Low · CVSS 3.6CVE-2026-77803

Front-end Desynchronization Vulnerability in Progress® Telerik® Fiddler® ClassicCVE-2026-77803

Answer in brief

CVE-2026-77803 records a Low severity (CVSS 3.6) vulnerability in Front-end Desynchronization Vulnerability in Progress® Telerik® Fiddler® Classic. The current sources do not mark it as known exploited. The current feed maps Progress Software/Progress® Telerik® Fiddler® Classic (generic). Check affected ranges and fixed versions before updating.

Analysis pending evidence review

HOL Guard separates source facts from reviewed analysis. See the methodology.

Published Oct 5, 2026Updated Oct 5, 2026Source checked Oct 5, 2026First seen by HOL Oct 5, 2026Material review Oct 5, 2026

Upstream Advisory

Key facts

Risk: Low · CVSS 3.6
Exploitation: Not marked as known exploited
Affected software: 1 mapped package or product
Fix availability: Available

Why this deserves its current priority

CVSS is 3.6. The current sources do not mark it as known exploited. Treat this as a source-backed prioritization signal, not a statement about your environment.

Analysis status

Analysis pending evidence review

Factual feed record only; HOL analysis is not approved for indexing. Read the methodology.

Affected scope and exposure questions

The current feed maps Progress Software/Progress® Telerik® Fiddler® Classic (generic). Check affected ranges and fixed versions before updating.

Mapped affected packages and fixed versions
Package	Affected range	Fixed version
Progress Software/Progress® Telerik® Fiddler® Classicgeneric	>=1.0.0 <6.0.20262.10021	6.0.20262.10021

Recommended response

Check inventory. Check lockfiles and deployed manifests for Progress Software/Progress® Telerik® Fiddler® Classic.
Review the reported fix. Source-reported workaround: 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.

Evidence timeline and material changes

Published upstream
Oct 5, 2026
Evidence: source:cvelist:source_dates:source-dates:record
Source modified
Oct 5, 2026
Evidence: source:cvelist:source_dates:source-dates:record
First seen by HOL
Oct 5, 2026

Sources and claim methodology

Source referencedocs.telerik.com

Upstream source description

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.

Quoted source text, attributed separately from HOL analysis.