WWBN AVideo's GIF poster fetch bypasses traversal scrubbing and exposes local files through public media URLs

Summary

objects/aVideoEncoderReceiveImage.json.php allowed an authenticated uploader to fetch attacker-controlled same-origin /videos/... URLs, bypass traversal scrubbing, and expose server-local files through the GIF poster storage path.

The vulnerable GIF branch could be abused to read local files such as /etc/passwd or application source files and republish those bytes through a normal public GIF media URL.

Details

The vulnerable chain was:

1. objects/aVideoEncoderReceiveImage.json.php accepted attacker-controlled downloadURL_gifimage
2. traversal scrubbing used str_replace('../', '', ...), which was bypassable with overlapping input such as ....//
3. same-origin /videos/... URLs were accepted
4. url_get_contents() and try_get_contents_from_local() resolved the request into a local filesystem read
5. the fetched bytes were written into the GIF destination
6. invalid GIF cleanup used the wrong variable, so the non-image payload remained on disk

This made the GIF poster path a local file disclosure primitive with public retrieval.

Proof of concept

1. Log in as an uploader and create an owned video row through the normal encoder flow.
2. Send:

POST /objects/aVideoEncoderReceiveImage.json.php
downloadURL_gifimage=https://localhost/videos/....//....//....//....//....//....//etc/passwd

3. Query:

GET /objects/videos.json.php?showAll=1

4. Recover the generated GIF URL from videosURL.gif.url.
5. Download that GIF URL.
6. Observe that the body matches the target local file, such as /etc/passwd, byte-for-byte.

Impact

An authenticated uploader can read server-local files and republish them through a public GIF media URL by supplying a crafted same-origin /videos/... path to downloadURL_gifimage. Because traversal scrubbing was bypassable and the fetched bytes were written to the GIF destination without effective invalid-image cleanup, successful exploitation allows disclosure of files such as /etc/passwd, readable application source code, or deployment-specific configuration accessible to the application.

Recommended fix

• Reject any remote image URL whose decoded path contains traversal markers
• Do not allow attacker-controlled same-origin /videos/... fetches to resolve into local file reads
• Constrain any local shortcut path handling with realpath() and strict base-directory allowlists
• Validate GIF content before saving it into public media storage
• Ensure invalid-image cleanup checks the correct destination path