Ars Technica have recently published a series of fascinating (at least for those of us interested in web security) articles on the ongoing difficulties between HBGary, a security consultancy firm, and Anonymous, a collective of computer hackers.

The articles are well written and are worth reading in sequence:

How one man tracked down Anonymous—and paid a heavy price

(Virtually) face to face: how Aaron Barr revealed himself to Anonymous

Anonymous speaks: the inside story of the HBGary hack

The last one in particular is well worth your attention: it provides a detailed breakdown of some common attacks like SQL injection, brute force password cracks, and social engineering.

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Updated 2022 : note that the above post was originally published in 2011, but is left here for archival purposes.