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Diving Deeper: Unmasking the Spotify Installer's Network Secrets (Or Not!)

My recent bug bounty adventure with the Spotify installer took an interesting turn. After thoroughly investigating potential DLL hijacking vulnerabilities and finding the installer to be surprisingly resilient, the next logical step was to peek into its network communications. After all, the installer prominently displayed a "downloading installer" message, implying it was reaching out to the internet.

This blog post chronicles our journey into capturing HTTP/HTTPS requests, battling DNS complexities, attempting local server interception, and ultimately, uncovering more about the installer's robust design.

The New Challenge: Capturing Network Traffic

Our trusted Process Monitor was fantastic for file system and registry activity, but it falls short when it comes to detailed HTTP/HTTPS requests. For that, we needed a dedicated network analysis tool. While Fiddler Classic is often my go-to for web traffic (especially with its easy HTTPS decryption), we opted for the powerful low-level packet capture capabilities of Wireshark for this leg of the journey.

First Glimpse: A Curious DNS Query

With Wireshark set to capture all traffic on my primary network interface, I launched SpotifySetup.exe. To narrow down the overwhelming amount of data, I applied a display filter: dns.qry.name contains "spotify". Immediately, one query caught my eye:

99530	10.982355	192.168.1.167	192.168.1.1	DNS	81	Standard query 0x00e4 A login.app.spotify.com

Several things stood out:

  1. Local DNS Resolution: The query was from my machine (192.168.1.167) to my router (192.168.1.1). This is normal for home networks, where the router acts as the local DNS resolver, forwarding requests upstream.
  2. Unusual Domain Name: login.app.spotify.com looked less like a traditional domain and more like a path or a dynamically constructed string within a domain name.
  3. type HTTPS Query: Expanding the DNS response, I noticed the query was specifically for type HTTPS (an SVCB record), not a standard A (IPv4) or AAAA (IPv6) record.

Crucially, the DNS response to this query was NXDOMAIN (No Such Domain). This meant the DNS server was explicitly stating that no HTTPS record existed for login.app.spotify.com.

The Hypothesis: Can We Trick the Installer with Local DNS Spoofing?

An NXDOMAIN response, while indicating non-existence, sparked an idea: what if we could force the installer to resolve this domain to our local machine? This technique, known as local DNS spoofing or DNS redirection, is common for security testing and understanding application behavior.

The plan was simple:

  1. Modify my hosts file to make login.app.spotify.com resolve to a local IP address (e.g., 127.0.0.2).
  2. Run a simple local web server on 127.0.0.2 to see if the installer would make any requests to it.

Implementing the Interception

First, the hosts file modification:

# Open C:\Windows\System32\drivers\etc\hosts as Administrator
127.0.0.2 login.app.spotify.com

(I used 127.0.0.2 as 127.0.0.1 was in use for other purposes, which is perfectly fine for loopback addresses).

Next, I fired up a simple Python HTTP server, bound specifically to our chosen loopback address on port 80:

Bash
python -m http.server 80 --bind 127.0.0.2

(I knew a simple HTTP server wouldn't handle HTTPS, which the installer likely uses, but this was a test to see if any connection was even attempted).

The Verdict: Silence on the Wire

With the hosts file modified and my Python server diligently listening, I relaunched SpotifySetup.exe. In Wireshark, I set a new filter: ip.dst == 127.0.0.2 to capture any traffic heading to my local server.

The result? Absolutely nothing.

Wireshark showed no packets destined for 127.0.0.2. My Python server's console remained stubbornly blank. The installer proceeded as usual, completely ignoring our carefully crafted local redirection.

Why It Didn't Work: Lessons Learned

This lack of activity, though a "failure" for interception, was a significant success in understanding the installer's design:

  1. Strict DNS Record Type Validation: The most probable reason for the failure. The installer didn't just want any IP address for login.app.spotify.com. It specifically queried for an HTTPS (or SVCB) DNS record. Since my hosts file could only provide a basic A record (an IPv4 address) and couldn't fake the HTTPS record type, the installer likely deemed the DNS resolution insufficient or invalid and decided not to proceed with a connection to that particular domain.
  2. Robust Network Logic & Fallback: This indicates the installer has strong, secure network handling. If its primary, preferred method of finding download servers (likely involving specific DNS record types or secure certificate validation) fails, it doesn't blindly connect to whatever IP is returned. It likely falls back to other, hardcoded, or securely determined URLs/CDNs that bypass simple local DNS redirection.
  3. Client-Side Resilience: The Spotify installer proved to be quite resilient against both DLL hijacking attempts and this network interception. It highlights that modern, well-developed applications integrate multiple layers of security to prevent tampering and ensure secure downloads.

Conclusion: A Secure Installer

This detailed network analysis, following our earlier DLL hijacking investigation, paints a clear picture: the SpotifySetup.exe installer (at least this version) is remarkably secure against these common client-side attack vectors. While it was a "no-bug" outcome, the journey provided invaluable insights into application security, advanced debugging, and the nuanced world of network protocols. Every failed exploit attempt is a success in learning and understanding robust software.

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