Front-end Desynchronization Vulnerability in Progress® Telerik® Fiddler® Classic
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.
CVE-2026-77803
Primary source · Revision 1
Imported from structured advisory data. Source references identify upstream records or fields; verify affected versions and your deployed configuration.
Security context
Source reference: cve:CVE-2026-77803
Progress® Telerik® Fiddler® Classic
Source reference: /containers/cna/affected/0/product
- Affected versions
- Not stated in this record
- Fixed versions
- Not stated in this record
- Prerequisites
- Not stated in this record
Structured affected-version data
Original package identities and version rules from this source.
[
{
"vendor": "Progress Software",
"product": "Progress® Telerik® Fiddler® Classic",
"versions": [
{
"status": "affected",
"version": "1.0.0",
"lessThan": "6.0.20262.10021",
"versionType": "custom"
}
],
"platforms": [
"Windows"
],
"defaultStatus": "unaffected"
}
]- Related CVEs
- CVE-2026-77803
Source reference:
/cveMetadata/cveId
Publisher severity
low
Source reference: /containers/cna/metrics
- Event
- #9497 · Revision 1
- Added to Heist Feed
- Publisher date