Front-end Desynchronization Vulnerability in Progress® Telerik® Fiddler® Classic

MITRE NVD CVE.ORG JSON API

Summary

CVECVE-2026-77803
StatePUBLISHED
AssignerProgressSoftware
Source PriorityCVE Program / NVD first with legacy fallback
Published2026-10-05 13:16:54 UTC
Updated2026-10-06 18:16:56 UTC
DescriptionIn 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.

Risk And Classification

Primary CVSS: v3.1 3.6 LOW from [email protected]

CVSS:3.1/AV:L/AC:H/PR:L/UI:N/S:U/C:L/I:L/A:N

EPSS: 0.000870000 probability, percentile 0.003220000 (date 2026-10-06)

Problem Types: CWE-444 | CWE-444 CWE-444: Inconsistent Interpretation of HTTP Requests ('HTTP Request/Response Smuggling')

VersionSourceTypeScoreSeverityVector
3.1[email protected]Secondary3.6LOWCVSS:3.1/AV:L/AC:H/PR:L/UI:N/S:U/C:L/I:L/A:N
3.1CNACVSS3.6LOWCVSS:3.1/AV:L/AC:H/PR:L/UI:N/S:U/C:L/I:L/A:N

CVSS v3.1 Breakdown

Attack Vector
Local
Attack Complexity
High
Privileges Required
Low
User Interaction
None
Scope
Unchanged
Confidentiality
Low
Integrity
Low
Availability
None

CVSS:3.1/AV:L/AC:H/PR:L/UI:N/S:U/C:L/I:L/A:N

Vendor Declared Affected Products

SourceVendorProductVersionPlatforms
CNA Progress Software Progress Telerik Fiddler Classic affected 1.0.0 6.0.20262.10021 custom Windows

References

ReferenceSourceLinkTags
docs.telerik.com/fiddler/knowledge-base/kb-security-front-end-desynchronizatio... [email protected] docs.telerik.com
CVE Program record CVE.ORG www.cve.org canonical
NVD vulnerability detail NVD nvd.nist.gov canonical, analysis

Vendor Comments And Credit

Discovery Credit

CNA: NATO Cyber Security Centre (NCSC) (en)

Additional Advisory Data

Workarounds

CNA: 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.

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