CVE-2018-7753 in Bleach
Summary
by MITRE
An issue was discovered in Bleach 2.1.x before 2.1.3. Attributes that have URI values weren't properly sanitized if the values contained character entities. Using character entities, it was possible to construct a URI value with a scheme that was not allowed that would slide through unsanitized.
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Analysis
by VulDB Data Team • 02/17/2023
The vulnerability identified as CVE-2018-7753 represents a critical security flaw in the Bleach library version 2.1.x prior to 2.1.3, specifically addressing improper sanitization of URI attributes within HTML content processing. This issue resides within the library's handling of attribute values that contain URI schemes, creating a pathway for malicious actors to bypass security controls through carefully constructed character entities.
The technical root cause of this vulnerability stems from insufficient validation of URI schemes when character entities are present within attribute values. When Bleach processes HTML content containing attributes with URI values, it fails to properly sanitize these values if they contain character entities such as < or &. This oversight allows attackers to craft URI values where the actual scheme portion of the URI is obfuscated through character entity encoding, enabling the bypass of allowed scheme restrictions. The vulnerability specifically manifests when the library processes URIs that appear to contain disallowed schemes but are actually constructed using character entities to hide the true nature of the URI.
This flaw creates significant operational impact as it allows for potential cross-site scripting attacks and arbitrary code execution through maliciously crafted HTML content. Attackers can exploit this vulnerability to inject unauthorized URI schemes that would normally be blocked by security policies, potentially leading to redirection to malicious domains or execution of harmful scripts. The vulnerability is particularly dangerous in web applications that rely on Bleach for HTML sanitization, as it undermines the core security assumptions of the library's content filtering capabilities. The impact extends beyond simple XSS scenarios to potentially enable more sophisticated attack vectors including phishing, data exfiltration, and privilege escalation within affected applications.
The vulnerability aligns with CWE-116, which addresses improper encoding or escaping of output, and demonstrates characteristics consistent with ATT&CK technique T1203, which involves the use of external payloads for exploitation. Organizations utilizing Bleach for HTML sanitization should immediately update to version 2.1.3 or later to address this issue. The mitigation strategy involves not only applying the official patch but also implementing additional monitoring for suspicious URI patterns and conducting thorough security reviews of applications that depend on this library. Security teams should also consider implementing network-level protections and web application firewalls to detect and block potentially malicious URI schemes that might bypass the patched library functionality. The vulnerability underscores the importance of proper input validation and the need for comprehensive testing of security controls under various encoding scenarios.