Technical Analysis of "SSRF Vulnerability via FFmpeg HLS Processing"

Overview:
Valeriy Shevchenko's write-up reveals an SSRF (Server-Side Request Forgery) vulnerability in handling HLS (HTTP Live Streaming) playlists through FFmpeg. The vulnerability is linked to how FFmpeg processes media files, specifically HLS playlists, which can be exploited to make arbitrary HTTP requests.

Key Technical Details:

1. FFmpeg and HLS:
   FFmpeg is a versatile multimedia framework commonly used for converting, streaming, and recording media files. HLS is a popular media streaming protocol developed by Apple. HLS streams comprise a series of small media segment files and a playlist file (M3U8 format) that directs the player to the content.

2. Understanding the HLS Playlist:
   The M3U8 playlist file contains links to media segment files and other playlists. FFmpeg processes this file to fetch the media segments specified within it.

m3u #EXTM3U #EXT-X-VERSION:3 #EXT-X-STREAM-INF:BANDWIDTH=1280000 http://example.com/segment1.ts

1. SSRF Vector in FFmpeg:
   FFmpeg, while processing HLS playlists, does not impose restrictions on the URLs specified in the M3U8 file. This means an attacker can craft a malicious M3U8 playlist containing URLs that point to internal services or resources within the same network (intranet).

2. Example of a malicious playlist: m3u #EXTM3U #EXT-X-VERSION:3 #EXT-X-STREAM-INF:BANDWIDTH=1280000 http://internal-service.local/segment1.ts

3. Exploiting the Vulnerability:
   An attacker could upload or provide a crafted M3U8 file to a service utilizing FFmpeg for media processing. Once FFmpeg processes this file, it attempts to retrieve the segments, inadvertently making HTTP requests to internal services based on the URLs specified. This can be used to perform SSRF attacks, extracting sensitive information or interacting with internal APIs.

4. SSRF Impact:
   The ability to direct FFmpeg to make arbitrary requests could have severe implications, such as:

5. Accessing internal administrative interfaces.
6. Performing unauthorized actions due to internal APIs expecting requests from within the network.
7. Bypassing IP-based access controls.

8. Leaking sensitive data from otherwise inaccessible endpoints.

9. Mitigating the Vulnerability:
   Proper mitigation requires implementing strict network and URL validation within FFmpeg or the consuming application. Recommendations include:

10. Whitelist allowed URLs/domains within the application processing HLS playlists.
11. Restrict internal network access for services handling untrusted M3U8 files.
12. Use isolation techniques (e.g., sandboxing) to limit the potential damage from SSRF.

Detailed Breakdown:

• HLS Playlist Structure: A typical HLS playlist (M3U8) file lists the locations of media segments or other playlist files using HTTP(S) URLs.

• Processing Behavior of FFmpeg: When FFmpeg reads an M3U8 file, it follows the listed URLs to retrieve necessary media segments. This behavior is exploited in the attack.

• Crafting the Malicious M3U8 File: The crafted M3U8 file exploits FFmpeg's trust by listing URLs pointing to internal network resources, which the attacker would not have direct access to.

m3u #EXTM3U #EXT-X-VERSION:3 #EXT-X-STREAM-INF:BANDWIDTH=1280000 http://localhost:8080/internal-api

• Impact Analysis:
• Data Exfiltration: Sensitive data can be retrieved from internal URLs.
• Intrusions: Unauthorized actions can be performed on internal services.

• Network Mapping: Revealing the internal network structure depending on error codes returned.

• Mitigation Strategies:

• Network Controls: Limit network exposure of services processing HLS files.
• URL Whitelisting: Only allow trusted URLs to be processed.
• Sanitization: Sanitize and validate URLs in M3U8 files before processing.
• Isolation: Process untrusted media files in isolated environments to prevent network access.

Conclusion:

SSRF vulnerabilities, facilitated by FFmpeg’s handling of HLS playlists, demonstrate the risks of trusting incoming media data from untrusted sources. This write-up underscores the importance of validating and controlling network access, especially within powerful tools like FFmpeg, to mitigate such vulnerabilities.

For the original write-up, visit here.