CVE-2019-10905 in Parsedown
Summary
by MITRE
Parsedown before 1.7.2, when safe mode is used and HTML markup is disabled, might allow attackers to execute arbitrary JavaScript code if a script (already running on the affected page) executes the contents of any element with a specific class. This occurs because spaces are permitted in code block infostrings, which interferes with the intended behavior of a single class name beginning with the language- substring.
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Analysis
by VulDB Data Team • 08/27/2023
The vulnerability identified as CVE-2019-10905 affects Parsedown versions prior to 1.7.2 and represents a sophisticated cross-site scripting flaw that exploits the interaction between safe mode configuration and HTML markup handling. This vulnerability specifically manifests when the library operates in safe mode with HTML markup disabled, creating a false sense of security that can be subverted through carefully crafted input. The flaw stems from the permissive handling of spaces within code block infostrings, which allows attackers to manipulate the parsing behavior in ways that were not anticipated by the security model. The vulnerability is particularly concerning because it leverages an existing script execution context on the page, meaning that an attacker does not need to inject new code but can instead manipulate existing JavaScript to execute malicious payloads through the parsing of specific class attributes.
The technical implementation of this vulnerability involves the manipulation of code block syntax within Parsedown's markdown parsing engine. When a code block is defined with an infostring containing spaces, the parser incorrectly interprets the content in a way that allows JavaScript execution. The vulnerability specifically targets elements with class names beginning with the language- prefix, which is typically used to indicate programming language syntax for code highlighting. However, due to the improper handling of spaces in infostrings, an attacker can craft a scenario where a legitimate class name like language-javascript gets parsed in a manner that bypasses the intended security restrictions. This parsing error creates a condition where the safe mode protection is effectively circumvented, allowing malicious JavaScript to be executed within the context of the page where the parsed content is displayed. The vulnerability aligns with CWE-79, which describes cross-site scripting flaws where untrusted data is improperly handled, and represents a case of improper input validation that allows code injection.
The operational impact of CVE-2019-10905 extends beyond simple XSS attacks as it can enable more sophisticated exploitation techniques that leverage existing trust relationships within web applications. When an attacker can manipulate code block parsing to execute JavaScript, they can potentially perform actions such as stealing user sessions, modifying page content, redirecting users to malicious sites, or exfiltrating sensitive data from the affected application. The vulnerability is particularly dangerous in environments where Parsedown is used to process user-generated content, as it allows attackers to leverage legitimate application functionality to deliver malicious payloads. The attack vector requires that a script already be executing on the page, which means that the vulnerability is most effectively exploited in contexts where the application's own JavaScript code is already present and capable of interacting with the parsed content. This makes the vulnerability particularly insidious as it can be used to escalate privileges within the existing application context rather than requiring external injection techniques.
The remediation for this vulnerability requires upgrading to Parsedown version 1.7.2 or later, where the parsing logic has been corrected to properly handle spaces within code block infostrings. The fix implemented in the newer versions ensures that class names beginning with language- are properly validated and that spaces do not interfere with the intended security behavior. Organizations should also implement additional defensive measures such as content security policies that restrict script execution and sanitize all user-generated content before processing it through markdown parsers. The vulnerability demonstrates the importance of thorough input validation and the potential security implications of seemingly minor parsing behaviors that can have significant security consequences. Security practitioners should also consider implementing runtime monitoring for unusual script execution patterns and maintain comprehensive logging of markdown parsing activities to detect potential exploitation attempts. This vulnerability serves as a reminder of how complex interactions between different security controls can create unexpected attack surfaces, particularly when dealing with text processing libraries that handle user input in potentially dangerous contexts. The ATT&CK framework would classify this vulnerability under T1203, which involves exploiting weaknesses in input validation and parsing, and potentially T1059 for the execution of malicious scripts through legitimate application interfaces.