CVE-2006-0800 in PostNuke
Summary
by MITRE
Interpretation conflict in PostNuke 0.761 and earlier allows remote attackers to conduct cross-site scripting (XSS) attacks via HTML tags with a trailing "<" character, which is interpreted as a ">" character by some web browsers but bypasses the blacklist protection in (1) the pnVarCleanFromInput function in pnAPI.php, (2) the pnSecureInput function in pnAntiCracker.php, and (3) the htmltext parameter in an edituser operation to user.php.
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Analysis
by VulDB Data Team • 06/14/2019
The vulnerability described in CVE-2006-0800 represents a critical cross-site scripting flaw in PostNuke versions 0.761 and earlier, exploiting an interpretation conflict in how web browsers process HTML entities. This vulnerability specifically targets the pnVarCleanFromInput function in pnAPI.php, the pnSecureInput function in pnAntiCracker.php, and the htmltext parameter handling within user.php's edituser operation. The core issue arises from the inconsistent handling of HTML tags where a trailing "<" character is interpreted as ">" by certain web browsers, creating a bypass mechanism that circumvents existing blacklist protection mechanisms. This interpretation conflict fundamentally undermines the security controls designed to sanitize user input and prevent malicious script execution.
The technical exploitation of this vulnerability occurs through carefully crafted HTML tags that contain trailing "<" characters, which browsers interpret as closing brackets due to their lenient HTML parsing behavior. This parsing inconsistency allows attackers to inject malicious scripts that would normally be blocked by the blacklist filters implemented in the three affected functions. The pnVarCleanFromInput function in pnAPI.php serves as the primary entry point for input sanitization, while pnSecureInput in pnAntiCracker.php provides additional protection layers that are rendered ineffective by this specific character manipulation technique. The htmltext parameter within the edituser operation of user.php represents another attack vector where user-provided content is processed without proper validation, creating multiple pathways for exploitation.
The operational impact of this vulnerability extends beyond simple XSS attacks, as it enables attackers to execute arbitrary scripts in the context of affected users' browsers, potentially leading to session hijacking, credential theft, and complete browser compromise. The vulnerability affects the entire PostNuke application ecosystem, as these three functions are fundamental components used throughout the application for input validation and security processing. Attackers can leverage this flaw to inject malicious JavaScript code that executes when other users view affected pages, creating persistent XSS attacks that can remain undetected for extended periods. The bypass of blacklist protection mechanisms undermines the application's core security architecture and demonstrates a critical failure in input sanitization implementation.
Security mitigations for this vulnerability require immediate patching of the affected PostNuke versions to address the specific interpretation conflict in the three identified functions. Organizations should implement comprehensive input validation that accounts for browser-specific HTML parsing behaviors and ensures consistent character handling across all security functions. The fix must address the root cause by modifying the blacklist filtering mechanisms to properly handle trailing "<" characters and ensure that all HTML entities are consistently processed regardless of browser interpretation. Additionally, implementing Content Security Policy headers and additional input sanitization layers can provide defense-in-depth protection against similar vulnerabilities. This vulnerability aligns with CWE-79 (Cross-site Scripting) and demonstrates techniques that map to ATT&CK tactics including TA0001 (Initial Access) and TA0002 (Execution) through the exploitation of web application vulnerabilities.