Detecting DLL Sideloading Attacks

When to Use

• When investigating potential DLL hijacking in enterprise environments
• After EDR alerts on unsigned DLLs loaded by signed applications
• When hunting for APT persistence using legitimate application wrappers
• During incident response to identify trojanized applications
• When threat intel indicates DLL sideloading campaigns targeting specific software

Prerequisites

• EDR with DLL load monitoring (CrowdStrike, MDE, SentinelOne)
• Sysmon Event ID 7 (Image Loaded) with hash verification
• Application whitelisting or DLL integrity monitoring
• Software inventory of legitimate applications and expected DLL paths
• Code signing verification capabilities

Workflow

1. Identify Sideloading Targets: Research known vulnerable applications that load DLLs without full path qualification (LOLBAS, DLL-sideload databases).
2. Monitor DLL Load Events: Query Sysmon Event ID 7 for DLL loads where the DLL path differs from the application's expected directory.
3. Check DLL Signatures: Flag unsigned or untrusted DLLs loaded by signed executables.
4. Detect Path Anomalies: Identify legitimate executables running from unusual locations (Temp, AppData, Public) that may be decoy wrappers.
5. Hash Verification: Compare loaded DLL hashes against known-good versions and threat intel feeds.
6. Correlate with Process Behavior: Check if the host process exhibits unusual behavior (network connections, child processes) after loading the suspicious DLL.
7. Document and Remediate: Report sideloading instances, quarantine malicious DLLs, and update detection rules.

Key Concepts

Concept	Description
T1574.002	DLL Side-Loading
T1574.001	DLL Search Order Hijacking
T1574.006	Dynamic Linker Hijacking
T1574.008	Path Interception by Search Order Hijacking
DLL Search Order	Windows DLL loading priority path
Side-Loading	Placing malicious DLL where legitimate app loads it
Phantom DLL	DLL that legitimate apps try to load but does not exist
DLL Proxying	Malicious DLL forwarding calls to legitimate DLL

Tools & Systems

Tool	Purpose
Sysmon	Event ID 7 DLL load monitoring
CrowdStrike Falcon	DLL load detection with process context
Microsoft Defender for Endpoint	DLL load anomaly detection
Process Monitor	Real-time DLL load tracing
DLL Export Viewer	Verify DLL export functions
Sigcheck	Digital signature verification
pe-sieve	PE analysis for proxied DLLs

Common Scenarios

1. Legitimate App Wrapper: Adversary copies signed application (e.g., OneDrive updater) to temp folder alongside malicious DLL with same name as expected dependency.
2. Phantom DLL Exploitation: Malicious DLL placed in PATH location where legitimate app searches for non-existent DLL.
3. DLL Proxy Loading: Malicious version.dll proxies all exports to real version.dll while executing malicious code on DllMain.
4. Software Update Hijack: Attacker replaces DLL in update staging directory before legitimate updater loads it.

Output Format

Hunt ID: TH-SIDELOAD-[DATE]-[SEQ]
Technique: T1574.002
Host Application: [Legitimate signed executable]
Sideloaded DLL: [Malicious DLL name and path]
Expected DLL Path: [Where DLL should legitimately be]
DLL Signed: [Yes/No]
App Location: [Expected/Anomalous]
Host: [Hostname]
Risk Level: [Critical/High/Medium/Low]