CVE-2026-118011 in Firefox
Summary
by MITRE • 10/27/2026
Frame poisoning in the CSS Parsing and Computation component. This vulnerability was fixed in Firefox 159.
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Analysis
by VulDB Data Team • 10/27/2026
The discovery of frame poisoning within the CSS parsing and computation subsystem represents a significant security concern for web browsers, particularly affecting Mozilla Firefox prior to version 159. At its core, this vulnerability stems from an improper handling of Cascading Style Sheets data during the rendering pipeline, where maliciously crafted stylesheets can manipulate internal state variables or object references in ways that were not anticipated by the developers. When a browser processes CSS, it must parse the textual rules and compute their final values to apply them to Document Object Model elements. In this specific instance, the flaw allows an attacker to inject malformed or specially constructed style properties that cause the parser to misinterpret memory structures or object states associated with frame objects. This misinterpretation is not merely a cosmetic rendering error but rather a logical failure in how the browser manages the relationship between CSS computed values and their corresponding DOM frames.
From a technical perspective, this issue aligns closely with CWE-787, which describes out-of-bounds write vulnerabilities, or potentially CWE-20 if it involves improper input validation leading to state corruption. The attack vector typically relies on social engineering or cross-site scripting scenarios where an attacker can force a victim's browser to load and parse a malicious stylesheet. By exploiting the frame poisoning mechanism, the attacker gains the ability to corrupt memory structures that control how web content is isolated from other parts of the browsing context. This corruption can lead to unexpected behavior in the rendering engine, potentially allowing for information disclosure or, more critically, facilitating further exploitation chains such as arbitrary code execution if combined with other vulnerabilities like type confusion bugs within the JavaScript engine. The severity lies in the fact that CSS parsing occurs early in the page load process, making it a potent entry point for attackers who seek to bypass security boundaries established by the browser's sandboxing mechanisms.
The operational impact of this vulnerability is substantial because it undermines the fundamental isolation principles that protect users from malicious web content. If an attacker successfully exploits frame poisoning, they may be able to escape the same-origin policy restrictions or manipulate the visual presentation in a way that facilitates phishing attacks while simultaneously compromising underlying system stability. In worst-case scenarios, the memory corruption resulting from this flaw could lead to browser crashes, causing denial of service for legitimate users, or serve as a stepping stone for more advanced exploits targeting kernel-level vulnerabilities through JIT compiler bugs often found in conjunction with rendering engine flaws. The fix implemented in Firefox 159 addresses these issues by hardening the CSS parser against such malformed inputs and ensuring that frame references are validated strictly before being used in critical operations. This patch likely involves adding rigorous bounds checking, improving input sanitization routines within the style computation module, and reinforcing the integrity checks on object lifecycles during parsing.
To mitigate the risks associated with this class of vulnerabilities, organizations should ensure that all instances of Firefox are updated to version 159 or later immediately upon availability. For environments where immediate patching is not feasible due to compatibility concerns, deploying web application firewalls can help filter out suspicious CSS payloads before they reach the browser client. Additionally, enabling strict Content Security Policy headers on websites helps restrict the sources from which stylesheets can be loaded, thereby reducing the attack surface for such injection-based exploits. From a broader security architecture standpoint, this incident reinforces the importance of continuous fuzzing and static analysis within browser development lifecycles to detect logic errors in complex subsystems like CSS parsing before they reach production builds. Security teams should also monitor for related advisories from other major browser vendors, as similar flaws often exist across different rendering engines due to shared architectural patterns in how web standards are implemented.