CVE-2026-77803Front-end Desynchronization Vulnerability in Progress® Telerik® Fiddler® Classic
Updated under an hour ago ()
Does this affect you?
It affects Progress Software Progress® Telerik® Fiddler® Classic. Add what you run to find out whether that includes you.
No exploitation reported and no EPSS estimate yet.
Do this next
Free account, 1 product
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.
- CVSS
- 3.6 LOW (v3.1, CNA)CVSS:3.1/AV:L/AC:H/PR:L/UI:N/S:U/C:L/I:L/A:N
- Weakness (CWE)
- CWE-444
- Known exploited
- Not in CISA's KEV catalog
- EPSS (exploitation in the next 30 days)
- Not loaded for this CVE
- Assigned by
- ProgressSoftware
- NVD analysis
- Awaiting Analysis
- Record last updated
- Oct 6, 2026
- First seen by InfoSec Signals
- Oct 9, 2026
Affected products
Progress Software Progress® Telerik® Fiddler® Classic progress_software:progress_telerik_fiddler_classic (from the name) | from 1.0.0 before 6.0.20262.10021 | CNA record |
References
- https://docs.telerik.com/fiddler/knowledge-base/kb-security-front-end-desynchronization-cve-2026-77803vendor-advisory
The CVE record on cve.org NVD's entry
The CVE record is authoritative. Products marked “from the name” were identified from the CNA's free-text names, not a CPE, and are matched less certainly. This product uses the NVD API but is not endorsed or certified by the NVD.