CVE-2026-19879
MediumCVSS 5.3Exploitation Probability (EPSS)
Low risk15th percentile - higher than 15% of all known CVEs
Summary
A flaw in Undertow, an HTTP server, in 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, leading to truncation 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.
Risk Assessment
The risk includes potential distortion of response headers, which could lead to header injection or information disclosure if the application does not sanitize input.
Recommendation
Update Undertow to a patched version and properly sanitize user input before placing it into response headers.
Original NVD description (English source)
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.