Summary
CSS Parser: Improper Certificate Validation allows MITM injection of remote CSS content
Full technical description
The CSS Parser gem does not validate HTTPS connections, allowing a Man-in-the-Middle (MITM) attacker to inject or modify CSS content when stylesheets are loaded via HTTPS. The connection is established with OpenSSL::SSL::VERIFY_NONE, meaning any HTTPS certificate, even entirely untrusted, will be accepted without validation.
Details
In lib/css_parser/parser.rb, the HTTP client sets:
https://github.com/premailer/css_parser/blob/3f91e8db7547fac50ab50cb7f9920f785f722740/lib/css_parser/parser.rb#L646
http.verify_mode = OpenSSL::SSL::VERIFY_NONE
As a result, the library does not validate the authenticity of HTTPS connections and does not protect against man-in-the-middle attacks. Any attacker in a position to intercept network traffic can inject or modify CSS loaded via HTTPS URLs without detection or warning.
PoC
- Set up a test Ruby project that uses the CSS Parser gem and loads an external stylesheet over HTTPS.
- Use a local proxy (such as mitmproxy or Burp Suite) to intercept outgoing HTTPS requests.
- Present a fake self-signed certificate to the client.
- Inject custom CSS into the intercepted HTTPS response.
The request will succeed and the injected CSS will be delivered to the application, as the connection is not validated.
References
https://github.com/premailer/css_parser/issues/185
Credit
This vulnerability was uncovered by @JLLeitschuh of the @braze-inc security team.
Impact
Applications using CSS Parser to load remote stylesheets over HTTPS are vulnerable to CSS injection and content manipulation, regardless of the trust status of the remote server. All users who use CSS Parser to fetch external CSS over HTTPS may be impacted.
CVE-2026-44312 has a CVSS score of 5.8 (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 (2.1.0, 1.22.0); upgrading removes the vulnerable code path.
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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css_parser to 2.1.0 or later; css_parser to 1.22.0 or later
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Frequently Asked Questions
- What is CVE-2026-44312? CVE-2026-44312 is a medium-severity security vulnerability in css_parser (rubygems), affecting versions >= 2.0.0, < 2.1.0. It is fixed in 2.1.0, 1.22.0.
- How severe is CVE-2026-44312? CVE-2026-44312 has a CVSS score of 5.8 (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.
- Which versions of css_parser are affected by CVE-2026-44312? css_parser (rubygems) versions >= 2.0.0, < 2.1.0 is affected.
- Is there a fix for CVE-2026-44312? Yes. CVE-2026-44312 is fixed in 2.1.0, 1.22.0. Upgrade to this version or later.
- Is CVE-2026-44312 exploitable, and should I be worried? Whether CVE-2026-44312 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-2026-44312 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-2026-44312?
- Upgrade
css_parserto 2.1.0 or later - Upgrade
css_parserto 1.22.0 or later
- Upgrade