Vulnerability in Progress Telerik Fiddler Classic for Windows (CVE-2026-77802)
Summary
Vulnerability in Progress Telerik Fiddler Classic for Windows (CVE-2026-77802). Confidential information can be exposed externally.
Original description
The description as published by the source.
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.
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-2026-77802 has been found in Progress Telerik Fiddler Classic for Windows.
Confidential information can be exposed externally. CVSS score: 6.3/10.
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-2026-77802" on the vendor's site.
CVE-2026-77802 (Progress Telerik Fiddler Classic for Windows) — CWE-444 / CVSS v3 6.3
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: Medium (CVSS 6.3/10). Confidential information can be exposed externally.
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