Disclosing software vulnerabilities can be bad for stock price, according to research carried out by security firm McAfee.
When McAfee researcher Anthony Bettini examines Microsoft's changing stock price, he found that the company underperformed on the days when it announced and released patches (days known as "Advanced Notification Thursday," which is the first Thursday of the month, and "Patch Tuesday," which is the second Tuesday of the month).
The data for Path Tuesday's is particularly interesting (note how the effect seems to be snowballing):
Compare this to any other normal Tuesday:
Or Microsoft average stock day in a particular year:
Here's more data:
What's equally interesting is that the stock price almost always recovers the day after Patch Tuesday. Bettini said that "this is probably because institutional investors or market makers feel Microsoft was oversold the day before because of the bad news and that, in reality, Microsoft's value as an investment was only negligibly affected."
Are there some shady investors taking advantage of this? I doubt it myself given how small the effect is, but in today's climate you never know.