# CVE-2026-66746: Rouille HTTP response splitting in response headers

Source: https://junglewise.ai/threats/cve-2026-66746-rouille-http-response-splitting-in-response-headers  
Publisher: Junglewise Threat Intelligence, https://junglewise.ai/threats  
Published: 2026-07-28. Updated: 2026-07-28  
License: CC BY 4.0 with a link back

CVE-2026-66746 · Severity: medium · CVSS 5.4 · Published 2026-07-28

Rouille, a web server library for the Rust programming language, contains a flaw in how it handles web traffic headers. An attacker can send specially crafted data that tricks the server into splitting a single response into two, allowing them to inject malicious instructions. This can be used to bypass security policies, hijack user sessions, or serve fraudulent content to other visitors.

Technologies: [Tomaka Rouille](https://junglewise.ai/threats/technologies/rouille). Vendors: [Tomaka](https://junglewise.ai/threats/vendors/tomaka).

## Quick answers

**Is CVE-2026-66746 exploited in the wild?** No. Junglewise Threat Intelligence has no report of CVE-2026-66746 being exploited in the wild.

**How severe is CVE-2026-66746?** Medium severity, with a CVSS score of 5.4, published 28 July 2026.

**Which products and versions are affected?** Tomaka Rouille 0.4.0 through 3.6.2.

**Is there a fix for CVE-2026-66746?** No fixed version is recorded yet. Check the vendor's advisory in the references below. If you run it, Junglewise drafts a mitigation that fits your setup and applies it once you approve, with senior security engineers behind it.

**How is CVE-2026-66746 attacked?** It is attacked over the network, in a software library.

**Am I affected by CVE-2026-66746?** Check whether you run Tomaka Rouille at an affected version. Junglewise does that check for you around the clock: connect your code, cloud and domains, and it matches new vulnerabilities like this one against what you actually run and tells you when one reaches you.

## Technical details

Rouille versions 0.4.0 through 3.6.2 are vulnerable to HTTP response splitting (CWE-113) due to improper neutralization of CRLF sequences (0x0D and 0x0A) in HTTP headers. The vulnerability occurs when percent-decoded query parameters or unvalidated Cookie header values are interpolated directly into response headers like Set-Cookie. A remote attacker can exploit this by providing input containing carriage return or line feed characters, allowing them to terminate the current header and inject arbitrary new headers or a second response body. This can lead to cache poisoning, session fixation, and the bypass of security mechanisms such as Content Security Policy (CSP) or Cross-Origin Resource Sharing (CORS). Exploitation typically requires some level of user interaction to trigger the malicious request.

## Affected products

- tomaka rouille 0.4.0 through 3.6.2

## Timeline

- 2026-07-28: disclosed
- 2026-07-28: advisory

## References

- [https://github.com/theopaid/HTTP-Response-Splitting-via-Unvalidated-Response-Header-Values-rouille-](https://github.com/theopaid/HTTP-Response-Splitting-via-Unvalidated-Response-Header-Values-rouille-)
- [https://www.vulncheck.com/advisories/rouille-http-response-splitting-via-header-injection](https://www.vulncheck.com/advisories/rouille-http-response-splitting-via-header-injection)

## Related threats

- [tomaka rouille HTTP request smuggling via header injection](https://junglewise.ai/threats/cve-2026-67182-tomaka-rouille-http-request-smuggling-via-header-injection)
- [Rouille HTTP request smuggling in proxy implementation](https://junglewise.ai/threats/cve-2026-67181-rouille-http-request-smuggling-in-proxy-implementation)
- [tomaka rouille reachable assertion in Request::remove_prefix](https://junglewise.ai/threats/cve-2026-66754-tomaka-rouille-reachable-assertion-in-request-remove-prefix)