CVE-2026-69205

Http4s is a Scala interface for HTTP services. Prior to 0.23.35 and 1.0.0-M47, Ember’s HeaderP.parse uses a case-sensitive substring test for the Transfer-Encoding value and decodes header bytes with the platform default charset. Values such as Chunked are not recognized, values such as notchunked are incorrectly accepted, and Unicode case folding can turn a Kelvin-sign byte sequence into a match when UTF-8 is used. Intermediaries that apply RFC-compliant token and charset rules can therefore disagree with Ember’s Content-Length or zero-length framing, enabling TE.CL or TE.0 request smuggling, access-control bypass, cross-user request hijacking, and cache poisoning on the server path. Response smuggling through an ember-client gateway requires a malicious or compromised upstream. This issue is fixed in versions 0.23.35 and 1.0.0-M47.

Http4s is a Scala interface for HTTP services. Prior to 0.23.35 and 1.0.0-M47, Ember’s HeaderP.parse uses a case-sensitive substring test for the Transfer-Encoding value and decodes header bytes with the platform default charset. Values such as Chunked are not recognized, values such as notchunked are incorrectly accepted, and Unicode case folding can turn a Kelvin-sign byte sequence into a match when UTF-8 is used. Intermediaries that apply RFC-compliant token and charset rules can therefore disagree with Ember’s Content-Length or zero-length framing, enabling TE.CL or TE.0 request smuggling, access-control bypass, cross-user request hijacking, and cache poisoning on the server path. Response smuggling through an ember-client gateway requires a malicious or compromised upstream. This issue is fixed in versions 0.23.35 and 1.0.0-M47.

Summary

Ember's HTTP/1.1 header parser matches the Transfer-Encoding header value with a case-sensitive substring test (hValue.contains("chunked")). RFC 9112 §7 requires transfer-coding names to be compared case-insensitively. A request carrying Transfer-Encoding: Chunked (capital C) is therefore not recognised as chunked, and Ember falls back to framing by Content-Length (or zero if absent) while a compliant intermediary frames the same bytes by chunked encoding. The two parsers then disagree on where the request body ends, enabling HTTP request smuggling (TE.CL / TE.0).

The same line of code admits two further variants:

• The substring test misfires on Transfer-Encoding: notchunked (an inverse desync — Ember treats it as chunked while a compliant intermediary rejects the unknown coding).
• Header field bytes are decoded with the platform-default charset. Under UTF-8 the wire bytes E2 84 AA decode to U+212A KELVIN SIGN, which String.equalsIgnoreCase Unicode-case-folds to k, so Transfer-Encoding: chun<U+212A>ed matches chunked once the comparison is made case-insensitive without also pinning the decode to ISO-8859-1.

Impact

Server

Request smuggling when ember-server is an origin behind an intermediary that honours Transfer-Encoding case-insensitively per RFC, forwards the header value verbatim, and reuses keep-alive connections to the backend:

• Front-end security bypass: the smuggled request reaches paths the intermediary's ACL/auth layer would have blocked, with attacker-chosen method and headers.
• Cross-user request hijack: a partial smuggled prefix left in Ember's connection buffer is concatenated with the next victim's request on the same pooled backend connection, exposing its headers (e.g. Cookie, Authorization) to the attacker.
• Cache poisoning: the smuggled response is associated with the next request key in a caching proxy.

Client

ember-client shares the same HeaderP.parse on the response path, enabling response smuggling when http4s is used as a gateway. This is less severe: it requires a malicious or compromised upstream rather than an anonymous remote client.

