If left unpatched, CVE-2026-77802 could lead to major system disruptions or data loss.
Status: Received on 05 Oct 2026, 13:16 UTC
Published on: 05 Oct 2026, 13:16 UTC
CVSS Release: version 3
security@progress.com
Secondary
CVSS:3.1/AV:L/AC:H/PR:L/UI:N/S:U/C:H/I:H/A:N
CVE-2026-77802: 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.
The exploitability of CVE-2026-77802 depends on two key factors: attack complexity (the level of effort required to execute an exploit) and privileges required (the access level an attacker needs).
This vulnerability, CVE-2026-77802, requires a high level of attack complexity and low privileges, making it difficult but not impossible to exploit. Organizations should ensure robust security configurations to mitigate risks.
A lower complexity and fewer privilege requirements make exploitation easier. Security teams should evaluate these aspects to determine the urgency of mitigation strategies, such as patch management and access control policies.
Attack Complexity (AC) measures the difficulty in executing an exploit. A high AC means that specific conditions must be met, making an attack more challenging, while a low AC means the vulnerability can be exploited with minimal effort.
Privileges Required (PR) determine the level of system access necessary for an attack. Vulnerabilities requiring no privileges are more accessible to attackers, whereas high privilege requirements limit exploitation to authorized users with elevated access.
Above is the CVSS Sub-score Breakdown for CVE-2026-77802, illustrating how Base, Impact, and Exploitability factors combine to form the overall severity rating. A higher sub-score typically indicates a more severe or easier-to-exploit vulnerability.
Below is the Impact Analysis for CVE-2026-77802, showing how Confidentiality, Integrity, and Availability might be affected if the vulnerability is exploited. Higher values usually signal greater potential damage.
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