Directory Traversal Affecting rembg package, versions [,2.0.82)

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Affected versions of this package are vulnerable to Directory Traversal via the sam_model parameter in rembg/sessions/sam.py, which is accepted from the public extras payload of /api/remove and interpolated directly into download URLs and local file paths without validation. An attacker can supply a value such as sam_vit_b_01ec64#/../../../evil to exploit a discrepancy between how HTTP clients drop URL fragments and how POSIX paths retain them, causing the legitimate asset to be fetched while the file is written outside the model directory. A malicious ONNX graph placed at the resulting path can declare an arbitrary readable file as external tensor data, encoding its contents into the returned mask and disclosing them to an unauthenticated caller.

The attached loopback-only PoC produced:

VULNERABLE: unauthenticated SAM model injection disclosed the synthetic /flag
first-http=500 second-http=200
decoded-value=REMBG_AUDIT_SYNTHETIC_FLAG_2026_08_28

The /etc/passwd variant produced:

VULNERABLE: disclosed /etc/passwd (839 bytes)
root:x:0:0:root:/root:/bin/bash
daemon:x:1:1:daemon:/usr/sbin:/usr/sbin/nologin
...
nobody:x:65534:65534:nobody:/nonexistent:/usr/sbin/nologin

The decoded contents matched /etc/passwd inside the owned container. Testing used a local TLS stand-in for GitHub; no public target was contacted.

Screenshot 3 — The decoder reconstructs all 839 bytes of /etc/passwd.

Note: The PoC was run against a local owned container using a local TLS stand-in for GitHub, with no non-default configuration stated beyond the loopback-only setup.

A Directory Traversal attack (also known as path traversal) aims to access files and directories that are stored outside the intended folder. By manipulating files with "dot-dot-slash (../)" sequences and its variations, or by using absolute file paths, it may be possible to access arbitrary files and directories stored on file system, including application source code, configuration, and other critical system files.

Directory Traversal vulnerabilities can be generally divided into two types:

• Information Disclosure: Allows the attacker to gain information about the folder structure or read the contents of sensitive files on the system.

st is a module for serving static files on web pages, and contains a vulnerability of this type. In our example, we will serve files from the public route.

If an attacker requests the following URL from our server, it will in turn leak the sensitive private key of the root user.

curl http://localhost:8080/public/%2e%2e/%2e%2e/%2e%2e/%2e%2e/%2e%2e/root/.ssh/id_rsa

Note %2e is the URL encoded version of . (dot).

• Writing arbitrary files: Allows the attacker to create or replace existing files. This type of vulnerability is also known as Zip-Slip.

One way to achieve this is by using a malicious zip archive that holds path traversal filenames. When each filename in the zip archive gets concatenated to the target extraction folder, without validation, the final path ends up outside of the target folder. If an executable or a configuration file is overwritten with a file containing malicious code, the problem can turn into an arbitrary code execution issue quite easily.

The following is an example of a zip archive with one benign file and one malicious file. Extracting the malicious file will result in traversing out of the target folder, ending up in /root/.ssh/ overwriting the authorized_keys file:

2018-04-15 22:04:29 .....           19           19  good.txt
2018-04-15 22:04:42 .....           20           20  ../../../../../../root/.ssh/authorized_keys

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