A critical flaw has been identified in the remote file reading functionality of a widely used CSS parsing library, exposing applications to severe exploitation risks. The vulnerability carries a high severity CVSS v4 score of 8.9, driven by the absence of network-layer input validation when processing external resources. When applications accept unvalidated stylesheets containing remote import directives, an attacker can coerce the server into issuing arbitrary outbound HTTP or HTTPS requests. This mechanism allows unauthorized access to internal network services, loops back to internal administration endpoints, and presents significant data exfiltration and resource exhaustion risks.
CVE-2026-53727:A Server-Side Request Forgery and Local File Disclosure vulnerability exists within the remote file handling mechanism of the parser component. An attacker can use specific style import directives to force the server to connect to arbitrary hosts, ports, or internal link-local addresses. Because the application recursively processes cross-scheme redirects without tracking protocol transformations, a secondary redirect can be leveraged to invoke the local file scheme. This allows an external actor to execute arbitrary reads against host files or map out filesystem architectures via error-tracking mechanisms.[/subscribe_to_unlock_form]
A critical flaw has been identified in the remote file reading functionality of a widely used CSS parsing library, exposing applications to severe exploitation risks. The vulnerability carries a high severity CVSS v4 score of 8.9, driven by the absence of network-layer input validation when processing external resources. When applications accept unvalidated stylesheets containing remote import directives, an attacker can coerce the server into issuing arbitrary outbound HTTP or HTTPS requests. This mechanism allows unauthorized access to internal network services, loops back to internal administration endpoints, and presents significant data exfiltration and resource exhaustion risks.
CVE-2026-53727:A Server-Side Request Forgery and Local File Disclosure vulnerability exists within the remote file handling mechanism of the parser component. An attacker can use specific style import directives to force the server to connect to arbitrary hosts, ports, or internal link-local addresses. Because the application recursively processes cross-scheme redirects without tracking protocol transformations, a secondary redirect can be leveraged to invoke the local file scheme. This allows an external actor to execute arbitrary reads against host files or map out filesystem architectures via error-tracking mechanisms.[emaillocker id="1283"]
Security administrators and developers are strongly advised to enforce strict network restrictions to neutralize these exploitation vectors. Upgrading to a remediated version ensures that network-layer operations are routed through secure, resolving filters that automatically drop requests targeting loopback, link-local, and private address blocks. Furthermore, cross-scheme redirection loops should be truncated by separating local and remote resource fetching pipelines entirely. If legacy implementations must remain active temporarily, all outbound traffic generated by the parsing library should be isolated via localized firewall rules or internal proxy definitions.
We recommend you to update css_parser to version 3.0.0.
The following reports contain further technical details:
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