CVE-2026-69216 in Http4s
Summary
by MITRE • 09/15/2026
Http4s is a Scala interface for HTTP services. Prior to 0.23.35 and 1.0.0-M47, Ember’s chunk decoder trims the chunk-size token and accepts leading plus or minus signs instead of requiring one or more hexadecimal digits followed by the required CRLF. When an intermediary forwards chunked data without re-encoding and interprets malformed chunk boundaries differently, an unauthenticated attacker can create TE.TE request smuggling that bypasses intermediary controls, poisons caches, or hijacks the request queue. The same response-path leniency can enable response smuggling against an ember-client used as a gateway when the upstream is malicious or compromised. This issue is fixed in versions 0.23.35 and 1.0.0-M47.
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Analysis
by VulDB Data Team • 09/15/2026
Http4s serves as a prominent functional HTTP server and client library for the Scala programming language, providing developers with tools to build robust web services. Within this ecosystem, the Ember module handles low-level HTTP parsing and connection management. A critical security flaw was identified in the chunked transfer encoding decoder used by Ember prior to versions 0.23.35 and 1.0.0-M47. This vulnerability stems from an overly permissive implementation of the HTTP/1.1 chunked coding mechanism, specifically regarding how the library parses the size token that precedes each data chunk in a stream. According to RFC 7230, the chunk-size field must consist exclusively of hexadecimal digits followed by optional whitespace and then a carriage return-line feed sequence. However, the vulnerable implementation incorrectly accepted leading plus or minus signs as valid prefixes for these hex values, effectively treating them as part of the numeric value rather than rejecting them as malformed input.
This deviation from standard protocol behavior creates significant risks in environments where HTTP requests are proxied through intermediaries such as load balancers, reverse proxies, or API gateways. When an unauthenticated attacker sends a request with chunked transfer encoding that includes these non-standard signs, the vulnerable Ember server may interpret the chunk boundaries differently than the intermediary processing the same traffic. This discrepancy in parsing logic leads to HTTP Request Smuggling, specifically classified under TE.TE smuggling where both the client and the server use Transfer-Encoding: chunked but disagree on how to parse it. By carefully crafting requests that exploit this leniency, an attacker can manipulate the order in which subsequent requests are processed by the backend application or the intermediary itself.
The operational impact of such request smuggling is severe and multifaceted. An attacker may successfully bypass security controls implemented at the network perimeter, as the smuggled request might be interpreted differently by a firewall or intrusion detection system compared to the actual server logic. Furthermore, this vulnerability enables cache poisoning attacks where an adversary can inject malicious responses into shared caches, causing other users to receive compromised content. In more advanced scenarios, it allows for session hijacking if authentication tokens are leaked through misrouted requests, or queue hijacking where sensitive operations intended for one user are executed on behalf of another due to the reordering of HTTP messages in the processing pipeline.
The vulnerability also extends beyond request handling into response management when Ember is utilized as a client acting as a gateway between internal services and external upstream servers. If an attacker compromises or controls an upstream server, they can send malformed chunked responses that exploit the same parsing leniency within the Ember client. This enables HTTP Response Smuggling, where the downstream intermediary misinterprets the response boundaries, potentially allowing the injection of malicious headers or content into internal systems. This bidirectional nature of the flaw underscores its severity across both server-side and gateway deployment patterns involving Http4s.
To mitigate this vulnerability, organizations must upgrade their dependencies to fixed versions 0.23.35 or 1.0.0-M47 and later releases where the chunk decoder strictly enforces hexadecimal-only parsing for size tokens. Security teams should also audit configurations that rely on HTTP/1.1 pipelining or complex proxy chains, as these environments are most susceptible to exploitation of such protocol ambiguities. Implementing strict input validation at the edge proxies can provide an additional layer of defense by rejecting requests with non-standard chunk formatting before they reach the application server. This issue is categorized under CWE-20 Improper Input Validation and maps to ATT&CK techniques related to HTTP Protocol Manipulation, highlighting the importance of adhering strictly to RFC standards in network-facing software components.