CVE-2018-5773 in markdown2
Summary
by MITRE
An issue was discovered in markdown2 (aka python-markdown2) through 2.3.5. The safe_mode feature, which is supposed to sanitize user input against XSS, is flawed and does not escape the input properly. With a crafted payload, XSS can be triggered, as demonstrated by omitting the final '>' character from an IMG tag.
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Analysis
by VulDB Data Team • 02/02/2023
The vulnerability identified as CVE-2018-5773 affects the markdown2 library version 2.3.5 and earlier, representing a critical security flaw in the library's implementation of safe mode functionality. This library serves as a popular markdown parser for python applications, widely used for converting markdown text into HTML format. The safe_mode feature was specifically designed to prevent cross-site scripting attacks by sanitizing user input before HTML generation, making it a critical security control for applications processing untrusted markdown content. The flaw manifests in the library's improper handling of HTML tag sanitization, particularly when processing image tags, where the implementation fails to properly escape or validate input parameters.
The technical implementation flaw stems from inadequate input validation within the safe_mode sanitization process, where the library fails to properly handle malformed HTML tags that could be exploited for XSS attacks. Specifically, the vulnerability demonstrates that when an attacker crafts a malicious payload by omitting the final '>' character from an IMG tag, the sanitization mechanism does not properly escape or neutralize the potentially dangerous input. This creates a condition where malicious script code can be injected into the generated HTML output, bypassing the intended security protections. The vulnerability operates at the input sanitization layer, where the library's HTML parsing and escaping mechanisms fail to account for all possible malformed tag scenarios, particularly those involving incomplete HTML constructs.
From an operational perspective, this vulnerability presents a significant risk to web applications that rely on markdown2 for processing user-generated content. Applications using this library for blog platforms, content management systems, wikis, or any service allowing user markdown input become vulnerable to XSS attacks when this flaw is exploited. Attackers can leverage the vulnerability to inject malicious JavaScript code that executes in the context of other users' browsers, potentially leading to session hijacking, credential theft, or redirection to malicious sites. The impact extends beyond simple XSS execution as it undermines the fundamental security assumptions of applications relying on the safe_mode feature, potentially exposing entire application ecosystems to further exploitation. The vulnerability is particularly dangerous because it can be triggered with relatively simple payloads that do not require complex exploitation techniques, making it accessible to attackers with basic knowledge of web security principles.
The vulnerability maps to CWE-79 which specifically addresses Cross-site Scripting flaws in web applications, and aligns with ATT&CK technique T1203 for Exploitation for Client Execution. Organizations should immediately upgrade to markdown2 version 2.3.6 or later, which contains the patched implementation of the safe_mode feature. Additionally, implementing multiple layers of defense including Content Security Policy headers, proper input validation at multiple points in the application stack, and regular security assessments of third-party libraries can help mitigate the risk. Security teams should also consider implementing runtime monitoring to detect potential exploitation attempts and establish incident response procedures for addressing similar vulnerabilities in other dependencies. The vulnerability highlights the critical importance of thorough security testing for input sanitization mechanisms and the need for comprehensive security reviews of all third-party components in application environments.