CVE-2020-26296: 
JavaScript vulnerability analysis and mitigation

CVE-2020-26296 is a cross-site scripting (XSS) vulnerability discovered in Vega, a visualization grammar package for creating interactive visualization designs. The vulnerability affects Vega versions prior to 5.17.3 and was disclosed on December 30, 2020. The issue exists in Vega expressions, where a specially crafted expression could allow attackers to execute arbitrary JavaScript code on a victim's machine (GitHub Advisory, NVD).

The vulnerability received a CVSS v3.1 base score of 8.7 (HIGH) with vector string CVSS:3.1/AV:N/AC:L/PR:L/UI:R/S:C/C:H/I:H/A:N. The issue was identified in the expression parser component where malicious Vega expressions could bypass intended security restrictions. The vulnerability was discovered during the hxp CTF competition and was demonstrated through a proof-of-concept that could trigger JavaScript alert dialogs through specially crafted JSON input (GitHub Issue).

The vulnerability allows attackers to execute arbitrary JavaScript code in the context of the victim's browser through specially crafted Vega expressions. This could potentially lead to theft of sensitive information, session hijacking, or other malicious actions typically associated with XSS attacks (NVD).

The vulnerability can be exploited by providing a specially crafted Vega expression through the transform filter functionality. A proof-of-concept exploit was demonstrated that could execute arbitrary JavaScript code through the expression parser (GitHub Issue).

The vulnerability was fixed in Vega version 5.17.3. The fix included updates to the expression parser and code generator to prevent unsafe code execution. Users are strongly recommended to upgrade to version 5.17.3 or later to address this security issue (GitHub Release).

Security researchers highlighted concerns about the overall approach to JavaScript sandboxing in Vega, suggesting that using parsing/rewriting for security controls might be insufficient. They recommended considering alternative approaches such as using sandboxed iframes for code execution (GitHub PR).