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TI-2026-67611 Aliases CVE-2026-77802GHSA-p3gc-87rm-gmjp

HTTP request smuggling in Progress Telerik Fiddler Classic

Priority LOW 29CWE-444Published · Updated

Key facts

Attack vector
Local
Privileges required
Low
User interaction
None
Attack complexity
High
Impact type
—
Confidentiality · integrity · availability
High · High · None
Fix status
Fixed

Attack profile from CVSS 3.1 · CNA. Fix status from CNA.

Summary

Progress Telerik Fiddler Classic for Windows from 1.0.0 before 6.0.20262.10021 is affected. Conflicting Content-Length headers are forwarded unchanged while Fiddler frames the body by the first value, causing request smuggling (CWE-444). A local user with low privilege sharing the proxy with another user can smuggle a request via a non-RFC 9110 compliant origin server that keeps connections alive. Poisoned responses are served to other users, who can receive responses intended for them.

Written with Claude Haiku 5.5 from the sources below · 2026-10-29

Description from the CNA

In 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.

Why it matters now

Exploitation None reported CISA SSVC · not in CISA KEV
EPSS 0.00103 0th percentileScored 2026-11-07
Fix Fixed Progress® Telerik® Fiddler® Classic (Progress Software): 6.0.20262.10021

SSVC decision points

CISA ADP · SSVC 2.0.3

Exploitation
None
Automatable
No
Technical impact
Total

The decision (Track, Attend, Act) also depends on your mission and well-being impact; see CISA's SSVC guide.

Affected products

VendorProductAffected versionsFixed inStatusSource
ProgressProgress® Telerik® Fiddler® Classic1.0.0 to before 6.0.20262.100216.0.20262.10021Patch availableCNA
  • Progress® Telerik® Fiddler® Classic VendorProgress Affected 1.0.0 to before 6.0.20262.10021 Fixed in 6.0.20262.10021 Patch available CNA

Remediation

Update Progress Telerik Fiddler Classic to 6.0.20262.10021 or later.

Written with Claude Haiku 5.5 from the sources below · 2026-10-29

  • Fix · CNA Progress® Telerik® Fiddler® Classic (Progress Software) Fixed in 6.0.20262.10021

Fixed in the same release

Records fixed by the same vendor advisory or release, as the CNA lists them.

  • Release ·

    Progress® Telerik® Fiddler® Classic 6.0.20262.10021

    4 records Highest priority Priority LOW 4 LOW

    Other records, highest priority first

    • TI-2026-67614 Weak helper tool signature checks in Telerik Fiddler ClassicPriority LOW 35
    • TI-2026-67613 Race condition in Progress Telerik Fiddler Classic certificate installationPriority LOW 30
    • TI-2026-67612 Telerik Fiddler Classic front-end desynchronization enables request smugglingPriority LOW 16
    Show all 4 records →

Timeline

  1. 2026-10-05 Threatist record created from CVE-2026-77802
  2. 2026-10-05 Alias added: GHSA-p3gc-87rm-gmjp
  3. 2026-10-06 CISA SSVC decision points changed
  4. 2026-10-06 Priority changed: MEDIUM → LOW

References

Advisory 1

  • TI-2026-07083 Command injection in Progress LoadMaster KEVCVSS 3.1CRITICAL 9.8
  • TI-2024-06182 WhatsUp Gold HasErrors SQL Injection Authentication Bypass Vulnerability KEVCVSS 3.1CRITICAL 9.8
  • TI-2024-04541 WhatsUp Gold GetFileWithoutZip Directory Traversal Remote Code Execution Vulnerability KEVCVSS 3.1CRITICAL 9.8
  • TI-2024-04060 Registration Authentication Bypass Vulnerability KEVCVSS 3.1CRITICAL 9.8
  • TI-2024-01127 LoadMaster Pre-Authenticated OS Command Injection KEVCVSS 3.1CRITICAL 10.0
  • TI-2023-20934 WS_FTP Server Ad Hoc Transfer Module .NET Deserialization Vulnerability KEVCVSS 3.1CRITICAL 10.0
See all 436 →

Sources

This product uses the NVD API but is not endorsed or certified by the NVD. Sources and licences