CVE-1999-1357 in Communicator
Summary
by MITRE
Netscape Communicator 4.04 through 4.7 (and possibly other versions) in various UNIX operating systems converts the 0x8b character to a "<" sign, and the 0x9b character to a ">" sign, which could allow remote attackers to attack other clients via cross-site scripting (CSS) in CGI programs that do not filter these characters.
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Analysis
by VulDB Data Team • 04/19/2026
The vulnerability described in CVE-1999-1357 represents a classic cross-site scripting issue that emerged in Netscape Communicator versions 4.04 through 4.7 on UNIX systems. This flaw stems from improper character encoding handling within the browser's rendering engine, specifically when processing certain byte values that were not properly sanitized during HTML content interpretation. The vulnerability operates at the application layer and demonstrates how seemingly benign character conversion can create significant security implications for web applications.
The technical mechanism of this vulnerability involves the conversion of specific byte values 0x8b and 0x9b to HTML special characters less than and greater than signs respectively. When these characters are processed by the Netscape browser, they are interpreted as HTML tags rather than literal characters, potentially allowing malicious actors to inject HTML content into web pages. This character mapping occurs during the browser's HTML parsing phase, where the 0x8b byte is converted to "<" and 0x9b to ">", creating conditions where attackers can manipulate HTML structure through carefully crafted input.
The operational impact of this vulnerability extends beyond simple HTML injection, as it creates opportunities for cross-site scripting attacks that can compromise user sessions and data integrity. When CGI programs running on UNIX systems process user input containing these specific byte sequences without proper filtering, the resulting HTML output can be manipulated to execute malicious scripts in the context of other users' browsers. This vulnerability specifically affects the client-side rendering environment rather than server-side processing, making it particularly dangerous in multi-user environments where different clients may be exposed to malicious content through shared applications.
This vulnerability aligns with CWE-79, which categorizes cross-site scripting flaws as a critical security weakness in web applications. The issue demonstrates how character encoding inconsistencies can create attack vectors that bypass traditional input validation mechanisms, as the malicious characters are not immediately recognizable as harmful content. From an ATT&CK framework perspective, this vulnerability maps to T1566.001, representing the use of malicious HTML content to conduct phishing attacks and session hijacking operations. The vulnerability also relates to T1211, which covers the exploitation of web application flaws through the injection of malicious code.
Mitigation strategies for this vulnerability require both client-side and server-side approaches to address the character encoding inconsistencies. Users should upgrade to newer browser versions that properly handle character encoding, while system administrators must implement comprehensive input filtering mechanisms in CGI applications to sanitize user input before processing. The recommended solution involves implementing strict character validation that prevents the processing of byte sequences that could be converted to HTML special characters, along with proper HTML escaping mechanisms for all user-generated content. Additionally, network security measures such as web application firewalls should be deployed to monitor and filter suspicious character patterns that could indicate exploitation attempts.
The broader implications of this vulnerability highlight the importance of proper character encoding handling in web applications and the need for comprehensive security testing across different browser implementations. This issue demonstrates how browser-specific quirks can create security vulnerabilities that are difficult to detect through conventional security scanning methods, as the problem only manifests when specific character sequences are processed through the browser's rendering engine. Organizations should implement regular security assessments that include browser compatibility testing to identify similar encoding-related vulnerabilities that could compromise user security.
This vulnerability serves as an important historical example of how security issues can emerge from seemingly innocuous character handling behaviors, emphasizing the critical need for robust input validation and output encoding practices in web application development. The issue underscores the importance of maintaining up-to-date software versions and implementing defense-in-depth strategies that protect against multiple attack vectors simultaneously. Modern security practices should incorporate comprehensive character encoding validation and proper content sanitization to prevent similar vulnerabilities from emerging in contemporary web applications.