CVE-2026-77803Front-end Desynchronization Vulnerability in Progress® Telerik® Fiddler® Classic
- 3.6LowCVSS base score
- 0.10%Exploitation in 30 daysEPSS · top 99% of all CVEs
- NoKnown exploitednot in CISA's KEV catalog
- NonePublic exploitsnone known yet
Description
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.
Timeline
ID reserved
By ProgressSoftware
- 45 days later
· 45 days later
Published
- 1 day later
· 1 day later
Record updated
Changes after publication (a new score, a public exploit, EPSS crossing 1, 10 or 50 %, leaving KEV) are kept since November 2026; earlier ones are not known.
Scores
| Scored by | CVSS | Score | Vector |
|---|---|---|---|
| ProgressSoftwareCNA (assigner) | 3.1 | Low3.6 | CVSS:3.1/AV:L/AC:H/PR:L/UI:N/S:U/C:L/I:L/A:N |
| NVDNVD · Awaiting Analysis | 3.1 | Low3.6 | CVSS:3.1/AV:L/AC:H/PR:L/UI:N/S:U/C:L/I:L/A:N |
- EPSS (exploitation in 30 days)
- 0.10% · top 99% of all CVEs (12 Nov 2026)
- CISA SSVC
- Exploitation: none · Automatable: no · Technical impact: partial
The list uses the assigner's score when there is one, then CISA's, then NVD's: Low 3.6.
Affected products
Progress Software · Progress® Telerik® Fiddler® Classic
- affected 1.0.0 – < 6.0.20262.10021
What to do
Workaround: Disable client connection reuse in Fiddler Classic: open Tools > Options > Connections and uncheck the "Reuse client connections" checkbox. Disabling client pipe reuse prevents the excess data of a malformed request from being parsed as a pipelined request.
References
Sources
The same CVE in the primary databases and aggregators, to check against.
- CVE.org record (opens in a new tab)
- CVE List JSON (GitHub) (opens in a new tab)
- NVD (opens in a new tab)
- FIRST EPSS (opens in a new tab)
- EUVD EUVD-2026-92426 (opens in a new tab)
- OSV.dev (opens in a new tab)
- GitHub Advisories (opens in a new tab)
- CIRCL Vulnerability-Lookup (opens in a new tab)
Badge for a README or a ticket
[](https://cveledger.app/cve/CVE-2026-77803)Record last updated .