Preconditions

• Unauthenticated remote attacker (server)
• ember-server as origin behind a keep-alive intermediary
• Intermediary treats Transfer-Encoding case-insensitively (per RFC) and forwards the header value without lowercasing it
• Malicious or compromised upstream (client)

Workarounds

• Intermediary fully buffers and re-encodes request bodies (e.g. nginx with default proxy_request_buffering on)
• Intermediary normalises the Transfer-Encoding value (lowercases the token) before forwarding
• Disable backend keep-alive between the intermediary and Ember

Http4s is a Scala interface for HTTP services. Prior to 0.23.35 and 1.0.0-M47, Ember’s HeaderP.parse uses a case-sensitive substring test for the Transfer-Encoding value and decodes header bytes with the platform default charset. Values such as Chunked are not recognized, values such as notchunked are incorrectly accepted, and Unicode case folding can turn a Kelvin-sign byte sequence into a match when UTF-8 is used. Intermediaries that apply RFC-compliant token and charset rules can therefore disagree with Ember’s Content-Length or zero-length framing, enabling TE.CL or TE.0 request smuggling, access-control bypass, cross-user request hijacking, and cache poisoning on the server path. Response smuggling through an ember-client gateway requires a malicious or compromised upstream. This issue is fixed in versions 0.23.35 and 1.0.0-M47.

Http4s is a Scala interface for HTTP services. Prior to 0.23.35 and 1.0.0-M47, Ember’s HeaderP.parse uses a case-sensitive substring test for the Transfer-Encoding value and decodes header bytes with the platform default charset. Values such as Chunked are not recognized, values such as notchunked are incorrectly accepted, and Unicode case folding can turn a Kelvin-sign byte sequence into a match when UTF-8 is used. Intermediaries that apply RFC-compliant token and charset rules can therefore disagree with Ember’s Content-Length or zero-length framing, enabling TE.CL or TE.0 request smuggling, access-control bypass, cross-user request hijacking, and cache poisoning on the server path. Response smuggling through an ember-client gateway requires a malicious or compromised upstream. This issue is fixed in versions 0.23.35 and 1.0.0-M47.

Summary

Ember's HTTP/1.1 header parser matches the Transfer-Encoding header value with a case-sensitive substring test (hValue.contains("chunked")). RFC 9112 §7 requires transfer-coding names to be compared case-insensitively. A request carrying Transfer-Encoding: Chunked (capital C) is therefore not recognised as chunked, and Ember falls back to framing by Content-Length (or zero if absent) while a compliant intermediary frames the same bytes by chunked encoding. The two parsers then disagree on where the request body ends, enabling HTTP request smuggling (TE.CL / TE.0).

The same line of code admits two further variants:

• The substring test misfires on Transfer-Encoding: notchunked (an inverse desync — Ember treats it as chunked while a compliant intermediary rejects the unknown coding).
• Header field bytes are decoded with the platform-default charset. Under UTF-8 the wire bytes E2 84 AA decode to U+212A KELVIN SIGN, which String.equalsIgnoreCase Unicode-case-folds to k, so Transfer-Encoding: chun<U+212A>ed matches chunked once the comparison is made case-insensitive without also pinning the decode to ISO-8859-1.

Impact

Server

Request smuggling when ember-server is an origin behind an intermediary that honours Transfer-Encoding case-insensitively per RFC, forwards the header value verbatim, and reuses keep-alive connections to the backend:

• Front-end security bypass: the smuggled request reaches paths the intermediary's ACL/auth layer would have blocked, with attacker-chosen method and headers.
• Cross-user request hijack: a partial smuggled prefix left in Ember's connection buffer is concatenated with the next victim's request on the same pooled backend connection, exposing its headers (e.g. Cookie, Authorization) to the attacker.
• Cache poisoning: the smuggled response is associated with the next request key in a caching proxy.

Client

ember-client shares the same HeaderP.parse on the response path, enabling response smuggling when http4s is used as a gateway. This is less severe: it requires a malicious or compromised upstream rather than an anonymous remote client.

Preconditions

• Unauthenticated remote attacker (server)
• ember-server as origin behind a keep-alive intermediary
• Intermediary treats Transfer-Encoding case-insensitively (per RFC) and forwards the header value without lowercasing it
• Malicious or compromised upstream (client)

Workarounds

• Intermediary fully buffers and re-encodes request bodies (e.g. nginx with default proxy_request_buffering on)
• Intermediary normalises the Transfer-Encoding value (lowercases the token) before forwarding
• Disable backend keep-alive between the intermediary and Ember