Criticism of Adobe Flash

Adobe Flash as a format has become very widespread on the desktop market and created a market dominance. Adobe Systems claims that 98% of all Internet desktop users worldwide (and 99.1% of US/Canada Web users) have the Flash Player installed, with 82% having the latest version. Numbers vary depending on the detection scheme and research demographics. The Flash Player has been criticized for requiring a lot of computing power and therefore not performing well on weaker machines.
Usability & high CPU usage
Many usability concerns regarding Flash concern Flash's numerous breaks with many conventions associated with normal HTML pages. Things like selecting text, scrollbars, form control and right-clicking act differently than with a regular HTML webpage, and it is argued that this contributes to a usability issue. Usability expert Jakob Nielsen published an Alertbox in 2000 entitled, Flash: 99% Bad which listed many of these issues. Much of this criticism is due to poor implementation. For example, bookmarking could be handled, but is often not implemented due to time, cost, or lack of knowledge.
Web pages which make heavy use of Flash can also cause difficulties for some users, such as those using old hardware or who can or will not, install or use Flash Player. As Flash elements often include a lot of graphics and sound, dial-up internet users are also affected by higher page load times, although there is a more general trend toward larger websites as high-speed internet becomes more common.
Large corporation websites which depend on Flash are typically available in a simpler HTML-version. However, the webserver typically decides automatically which version to send to the browser, and the user may not be presented with the choice. This way, the HTML page does not work as a fallback for the Flash page. It is not uncommon for users of alternative flash players to find flash files unplayable, effectively turning such a website into a useless dead end. Since this is a server side decision, the existence of any HTML version is hidden, leaving no clue to the user that removing the flash plugin solves the problem.
In many browsers, it is not possible to scroll a web page with the mouse while the cursor is held over flash content. Scrolling with the arrow keys may require a click on the page outside the flash. News sites may be troublesome to scroll through, as they are often cluttered with flash advertisements, so that one must slalom the cursor around the flash content while scrolling. This way, Flash appears as an unnecessary obstacle, contributing to the impression of poor integration and provides a source of irritation when using a not so ideal mouse, such as a laptop touchpad.
Accessibility issues
The US Justice Department has stated in regard to the Americans with Disabilities Act of 1990:

Covered entities under the ADA are required to provide effective communication, regardless of whether they generally communicate through print media, audio media, or computerized media such as the Internet. Covered entities that use the Internet for communications regarding their programs, goods, or services must be prepared to offer those communications through accessible means as well.

Although long since fixed with accessibility functions since Flash Player 6, Internet users who are visually-impaired, and who may rely on a screen reader, braille display, or using larger text sizes and/or high-contrast color schemes may find sites that make extensive use of Flash difficult or impossible to use.
Content control
Many content producers use Flash as a way to limit user's access to the media displayed in their browsers, and/or gain clicks by forcing extra steps to display. For example, in Windows, Shockwave/Flash (.swf) files cannot be right-clicked and saved. Famously, YouTube furnishes all video in flash video format (.flv). This prevents users from easily downloading copyrighted material and avoiding advertisements. The usage is now spreading to photo sharing websites such as Webshots. A Flash overlay exists over the initial photo displayed, requiring a second click to retrieve the photo, slowing the experience considerably. However, if Flash is not installed, the image displays normally.
Local Shared Objects
Flash Players since version 6 can store and retrieve persistent data without offering any visible signs to the user, similarly to cookies. It is possible to clear the temporary files that Flash stores on a computer either through the Flash website, or manually. The default storage location for LSOs is operating-system dependent:
* For Windows XP, the location is within each user's Application Data directory, under Macromedia\Flash Player\#SharedObjects.
* For Windows Vista it is in each user's AppData directory under Roaming\Macromedia\Flash Player\#SharedObjects.
* For Mac OS X it is in each user's Library directory under Preferences/Macromedia/Flash Player/#SharedObjects.
* On Linux it is in each user's directory: ~/.macromedia/Flash_Player/#SharedObjects.
Freeness
Another issue of great concern to many advocates of free software is the proprietary nature of Flash. The widespread use of Flash has according to many observers from the free software camps harmed the otherwise open nature of the World Wide Web.
Security issues
Specially crafted files have been shown to cause Flash applications to malfunction, by allowing the execution of malevolent code. Users who have not updated their Flash Player to the most recent version may be vulnerable to such an attack.
In addition to over 100 entries in the Open Source Vulnerability Database, security advisories published in August 2002, December 2002 and November 2005 highlight three examples of reports about various Flash Player versions that allowed remote code execution.
Flash Player on various platforms
The Adobe Flash Player is mainly optimized for the Windows 32 bit platform. 32 bit editions of version 9 are also available for Mac OS X, Linux, and Solaris. Although Flash Player 9 resolved important performance and compatibility issues on non-Microsoft platforms, it still has lower performance on some operating systems and may have problems with working properly. In some cases, Gnash may actually work better.
On Linux, sound output from Flash Player 9 may not work on certain combinations of Linux distribution and hardware. The PS3 web browser uses an obsolete version of Flash (licensed from Macromedia) that is not compatible with most modern websites.
As of December 2008, Adobe does not officially support Flash on any 64-bit platform. In November 2008, an experimental 64-bit Flash plugin for Linux was released by Adobe, with 64-bit support for Microsoft Windows and Mac OS X to come later.
Resolved issues
Adobe has rewritten the bitmap drawing routines in Flash Player 8 for Mac, using OpenGL planes via Quartz to draw the surfaces. The new drawing code is reported to be actually faster than its Windows counterpart, where JPEG, TIFF or other bitmap images are composited into the animation.
The Linux version of the Flash Player requires the Advanced Linux Sound Architecture (ALSA) to output sound. ALSA was introduced in Linux 2.5 and may be difficult to use with very old sound hardware. Users of the legacy Open Sound System must either compile and install the abstraction layer flashsupport provided by Adobe, run the Windows Flash Player in a Windows browser through WINE, or switch to ALSA, which may involve recompiling or upgrading the kernel and/or installing additional drivers.
Flash Player 7 for Linux was very CPU hungry in fullscreen mode, resulting in low frame rates. Also, the sound could lag about a second behind the picture; this issue was resolved in Flash Player 9. Flash Player 8 was never released for Linux, Adobe stated that they would skip that version and instead focus on preparing Flash Player 9. Flash Player 9 for Linux was released in January 2007.
Search engine indexability
Flash files are binary data, and so are not as easily indexable as other document formats. Also, due to their dynamic nature, it is often not possible for a search engine to link to a specific section in an all-Flash site.
Some methods have emerged to try deal with this problem. Adobe has a Flash Search Engine SDK, and Flash CS3 creates additional meta information that is indexable by search engines. One approach which has become popular is creating a base HTML page with indexable content, and adding an additional flash layer. This method of 'progressive enhancement' has become more popular in recent years.
Digital rights management
The latest iteration of Flash allows copyright holders to embed ads within videos, as well as control how those videos are used. With this latest piece of software, companies will be able to quickly remove any video that they feel violates copyright and force advertisements to play prior to the start of the video.

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