Description

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:

objects/aVideoEncoderReceiveImage.json.php accepted attacker-controlled downloadURL_gifimage

traversal scrubbing used str_replace('../', '', ...), which was bypassable with overlapping input such as ....//

same-origin /videos/... URLs were accepted

url_get_contents() and try_get_contents_from_local() resolved the request into a local filesystem read

the fetched bytes were written into the GIF destination

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

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

Send:

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

Query:

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

Recover the generated GIF URL from videosURL.gif.url.

Download that GIF URL.

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

Mitigation

Aliases

References