CVE-2026-77803 json
Front-end Desynchronization Vulnerability in Progress® Telerik® Fiddler® Classic
published 2026-10-05 35 d ago modified 2026-10-06 33 d ago also EUVD-2026-92426
- cvss
- 3.6
- version 3.1, CNA
- cisa kev
- Not listed
- not in CISA's catalog
- epss
- 0.1%
- EPSS 0.1% chance of exploitation in 30 days, percentile 0%
- public exploits
- 0
- none known
Summary
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.
Patch / fix
- Progress Software Progress® Telerik® Fiddler® Classic 6.0.20262.10021
CISA SSVC
- Exploitation
- none
- Automatable
- no
- Technical impact
- partial
- Assessed
- 33 d ago
Exploit prediction
0.10% chance of exploitation in the next 30 days, higher than 0% of all scored CVEs (FIRST EPSS, 2026-11-08).
CVSS scores
| assigned by | version | score | vector |
|---|---|---|---|
| CNA | 3.1 | 3.6 | CVSS:3.1/AV:L/AC:H/PR:L/UI:N/S:U/C:L/I:L/A:N |
Affected products
- Progress Software Progress® Telerik® Fiddler® Classic
Classification
- Weakness
-
- CWE-444 Inconsistent Interpretation of HTTP Requests
References
Sources
- CVE List CVE-2026-77803
- NVD CVE-2026-77803
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