What Are Debug Logs?

Debug logs in Linux systems are files that store detailed information about the behavior of applications, services, or the system itself. These logs are primarily used for troubleshooting and debugging. They often include:

• Application errors and warnings.
• System failures (e.g., segmentation faults).
• Debugging information left by developers.

These logs are typically stored in specific directories, such as /var/log/, or custom locations defined by the application.

How Are Debug Logs Manipulated?

Attackers use tools like sed (stream editor) or direct file editing to alter or remove specific entries in debug logs. For example:

sed -i '/segfault/d' debuglog

This command does the following:

• Searches for a Pattern: Looks for lines containing the word segfault (indicative of segmentation faults or crashes).
• Deletes Matching Lines: Removes all occurrences of lines matching the pattern from the file debuglog.
• Preserves the Rest of the File: Keeps non-matching entries intact, maintaining the overall structure of the log.

The -i option modifies the file in place without leaving a backup, ensuring minimal traces of the modification.

How It Works Internally

• Pattern Matching: The sed tool processes the log file line by line, comparing each line to the specified pattern (e.g., segfault).
• Line Deletion: When a match is found, sed excludes the line from the output.
• File Overwriting: With the -i option, sed rewrites the original file with the modified content, effectively erasing the targeted entries.

This process is lightweight and does not generate new log entries unless monitored by other security mechanisms.

How It Can Be Abused

Attackers manipulate debug logs to:

• Erase Evidence of Exploitation: Remove entries showing application crashes, segmentation faults, or other errors that may indicate the use of malicious payloads or exploits.
• Avoid Detection: By selectively deleting specific entries (e.g., those containing error messages or traces of exploitation), attackers can make logs appear normal and avoid raising suspicion.
• Mislead Investigators: By keeping the rest of the log file intact, attackers create the illusion that the system is functioning correctly, potentially leading investigators to incorrect conclusions.

Example Abuse:

• Removing traces of failed exploitation attempts prevents investigators from identifying the attack vector.
• Erasing entries showing malicious behavior, such as unauthorized commands or data access, conceals the attacker's presence.