CVE-2019-11837 in njs
Summary
by MITRE
njs through 0.3.1, used in NGINX, has a segmentation fault in String.prototype.toBytes for negative arguments, related to nxt_utf8_next in nxt/nxt_utf8.h and njs_string_offset in njs/njs_string.c.
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Analysis
by VulDB Data Team • 09/15/2023
The vulnerability identified as CVE-2019-11837 represents a critical memory corruption issue within the njs JavaScript engine version 0.3.1 and earlier, which is integrated into the NGINX web server ecosystem. This flaw manifests as a segmentation fault when the String.prototype.toBytes method is invoked with negative argument values, creating a scenario where the application crashes unpredictably and potentially exposes system stability risks. The vulnerability specifically targets the internal UTF-8 processing functions within the njs engine, particularly affecting nxt_utf8_next in nxt/nxt_utf8.h and njs_string_offset in njs/njs_string.c, which are responsible for character encoding and string manipulation operations.
The technical root cause of this vulnerability stems from inadequate input validation within the string processing pipeline of the njs engine. When negative values are passed to the toBytes method, the underlying UTF-8 parsing functions fail to properly handle the boundary conditions, leading to improper memory access patterns that result in segmentation faults. This type of vulnerability falls under CWE-129, Input Validation and Representation, and more specifically aligns with CWE-119, Improper Restriction of Operations within the Bounds of a Memory Buffer, as the flaw occurs during buffer manipulation when processing UTF-8 encoded strings. The flaw demonstrates characteristics consistent with buffer over-read conditions where the engine attempts to access memory locations beyond the allocated bounds, causing the system to crash.
From an operational perspective, this vulnerability poses significant risks to NGINX deployments that utilize njs for server-side JavaScript processing or dynamic content generation. Attackers could potentially exploit this weakness to cause denial of service attacks by sending malicious requests containing negative arguments to string manipulation functions, resulting in service disruption and potential system instability. The impact extends beyond simple crashes as such vulnerabilities often provide attackers with opportunities to escalate privileges or gain unauthorized access to system resources, particularly when the affected web server processes untrusted input from clients. This vulnerability particularly affects environments where JavaScript-based dynamic content generation is enabled, making it a critical concern for web applications relying on NGINX with njs functionality.
The mitigation strategy for CVE-2019-11837 requires immediate patching of the njs engine to version 0.3.2 or later, which includes proper input validation and boundary checking for negative arguments in string manipulation functions. System administrators should also implement input sanitization measures at the NGINX level to validate and reject malformed arguments before they reach the JavaScript engine. Additionally, organizations should consider implementing network-level protections such as web application firewalls that can detect and block suspicious patterns targeting this specific vulnerability. The ATT&CK framework categorizes this vulnerability under T1499.004, Network Denial of Service, as it enables adversaries to cause service disruption through controlled input manipulation. Regular security assessments and vulnerability scanning should be conducted to identify any potential variants or similar issues within the NGINX ecosystem, particularly focusing on string handling functions and UTF-8 processing modules that may present similar boundary condition vulnerabilities.