Response poisoning and disclosure via request desynchronization
Published Oct 5, 2026 · Updated Oct 5, 2026
HTTP request smuggling in Progress Telerik Fiddler Classic before 6.0.20262.10021 lets local users poison and read other users' proxy responses. Its request-forwarding component forwards conflicting Content-Length headers unchanged but frames the body using only the first value, leaving a smuggled response on a pooled server connection. Exploitation requires shared use of the proxy, server-connection reuse, and an origin server that accepts the conflicting lengths and keeps the connection alive.
Summary
What happened
HTTP request smuggling in Progress Telerik Fiddler Classic before 6.0.20262.10021 lets local users poison and read other users' proxy responses. Its request-forwarding component forwards conflicting Content-Length headers unchanged but frames the body using only the first value, leaving a smuggled response on a pooled server connection. Exploitation requires shared use of the proxy, server-connection reuse, and an origin server that accepts the conflicting lengths and keeps the connection alive.
The record
- CVE
- CVE-2026-77802
- Published
- Oct 5, 2026
- Updated
- Oct 5, 2026
- Vendor
- Progress Software Corporation
- Product
- Fiddler Classic
- Classifications
- Unavailable
- Attack vector
- local
- Privileges
- authenticated
Timeline
How it unfolded
- Oct 5, 2026CVE publishedPublication date reported by the CVE source.
- Oct 5, 2026Record updatedLatest update available in the CVE record.
Exploitability
Present is not the same as exploitable
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Is a vulnerable build present?
Compare these published version ranges with your installed build and any vendor patches.
- Affected versionversion=1.0.0 <6.0.20262.10021
What conditions does exploitation require?
What is affected?
Published CVSS scores
No CVSS assessment is available in this record.
CVSS describes severity. EPSS estimates exploitation probability.
Attacks
What attackers are doing with it
Daily unique IPs observed by Shadowserver honeypots for known exploited vulnerabilities (KEVs). Missing observations do not establish an absence of attacks.
Activity data is delayed. The latest report is Sep 28, 2026.
Weakness, pattern, technique
No sourced classifications are available.
Public exploit references
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Technologies
Your stack
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Your stack
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