HTTP Request Smuggling Vulnerability in Progress® Telerik® Fiddler® Classic

MITRE NVD CVE.ORG JSON API

Summary

CVECVE-2026-77802
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, HTTP request smuggling is possible in the proxy request forwarding component. Requests containing multiple Content-Length headers with conflicting values are forwarded verbatim to the origin server, while Fiddler frames the request body using only the first Content-Length value. A local threat actor with low privileges who is able to send requests through the same Fiddler proxy instance as another user can exploit this desynchronization against a non-RFC-9110-compliant origin server that keeps the connection alive to smuggle an additional request. Because Fiddler returns the server connection to its pipe pool after reading only the first response, the unread smuggled response remains buffered on the socket and is served to the next session that reuses that connection, allowing the attacker to poison responses delivered to other users and to obtain responses intended for them.

Risk And Classification

Primary CVSS: v3.1 6.3 MEDIUM from [email protected]

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

EPSS: 0.000920000 probability, percentile 0.004950000 (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]Secondary6.3MEDIUMCVSS:3.1/AV:L/AC:H/PR:L/UI:N/S:U/C:H/I:H/A:N
3.1CNACVSS6.3MEDIUMCVSS:3.1/AV:L/AC:H/PR:L/UI:N/S:U/C:H/I:H/A:N

CVSS v3.1 Breakdown

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

CVSS:3.1/AV:L/AC:H/PR:L/UI:N/S:U/C:H/I:H/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-http-request-smuggling-cve... [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 server connection reuse in Fiddler Classic: open Tools > Options > Connections and uncheck the "Reuse server connections" checkbox. Disabling server pipe reuse prevents a poisoned server connection from being handed to another session.

© CVE.report 2026

Use of this information constitutes acceptance for use in an AS IS condition. There are NO warranties, implied or otherwise, with regard to this information or its use. Any use of this information is at the user's risk. It is the responsibility of user to evaluate the accuracy, completeness or usefulness of any information, opinion, advice or other content. EACH USER WILL BE SOLELY RESPONSIBLE FOR ANY consequences of his or her direct or indirect use of this web site. ALL WARRANTIES OF ANY KIND ARE EXPRESSLY DISCLAIMED. This site will NOT BE LIABLE FOR ANY DIRECT, INDIRECT or any other kind of loss.

CVE, CWE, and OVAL are registred trademarks of The MITRE Corporation and the authoritative source of CVE content is MITRE's CVE web site. This site includes MITRE data granted under the following license.

Free CVE JSON API cve.report/api

CVE.report and Source URL Uptime Status status.cve.report