What’s Vulnerable › Vulnerabilities

CVE-2026-69217: Http4s: Ember Server accepts duplicate Content-Length headers

• Severity: high (CVSS 8.7 v3.1)
• Exploited in the wild: not listed in CISA KEV
• EPSS: 0.5% probability of exploitation in the next 30 days
• Public exploit or detection template: none known
• Published: 2026-09-15 · updated 2026-09-16
• Weakness: CWE-444

Description

Http4s is a Scala interface for HTTP services. Prior to 0.23.35 and 1.0.0-M47, Ember’s HTTP/1.1 parser accepts differing duplicate Content-Length headers and uses the last value instead of rejecting the message. When an Ember server is behind a keep-alive intermediary that selects a different occurrence, an unauthenticated attacker can create CL.CL request smuggling that bypasses front-end controls, captures a later user’s headers, or poisons a cache. The shared client parser can also misframe responses from a malicious or compromised upstream when the client acts as a proxy for multiple downstream consumers. This issue is fixed in versions 0.23.35 and 1.0.0-M47.

Affected products

• http4s: before 0.23.35; from 1.0.0-M1, < 1.0.0-M47

References

• https://github.com/http4s/http4s/commit/24e8d34e3ea89f8f23e0053ec9944f6467e8086d
• https://github.com/http4s/http4s/releases/tag/v0.23.35
• https://github.com/http4s/http4s/releases/tag/v1.0.0-M47
• https://github.com/http4s/http4s/security/advisories/GHSA-p83c-4wj9-p6w9

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