as part of Google's program. "Our vulnerability reward programs have been very successful in helping us fix more bugs and better protect our users, while also strengthening our relationships with security researchers," the Google team says.

The increase in payment for vulnerability finds comes on the heels of Google's announcement late last month of a more aggressive disclosure policy that would allow its own researchers to provide details on zero-day bugs they find in other vendors' software within seven days if the affected vendor hasn't provided an advisory or patch. That change in disclosure policy was aimed at pressuring vendors to more quickly fix bugs.

Google researchers had recently discovered attacks using a zero-day in another unnamed vendor's software. "We recently discovered that attackers are actively targeting a previously unknown and unpatched vulnerability in software belonging to another company. This isn't an isolated incident -- on a semi-regular basis, Google security researchers uncover real-world exploitation of publicly unknown ("zero-day") vulnerabilities. We always report these cases to the affected vendor immediately, and we work closely with them to drive the issue to resolution," the Googlers said in that post.

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