CVE-2026-73258 in Mongoose
Summary
by MITRE • 08/20/2026
Mongoose is an embedded web server and network library. Prior to 7.22, a remote attacker can place a lone carriage return or line feed in multipart input processed by mg_http_next_multipart() in src/http.c. The loops comparing s[b] and s[b + 1], and s[h2] and s[h2 + 1], use an incorrect AND condition and stop when either character resembles part of a CRLF terminator. This truncates headers, filenames, or boundaries and can cause an application to accept dangerous content after seeing a misleading Content-Type value. This issue is fixed in version 7.22.
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Analysis
by VulDB Data Team • 08/21/2026
Mongoose serves as a widely adopted embedded web server and network library designed for resource-constrained environments such as IoT devices and microcontrollers. The vulnerability identified affects versions prior to 7.22, specifically within the HTTP parsing logic located in the src/http.c file. This component is responsible for processing multipart form data, which is commonly used when clients upload files or submit complex forms containing multiple parts with distinct headers and content types. The core of the issue lies in the implementation of the mg_http_next_multipart function, which iterates through input buffers to identify boundaries between different sections of a multipart message.
The technical flaw stems from an incorrect logical condition within the loop structures that compare adjacent characters to detect CRLF sequences, which signify line breaks and part separators in HTTP protocols. Specifically, the code uses an AND operator instead of an OR operator when checking if two consecutive bytes resemble parts of a carriage return or line feed terminator. This logic error causes the parser to terminate its search prematurely whenever either character matches a component of the expected CRLF pattern, rather than requiring both characters to match correctly. Consequently, the parsing routine fails to properly validate the integrity of multipart boundaries and headers during input processing.
This logical defect allows a remote attacker to craft maliciously formatted HTTP requests containing lone carriage return or line feed characters within the multipart data. By exploiting this truncation behavior, an attacker can effectively break the expected structure of the multipart message. This manipulation leads to the truncation of critical header fields, filenames, and boundary markers that are essential for correctly interpreting the content type and disposition of each part. The parser may incorrectly conclude that a section has ended or misinterpret the boundaries, leading to significant parsing errors in how subsequent data is handled by the application layer.
The operational impact of this vulnerability is severe because it can lead to security bypasses related to file uploads and content validation. When headers such as Content-Type are truncated or malformed due to the premature loop termination, an application may fail to correctly identify the nature of the uploaded content. This discrepancy enables attackers to upload files with dangerous extensions or executable code while disguising them under misleading Content-Type values that suggest benign formats like images or text documents. The server might then process these files based on their deceptive headers rather than their actual binary structure, potentially leading to remote code execution, cross-site scripting, or other injection attacks depending on how the application handles the misidentified content downstream.
This vulnerability aligns with CWE-20 Improper Input Validation and CWE-74 Injection in external control of stream boundaries. From a threat modeling perspective using MITRE ATT&CK techniques, this flaw facilitates input validation bypasses that allow attackers to evade security controls designed to filter malicious file uploads or sanitize user-supplied data. The ability to manipulate how the server interprets multipart boundaries is particularly dangerous in environments where automated processing relies heavily on accurate header information for routing and sanitization decisions.
Mitigation strategies primarily involve upgrading the Mongoose library to version 7.22 or later, which corrects the logical operator error within the HTTP parsing routines. For organizations unable to immediately upgrade, implementing strict input validation at a higher application layer can provide some protection by enforcing expected formats for filenames and content types before they reach the vulnerable parser functions. Additionally, deploying web application firewalls that inspect multipart boundaries and enforce canonicalization of CRLF sequences may help detect and block attempts to exploit this truncation behavior. Regular security audits focusing on HTTP parsing logic in embedded systems are recommended to identify similar logical errors in custom network stacks or third-party libraries used within the infrastructure.