CVE-2026-77802 — HTTP Request Smuggling Vulnerability in Progress® Telerik® Fiddler® Classic
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.
Who exploits it and how urgent it is
A 0–100 score combining confirmed exploitation, ransomware and APT use, public exploits, EPSS and severity.
How the score was calculated
- EPSS 0.1% (30-day exploitation probability)+0
- Technical severity (medium)+3
The score orders the patch queue; it does not replace your asset context (internet exposure, criticality, compensating controls).
Triage
Is it exploited, how likely is exploitation, what does it touch, and how severe do the sources call it.
CISA SSVC assessment CISA
Three decision points CISA publishes for the CVEs it assesses (SSVC 2.0.3). A stakeholder decision, not a severity score.
Remediation
Fixes and mitigations documented by the sources — linked to the original advisory.
No fix documented by the sources we poll. That does not mean none exists: check the vendor advisory.
First 24 hours
Ordered from the record’s own fields — exposure first, because you cannot patch what you have not found. A rule-generated starting point.
- Identify exposed assets running the affected vendor/product/version combinations (Progress Software Progress® Telerik® Fiddler® Classic).
- No fix documented in the sources: reduce exposure and monitor the vendor advisory.
- Search available logs for exploit probes matching the vulnerability class (injection), errors and authentication anomalies.
Affected scope
Vendor, product and versions as the sources word them.
| Vendor | Product | Versions | Status | Source |
|---|---|---|---|---|
| Progress Software | Progress® Telerik® Fiddler® Classic | ≥ 1.0.0 and < 6.0.20262.10021 | Vulnerable | CVE Program (cve.org) e CISA ADP |
Attack characteristics
The CVSS vector, decoded. It describes the attack, not your exposure to it.
Every base score collected
Sources score independently and may disagree; each row says who scored it and under which version.
| Score | Version | Severity | Expl. | Impact | Scored by |
|---|---|---|---|---|---|
| 6.3 | CVSS 3.1 | Medium | 1.0 | 5.2 | security@progress.com via NIST National Vulnerability Database |
| 6.3 | CVSS 3.1 | Medium | — | — | ProgressSoftware via CVE Program (cve.org) e CISA ADP |
CVSS:3.1/AV:L/AC:H/PR:L/UI:N/S:U/C:H/I:H/A:N (score shown at the top: security@progress.com) Weakness MITRE
Weakness class according to MITRE’s CWE catalog.
EPSS over time FIRST
Oct 13, 2026 → Oct 14, 2026 · 2 reading(s) · max 0.09%
Detection
Read off the CVSS vector and the weakness class. Starting points, not tested rules.
- Local/physical vector: focus on endpoint telemetry (EDR) from affected hosts.
- Look for parameters with command/query metacharacters and unexpected child processes of the service.
Timeline
What happened to this CVE, newest first.
- 2026
- Affected products added · progress software/progress® telerik® fiddler® classic
- SSVC assessment updated
- Record published
- CVSS changed · 6.3
- Source status changed · → Awaiting Analysis
- Record created in this database
- CVE published
Source activity
Repeated readings are counted rather than listed.
- 1× Official description updated · Oct 13, 2026 – Oct 13, 2026
References
1 on the record
Elsewhere on this site
Related CVEs (same products)
- High CVE-2026-77805 Weak Executable Signature Verification Vulnerability in Progress® Telerik® Fiddler® Classic 10d
- Low CVE-2026-77803 Front-end Desynchronization Vulnerability in Progress® Telerik® Fiddler® Classic 10d
- Medium CVE-2026-77804 Time-of-check Time-of-use (TOCTOU) Race Condition in Root Certificate Installation in Progress® Telerik® Fiddler® Classic 10d
Not in any source we poll
Listed rather than left blank: an empty field and an unmeasured one look identical on screen, and only one is a reason to look elsewhere.
- No fixed version, patch or mitigation documented.
- No confirmed IOCs, IP addresses, domains, file hashes, or malware artifacts supplied.
- No organization-specific asset inventory, compensating-control status, or patch deployment evidence supplied.
- No exploit packet captures, log samples, or incident case IDs supplied.
Answered from this record
Text assembled only from the fields above — no additional claims.
What should defenders know first?
CVE-2026-77802 is "HTTP Request Smuggling Vulnerability in Progress® Telerik® Fiddler® Classic", rated Medium. It affects Progress® Telerik® Fiddler® Classic (Progress Software); versions are in the scope table. It is not in CISA KEV, which does not rule out exploitation. CVSS 6.3 describes technical severity. EPSS 0.09% estimates the probability of exploitation in 30 days; neither replaces your asset context. First verify whether exposed or business-critical assets run the affected versions, apply vendor fixes or mitigations, and preserve logs for review.
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