Summary
S3-Proxy allows Reflected Cross-site Scripting (XSS) in template implementation
A Reflected Cross-site Scripting (XSS) vulnerability enables attackers to create malicious URLs that, when visited, inject scripts into the web application. This can lead to session hijacking or phishing attacks on a trusted domain, posing a high risk to all users.
Details
Give all details on the vulnerability. Pointing to the incriminated source code is very helpful for the maintainer.
It's possible to inject html elements, including scripts through the folder-list template. It seems like the .Request.URL.Path variable is not escaped.
I did some research and found it might be due to the text/template import being used in the template implementation, instead of the safer html/template.
PoC
Complete instructions, including specific configuration details, to reproduce the vulnerability.
Using the default template configuration, the vulnerability can be reproduced with the following steps.
Navigate to
https://your-s3-proxy.com/path-not-foundand confirm the page looks as follows:Try inserting an HTML element by changing
/path-not-foundto/<img src="x">and confirm the page looks as follows:Now it should be possible to run any JavaScript by manipulating the
onerrorproperty of the img element. This should make the link look likehttps://your-s3-proxy.com/<img src="x" onerror="alert(1)">. Confirm that going to this URL will in fact shows an alert in the browser.
Impact
The affected template allows users to interact with the URL path provided by the Request.URL.Path variable, which is then rendered directly into the HTML without proper sanitization or escaping. This can be abused by attackers who craft a malicious URL containing injected HTML or JavaScript. When users visit such a URL, the malicious script will be executed in the user's context, leading to potential risks such as:
- Session Hijacking: Malicious scripts could be used to steal session cookies or other sensitive information.
- Phishing Attacks: JavaScript could be injected to trick users into submitting sensitive information, such as login credentials.
This vulnerability can be exploited by attackers who craft URLs containing malicious payloads, which would then execute in the user's browser when they access the affected page. This poses a high risk to all users who visit such URLs.
Untrusted input is rendered as active markup in a victim's browser, which can run script in their session. Typical impact: session or credential theft, and actions taken as the user.
Affected versions
Security releases
Kodem intelligence
Severity tells you how bad this could be in the worst case. It does not tell you whether you are exposed. Exploitability and impact are functions of runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A vulnerable package can sit in your dependency tree and never run.
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. Kodem's runtime-powered SCA identifies whether this CVE is reachable in your applications.
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Frequently Asked Questions
- What is CVE-2025-27088? CVE-2025-27088 is a high-severity cross-site scripting (XSS) vulnerability in github.com/oxyno-zeta/s3-proxy/cmd/s3-proxy (go), affecting versions <= 0.0.0-20250212215955-2873011f7c42. It is fixed in 0.0.0-20250220214310-c611c741ed48. Untrusted input is rendered as active markup in a victim's browser, which can run script in their session.
- Which versions of github.com/oxyno-zeta/s3-proxy/cmd/s3-proxy are affected by CVE-2025-27088? github.com/oxyno-zeta/s3-proxy/cmd/s3-proxy (go) versions <= 0.0.0-20250212215955-2873011f7c42 is affected.
- Is there a fix for CVE-2025-27088? Yes. CVE-2025-27088 is fixed in 0.0.0-20250220214310-c611c741ed48. Upgrade to this version or later.
- Is CVE-2025-27088 exploitable, and should I be worried? Whether CVE-2025-27088 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
- What actually determines whether CVE-2025-27088 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.
- How do I fix CVE-2025-27088? Upgrade
github.com/oxyno-zeta/s3-proxy/cmd/s3-proxyto 0.0.0-20250220214310-c611c741ed48 or later.