CVE-2026-44312: CSS Parser Improper Certificate Validation Leads to MITM CSS Injection
Summary
CVE-2026-44312: CSS Parser: Improper Certificate Validation allows MITM injection of remote CSS content. Affects css_parser versions < 2.1.0. Fixed in 2.1.0. How to fix ↓
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Background
The CSS Parser gem is a Ruby library designed to parse and manipulate CSS stylesheets. It allows developers to load and manage CSS from various sources, including remote URLs. However, the library has a critical vulnerability that arises from its failure to validate HTTPS connections, making it susceptible to man-in-the-middle (MITM) attacks.
Vulnerability Detail
The CSS Parser gem does not perform proper validation of HTTPS connections, as it sets the OpenSSL verify mode to VERIFY_NONE. This means that any HTTPS certificate, even untrusted ones, will be accepted without validation. Consequently, an attacker positioned to intercept network traffic can inject or modify CSS content loaded via HTTPS, leading to potential CSS injection and content manipulation attacks. This vulnerability affects all applications using the CSS Parser gem to fetch external stylesheets over HTTPS.
How To Fix
Minimum fixed version
2.1.0
This is the first release that contains the fix.
gem install css_parser -v 2.1.0Tip — Not sure which version your app runs? Vulert can match this fix to it. Scan free →
Workaround
As a temporary workaround, you can manually set the verify mode to a more secure option in your code, although this is not recommended as a long-term solution. Example code snippet:
http.verify_mode = OpenSSL::SSL::VERIFY_PEER
Conclusion
The CSS Parser gem has a significant vulnerability due to improper certificate validation, allowing for potential MITM attacks. Users are strongly advised to update to the latest version to mitigate risks. For those unable to update immediately, a temporary workaround is available, but it is not a substitute for a proper fix.
See every vulnerability recorded for css_parser →
FAQ
The vulnerability allows attackers to inject or modify CSS content over HTTPS, potentially leading to unauthorized content manipulation and security breaches.
You can use the Vulert playground at Vulert Scanner to check if your application is affected by this vulnerability without needing to sign up.
All versions prior to 2.1.0 are affected by this vulnerability.
Yes, the vulnerability has been patched in version 2.1.0 of the CSS Parser gem.
If an immediate update is not possible, consider implementing the workaround to set the verify mode to VERIFY_PEER temporarily.
References
Related vulnerabilities
Recent vulnerabilities recorded for css_parser.
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