CVE-2026-19879 in Undertow
Summary
by MITRE • 08/14/2026
A flaw was found in Undertow, an HTTP server, within its HTTP response header writing path. The `writeString()` method performs a silent narrowing cast from 16-bit Unicode characters to 8-bit bytes when writing HTTP response header values. A remote attacker can exploit this by supplying specific Unicode characters in user-controlled input that an application places into response headers. This can lead to the truncation of these characters into ASCII control characters or special symbols, potentially resulting in limited integrity impact or information disclosure if the application does not properly sanitize user input.
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Analysis
by VulDB Data Team • 08/14/2026
This vulnerability exists within the Undertow HTTP server implementation where the writeString() method executes a silent narrowing cast from 16-bit Unicode characters to 8-bit bytes during HTTP response header processing. The flaw occurs specifically in the header writing path and represents a classic data truncation issue that can be exploited remotely by attackers who provide carefully crafted Unicode input into response headers. According to CWE-194, this constitutes an unintended data truncation vulnerability where the system silently converts data types without proper validation or error handling, creating potential security implications for applications relying on Undertow's HTTP processing capabilities.
The technical exploitation of this vulnerability relies on the implicit conversion behavior within the writeString() method that does not properly validate or sanitize Unicode characters before casting them to 8-bit byte representations. When user-controlled input containing specific Unicode characters is placed into HTTP response headers, these characters can be silently truncated and converted into ASCII control characters or special symbols. This occurs because the method performs a narrowing cast without checking whether the 16-bit Unicode values can be properly represented in the 8-bit target format, leading to potential corruption of header values that may inadvertently contain control sequences or maliciously crafted characters.
The operational impact of this vulnerability extends beyond simple data truncation since it can affect application integrity and potentially enable information disclosure scenarios. Applications that do not implement proper input sanitization for HTTP response headers become vulnerable to attackers who can manipulate header content through Unicode character injection. The consequences may include header value corruption that could interfere with normal application behavior, or in some cases, the insertion of control characters that might be interpreted by downstream systems or client applications in unexpected ways, potentially leading to further security implications or service disruption.
Mitigation strategies should focus on implementing proper input validation and sanitization for all user-controlled data that enters HTTP response headers, ensuring that Unicode characters are properly handled before any casting operations occur. Organizations should consider upgrading to patched versions of Undertow where the narrowing cast behavior has been addressed through explicit type checking and validation mechanisms. Additionally, security monitoring should be implemented to detect unusual header values or character sequences that might indicate exploitation attempts, while following ATT&CK technique T1071.004 for application layer protocol manipulation. The fix typically involves modifying the writeString() method to either reject invalid Unicode sequences or properly handle character encoding conversions with appropriate error handling rather than allowing silent truncation that can compromise header integrity and potentially enable further exploitation scenarios.
The vulnerability demonstrates a critical weakness in HTTP server implementations where implicit type conversions are not properly validated, creating potential attack vectors through seemingly benign user input fields. This flaw aligns with common security patterns where applications fail to account for the full range of character encodings that might be encountered during processing, particularly in systems that handle internationalized content or user-supplied data in HTTP headers. Proper implementation should include explicit validation of character ranges and encoding requirements before any data is written to response headers, ensuring that applications maintain integrity of HTTP protocol elements while preventing unintended data corruption through type conversion artifacts.