Red Hat Undertow HTTP Response Header Writing writeString information disclosure

Summaryinfo

A vulnerability, which was classified as problematic, was found in Red Hat Undertow. Affected by this issue is the function writeString of the component HTTP Response Header Writing. Executing a manipulation can lead to information disclosure. This vulnerability appears as CVE-2026-19879. The attack may be performed from remote. There is no available exploit.

Detailsinfo

A vulnerability has been found in Red Hat Undertow (affected version not known) and classified as problematic. This vulnerability affects the function writeString of the component HTTP Response Header Writing. The manipulation with an unknown input leads to a information disclosure vulnerability. The CWE definition for the vulnerability is CWE-200. The product exposes sensitive information to an actor that is not explicitly authorized to have access to that information. As an impact it is known to affect confidentiality. CVE summarizes:

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.

The advisory is shared for download at access.redhat.com. This vulnerability was named CVE-2026-19879 since 08/14/2026. The exploitation appears to be easy. The attack can be initiated remotely. No form of authentication is required for a successful exploitation. There are known technical details, but no exploit is available. The current price for an exploit might be approx. USD $5k-$25k (estimation calculated on 08/14/2026). The MITRE ATT&CK project declares the attack technique as T1592.

There is no information about possible countermeasures known. It may be suggested to replace the affected object with an alternative product.

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Productinfo

Vendor

• Red Hat

Name

• Undertow

License

• free

Website

• Vendor: https://www.redhat.com/

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