Vulnerability in Progress Telerik Fiddler Classic for Windows (CVE-2026-77803)
Summary
Vulnerability in Progress Telerik Fiddler Classic for Windows (CVE-2026-77803). Risk of unauthorized operations or information disclosure.
Original description
The description as published by the source.
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.
Source: NIST National Vulnerability Database NVD GitHub Advisory
Auto-generated summary
Assembled by SecPulse from the source data (CVSS, CWE, affected versions) using templates — no AI involved. Check the original description and references below for exact details.
A vulnerability tracked as CVE-2026-77803 has been found in Progress Telerik Fiddler Classic for Windows.
Risk of unauthorized operations or information disclosure. CVSS score: 3.6/10.
What to do: apply the vendor's official patched release.
If unsure, ask your IT team or search "Progress Telerik Fiddler Classic for Windows CVE-2026-77803" on the vendor's site.
CVE-2026-77803 (Progress Telerik Fiddler Classic for Windows) — CWE-444 / CVSS v3 3.6
Attack vector: local / no user interaction
Plan: 1) Audit SBOM/dependencies, 2) Stage→prod upgrade, 3) Add WAF/proxy monitoring on affected endpoints, 4) Hunt IOCs in logs.
Refs: see the GHSA / vendor advisory / patched release linked on this page.
A vulnerability (CWE-444) exists in Progress Telerik Fiddler Classic for Windows.
Check the vendor advisory for the affected versions of Progress Telerik Fiddler Classic for Windows.
Severity: Low (CVSS 3.6/10). Risk of unauthorized operations or information disclosure.
Update to the patched release per vendor advisory.
Until the patch is applied: disable the affected feature, apply WAF rules, or restrict access via network ACLs.
Search webserver/proxy logs for unusual request patterns matching this CVE's known IOCs. Audit SBOM/dependencies to find affected services.
Response actions
Concrete steps and command examples for SOC/SRE teams to execute in order
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1Identify exposure identify
Audit SBOM/dependencies for affected components.資産台帳・SBOM・依存マニフェストで Progress Telerik Fiddler Classic for Windows を使っているサービスとバージョンを洗い出す。
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2Post-deployment verification verify
Confirm patched version is live in productionパッチ適用後、ステージングで PoC または同等の悪用パターンを再現して脆弱性が閉じたことを確認。本番ではアクセスログ・WAF でアラート再発が無いか継続監視。
References
- web security@progress.com
- web nvd.nist.gov
- web github.com