CVE-2026-77803
CVE-2026-77803
In Progress® Telerik® Fiddler® Classic for Windows, versions prior to v6.0.20262.10021, front-end request desynchronization is possible in the proxy request forwarding component. A request that contains both a Content-Length and a Transfer-Encoding header is forwarded with both headers present, while Fiddler frames the body using Transfer-Encoding only. The remaining bytes on the reused client connection are then parsed as a separate pipelined request, so a local threat actor with low privileges can cause a single malformed request to be split into two requests forwarded to the origin server and receive an additional smuggled response, without requiring a vulnerable server.
Vulnerability class: HTTP Request Smuggling
Published .
CVSS v3 metric
CVSS v3 base score 3.6 (Low). Vector: CVSS:3.1/AV:L/AC:H/PR:L/UI:N/S:U/C:L/I:L/A:N.
EPSS exploit prediction
No EPSS score from FIRST.org yet. New CVEs usually get one within a few days; no score means no signal yet, not low risk. How EPSS works.
Weakness classification (CWE)
References
Frequently asked questions
- What is CVE-2026-77803?
- CVE-2026-77803 is a low-severity vulnerability, classified under Inconsistent Interpretation of HTTP Requests (HTTP Request/Response Smuggling). CVSS score: 3.6/10. Published 2026-10-05.
- How severe is CVE-2026-77803?
- Low severity. CVSS v3 base score is 3.6 out of 10.