CVE-2026-77803: Front-end Desynchronization Vulnerability in Progress® Telerik® Fiddler® Classic
Progress Software · Progress® Telerik® Fiddler® Classic
Vendor fix available · Affected: Progress Software Progress® Telerik® Fiddler® Classic · Exploitation: unlikely (under 1%)
Not in CyberSIXT coverage yet: no article names this CVE. This page shows the public record.
Higher than 0% of scored CVEs
DescriptionCVE.org
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.
Affected products1
From the CVE record. A product or version that is not listed is unknown, not unaffected.
- Progress Software · Progress® Telerik® Fiddler® Classicfrom 1.0.0 before 6.0.20262.10021
On the Atlas
Every published CVE is on the CyberSIXT Atlas, grouped by weakness, vendor and month.
- Exploitation
- Unlikely (under 1%). EPSS under 1%, or no score yet.
- CISA KEV
- Not listed
- Public check
- No public nuclei template
- Vendor
- Progress Software
- Published
- October 2026
CoverageNo articles yet
No CyberSIXT article names CVE-2026-77803 yet, so there is no coverage timeline, action brief or change log for it. Everything on this page comes from the public CVE record, CISA KEV, EPSS and the nuclei template index. Search the archive or see the CVEs in the news.