Malware Analysis

ThreatForge Challenge 01, First Contact: Static Analysis Report

My first malware analysis writeup: a static-only triage of an NSIS-packaged sample, focused on structural evidence and what remains unknown.

malware-analysisstatic-analysisthreatforge

Scope: Static analysis only. No execution, no dynamic instrumentation.

Sample source: MalwareBazaar. Hash and original filename are intentionally withheld. This is a teaching writeup, not a sample index.

1. Scenario

An executable of unknown provenance was recovered from a host reported to be running noticeably slower than normal. The task was to determine, from static examination alone, what the file is and whether it warrants escalation.

2. Environment

Analysis was performed in an isolated REMnux VM under VirtualBox on a Kali Linux host. The VM used a host-only adapter with no route to the internet or the host network. A clean baseline snapshot was taken before the sample was introduced, and the sample was transferred via a shared folder. The host machine performed no analysis work at any point. Nothing about the sample should be handled outside the isolated guest.

Tooling: diec (Detect It Easy), pefile, 7z, standard REMnux string and PE utilities.

3. Findings

3.1 File identification

Property Value
Format PE32 executable
Architecture 32-bit
Target OS Microsoft Windows
Subsystem GUI
Compiler Microsoft C/C++
Packaging Nullsoft Scriptable Install System (NSIS) self-extracting installer
PE sections 5

The file is not a bare executable. It is an NSIS installer stub with a compressed archive appended to it.

3.2 Compile timestamp

The PE header records a compile timestamp of Sunday, 2 July 2023, 02:09 UTC. This value is reported but not trusted: PE timestamps are trivially forgeable, and in an NSIS package the stub is a prebuilt component supplied by the NSIS toolchain, so its timestamp reflects when the stub was built, not when this sample was assembled. It is recorded for correlation only and carries no weight in the assessment.

3.3 Structure and entropy

The five PE sections are the NSIS engine itself, unpacked, sitting in the open and reading as ordinary compiled code.

The bulk of the file by size is an overlay: data appended after the formal end of the PE structure. The overlay measures approximately 7.99 entropy, effectively maximum randomness. This is the central structural finding: the PE sections are the delivery mechanism, and the overlay is the cargo. Near-maximum entropy across the majority of a file indicates compressed or encrypted content. It does not by itself indicate packed code, and it does not by itself indicate malice. NSIS compresses its payload as a matter of normal operation.

3.4 Capability inferred from imports

The import table of the NSIS stub supports registry read/write, filesystem operations, process creation, and privilege elevation requests.

Critical caveat: every one of these capabilities is present in a legitimate NSIS installer. An installer, by definition, writes to the registry, drops files, elevates, and launches things. These imports are indistinguishable from a clean software installer and constitute no evidence of malice on their own. They describe what the stub is capable of, not what this sample does. Actual behavior is determined by the NSIS script and the payload inside the overlay, neither of which are visible in the import table.

3.5 Anti-analysis posture

The payload is compressed and the file’s structure obscures its contents from casual inspection. This structure is identical to that of any legitimate NSIS-packaged application. The compression is not, on its own, evidence of deliberate anti-analysis intent. No packer beyond NSIS was detected, and no obfuscated sections or anti-debug artifacts were identified at this stage.

3.6 Payload extraction

Extraction of the overlay produced multiple components with non-standard extensions: .for, .pre, .eeq, .col. These extensions are meaningless to Windows and do not correspond to any recognized format, which is consistent with an installer that drops inert-looking staging files, later reassembled, decoded, or side-loaded by a script or loader component.

Triage of these files is not yet complete. No file-type identification, entropy measurement, or content inspection has been performed on them, and nothing in this report characterizes what they contain.

4. Assessment

The sample is an NSIS installer carrying a high-entropy compressed payload of unknown content, which drops a set of files with non-standard extensions on extraction.

Taken individually, none of the observed properties are inherently malicious. NSIS packaging, unpacked stub sections, installer-typical imports, and a compressed overlay all appear in legitimate software. What raises concern is the combination: a self-extracting installer sourced from a malware repository, delivering payload components with deliberately meaningless extensions, in the context of a host reported to be degraded in performance.

Judgment: treat as suspicious pending payload analysis. The file is a delivery mechanism. Everything that determines whether it is harmful lives in the overlay contents, which have not yet been analyzed. A confident statement of capability or intent is not supportable from the evidence gathered so far.

Confidence: low-to-moderate, and confined to the structural findings above.

5. What this report does not establish

Stated explicitly, because the distinction matters more than the findings:

  • Malware family: not determined by this analysis. Any family attribution attached to this sample originates from the repository it was sourced from, not from work performed here.
  • Payload capability: unknown, not observed.
  • Persistence, C2, or credential access: not observed, not ruled out.
  • Impact on the reported host: no link established between this file and the reported slowdown. The scenario is context, not evidence.

6. Gaps and next steps

  1. String extraction: run strings against both the stub and the extracted components; look for URLs, registry paths, filenames, and mutex names.
  2. Triage the four extracted files: file, entropy per file, header inspection. Determine which is the loader and which are data blobs.
  3. Recover the NSIS script: 7z extraction of an NSIS archive commonly yields the install script, which states the drop paths and execution order directly. This is the fastest route to actual behavior.
  4. Ghidra: basic code analysis of whichever extracted component turns out to be executable.
  5. Re-assess once payload contents are known, and revise the assessment accordingly.

7. Methodological notes

Analysis is not narrative. Where evidence stops, the report stops. Absence of a finding is recorded as absence, not filled with a plausible-sounding inference. Legitimate-looking indicators do not confirm benign intent, and suspicious-looking indicators do not confirm malicious intent; both were treated as neutral until corroborated. The payload is the analysis. Everything upstream of it is packaging.