CVE-2026-66731 in facil.io
Summary
by MITRE • 07/27/2026
facil.io 0.7.5 through 0.7.6 contains a denial-of-service vulnerability in the HTTP/1.1 chunked transfer encoding parser that allows unauthenticated remote attackers to crash the server by sending a negative chunk size value. Attackers can send a single POST request with a Transfer-Encoding: chunked header containing a leading minus sign in the chunk size field, causing the parser in http1_parser.h to compute a large positive integer from the negated value, corrupting internal state and moving the read pointer into unmapped memory resulting in a fault.
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Analysis
by VulDB Data Team • 07/27/2026
The vulnerability in facil.io versions 0.7.5 through 0.7.6 represents a critical denial-of-service weakness that exploits the HTTP/1.1 chunked transfer encoding parser implementation. This flaw exists within the http1_parser.h component where the parser fails to properly validate chunk size values during the parsing process. The vulnerability specifically targets the handling of negative numbers in chunk size fields, which should never occur in legitimate HTTP traffic according to RFC 7230 specifications governing HTTP message syntax and routing.
The technical exploitation mechanism involves sending a single POST request with a Transfer-Encoding: chunked header containing a negative chunk size value. When the parser encounters a leading minus sign in the chunk size field, it performs arithmetic operations that convert the negative value into a large positive integer through negation. This mathematical transformation causes the parser to compute an invalid memory offset that exceeds the bounds of the allocated buffer space. The read pointer within the parser moves into unmapped memory regions, triggering a segmentation fault or access violation that crashes the entire server process.
This vulnerability falls under CWE-129 Input Validation and CWE-704 Incorrect Calculation categories, demonstrating how improper validation of input data can lead to severe system instability. From an operational perspective, this issue represents a remote code execution risk that allows unauthenticated attackers to completely disrupt service availability without requiring any credentials or privileged access. The attack surface is particularly concerning as it only requires sending a single malformed HTTP request to cause the server to crash, making it an ideal target for volumetric denial-of-service attacks.
The impact extends beyond simple service disruption as this vulnerability can be leveraged in coordinated attacks that overwhelm system resources through repeated exploitation attempts. Security practitioners should note that this issue aligns with ATT&CK technique T1499.004 for Network Denial of Service, where attackers manipulate network protocols to cause system failures. The vulnerability's location within the core HTTP parser means that any application using facil.io for web services becomes immediately susceptible to this attack vector regardless of the specific application logic implemented above the protocol layer.
Mitigation strategies should focus on immediate version upgrades to 0.7.7 or later where the chunk size validation has been properly implemented. Organizations should also consider implementing network-level protections such as rate limiting and request filtering to prevent malformed chunked requests from reaching vulnerable servers. Additionally, deploying intrusion detection systems that can identify and block suspicious Transfer-Encoding headers containing negative values would provide an additional layer of defense. The fundamental fix requires proper input validation within the parser to reject any chunk size values that are not positive integers, ensuring that mathematical operations on these fields cannot result in invalid memory access patterns.
The vulnerability demonstrates how seemingly minor parsing errors in protocol implementations can create significant security risks, particularly when dealing with low-level network components that handle untrusted input. This issue highlights the importance of rigorous testing for edge cases in protocol parsers and proper validation of all input data against established standards and specifications. Organizations should conduct thorough vulnerability assessments to identify similar issues in other HTTP implementations and ensure that their systems maintain robust defenses against malformed request exploitation attempts.