CVE-2023-29159

When using StaticFiles, if there's a file or directory that starts with the same name as the StaticFiles directory, that file or directory is also exposed via StaticFiles which is a path traversal vulnerability.

Details

The root cause of this issue is the usage of os.path.commonprefix():
https://github.com/encode/starlette/blob/4bab981d9e870f6cee1bd4cd59b87ddaf355b2dc/starlette/staticfiles.py#L172-L174

As stated in the Python documentation (https://docs.python.org/3/library/os.path.html#os.path.commonprefix) this function returns the longest prefix common to paths.

When passing a path like /static/../static1.txt, os.path.commonprefix([full_path, directory]) returns ./static which is the common part of ./static1.txt and ./static, It refers to /static/../static1.txt because it is considered in the staticfiles directory. As a result, it becomes possible to view files that should not be open to the public.

The solution is to use os.path.commonpath as the Python documentation explains that os.path.commonprefix works a character at a time, it does not treat the arguments as paths.

PoC

In order to reproduce the issue, you need to create the following structure:

├── static
│   ├── index.html
├── static_disallow
│   ├── index.html
└── static1.txt

And run the Starlette app with:

import uvicorn
from starlette.applications import Starlette
from starlette.routing import Mount
from starlette.staticfiles import StaticFiles


routes = [
    Mount("/static", app=StaticFiles(directory="static", html=True), name="static"),
]

app = Starlette(routes=routes)


if __name__ == "__main__":
    uvicorn.run(app, host="0.0.0.0", port=8000)

And running the commands:

curl --path-as-is 'localhost:8000/static/../static_disallow/'
curl --path-as-is 'localhost:8000/static/../static1.txt'

The static1.txt and the directory static_disallow are exposed.

Credits

Security researcher Masashi Yamane of LAC Co., Ltd reported this vulnerability to JPCERT/CC Vulnerability Coordination Group and they contacted us to coordinate a patch for the security issue.

Confidentiality is breached: An attacker may obtain files that should not be open to the public.

Input manipulates file paths to reach files outside the intended directory, such as configuration or credential files. Typical impact: unauthorized file read or write outside the intended directory.

CVE-2023-29159 has a CVSS score of 3.7 (Medium). The vector is network-reachable, no privileges required, and no user interaction. A CVSS score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether this affects your application depends on whether the vulnerable code is present and reachable in your environment. A fixed version is available (0.27.0); upgrading removes the vulnerable code path.

Kodem intelligence

1. What is CVE-2023-29159? CVE-2023-29159 is a medium-severity path traversal vulnerability in starlette (pip), affecting versions >= 0.13.5, < 0.27.0. It is fixed in 0.27.0. Input manipulates file paths to reach files outside the intended directory, such as configuration or credential files.
2. How severe is CVE-2023-29159? CVE-2023-29159 has a CVSS score of 3.7 (Medium). This score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether it represents real risk in your environment depends on whether the vulnerable code is present and reachable.
3. Which versions of starlette are affected by CVE-2023-29159? starlette (pip) versions >= 0.13.5, < 0.27.0 is affected.
4. Is there a fix for CVE-2023-29159? Yes. CVE-2023-29159 is fixed in 0.27.0. Upgrade to this version or later.
5. Is CVE-2023-29159 exploitable, and should I be worried? Whether CVE-2023-29159 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
6. What actually determines whether CVE-2023-29159 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
7. How do I fix CVE-2023-29159? Upgrade starlette to 0.27.0 or later.