IT Forensics · Linux

Forensic analysis of a btmp file – analysing failed login attempts

The btmp file logs failed login attempts and thus provides important evidence of potential brute-force attacks or unauthorised attempts to access individual user accounts.

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As btmp records only failed attempts, it is necessary to cross-reference the data with successful login attempts from wtmp in order to obtain a complete picture of the possible course of an attack.

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Since its foundation, LanCologne has specialised in professional IT forensics. Our staff have decades of experience in the field of information technology and assist companies, solicitors, private individuals and, on a regular basis, the courts in the technical investigation of complex digital matters.

The examination is, as a matter of principle, carried out exclusively on a forensic copy, a forensic image or a data source captured in a technically equivalent manner that preserves the integrity of the evidence. The original evidence remains unchanged and is stored in a manner that preserves its integrity.

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We extract failed login attempts from btmp, organise them chronologically and by affected user account, and cross-reference them with successful logins and other security logs.

Typical areas of application

•Evidence of attempted brute-force attacks
•Investigation into targeted attacks on individual user accounts
•Reconstruction of the sequence of events during the attack
•Assessment of the effectiveness of existing protective measures
•Incident Response on Linux Systems
•Judicial and non-judicial expert reports

This is how a btmp analysis works

Once the backup has been completed, the failed login attempts recorded in btmp are extracted and organised chronologically. The results are compared with successful logins from wtmp, as well as SSH and PAM logs, in order to reconstruct the actual sequence of events as fully as possible.

Why is btmp analysis relevant in a forensic context?

A pattern of repeated failed login attempts can be a strong indication of a targeted attack, particularly if a successful login followed.

However, considering btmp in isolation, without context, can lead to misinterpretations, for example in the case of accidental input errors by legitimate users. For this reason, an overall assessment is always carried out in the context of other sources.

Frequently Asked Questions

What is logged in btmp?+
Failed login attempts, including the time and, where available, the user account affected and the source.
Does an entry in btmp automatically indicate an attack?+
Not necessarily. Even accidental input errors made by legitimate users are logged, which is why an overall assessment is required.
Can btmp reliably detect brute-force attacks?+
When used in conjunction with other sources, such as SSH logs, btmp can provide strong evidence, but on its own it is usually not conclusive.

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