CVE-2026-28898: Swift-NIO-HTTP2 Pseudo-Header Validation Bypass – Header Injection Risk

Technical summary

The vulnerability resides in swift-nio-http2's HPACK header processing and HTTP/2-to-HTTP/1.1 translation logic. HTTP/2 uses pseudo-headers (prefixed with ':') that carry critical metadata; these must be sanitized before translation to HTTP/1.1's header format. The codec did not validate pseudo-header values for CR (0x0D), LF (0x0A), or NUL (0x00) bytes, which are line-breaking or string-terminating control characters. In HTTP/1.1, these characters can be interpreted as header delimiters or terminators, potentially allowing header injection or request smuggling attacks when processed by downstream systems. The fix implements validation at two layers: HPACK validation during frame parsing and an additional check during the HTTP/2-to-HTTP/1.1 translation step. Non-compliant messages now trigger immediate connection closure with a protocol error.

Business impact

HTTP/2-to-HTTP/1.1 conversion gateways using swift-nio-http2 could become conduits for header injection attacks. If a reverse proxy or load balancer built on this library sits between untrusted clients and backend HTTP/1.1 services, attackers could inject CR/LF bytes to craft malicious headers that bypass authentication, inject content, or confuse request routing. The impact depends on downstream application behavior; some systems may ignore malformed headers, while others may misinterpret them. Organizations relying on swift-nio-http2 for API gateways, proxies, or protocol translation should prioritize patching to prevent request smuggling and header injection scenarios.

Affected systems

Apple's swift-nio-http2 library versions prior to 1.44.1 are affected. Any application or service that uses swift-nio-http2 for HTTP/2 handling is potentially vulnerable, including custom HTTP/2 servers, reverse proxies, API gateways, and protocol converters built in Swift. The vulnerability is most concerning in deployments where swift-nio-http2 translates HTTP/2 traffic from untrusted external clients to HTTP/1.1 backend services. Server applications are at higher risk than client libraries, though client-side translation could theoretically be exploited if a malicious HTTP/2 server responds with crafted pseudo-headers.

Exploitability

The vulnerability requires no authentication, no user interaction, and no special network positioning beyond the ability to send HTTP/2 requests to the affected service. The attack surface is network-adjacent (AV:N, AC:L per CVSS). However, successful exploitation depends on downstream system behavior: some HTTP/1.1 parsers may silently drop malformed headers, limiting impact. The CVSS score of 5.3 (MEDIUM) reflects the confidentiality impact (C:L) from potential header-based information disclosure or bypass of security controls, but no integrity or availability impact in the base case. Real-world risk scales with the trust relationship between the HTTP/2 entry point and backend services.

Remediation

Upgrade swift-nio-http2 to version 1.44.1 or later immediately. This applies to any Swift-based application or service using the library. Verify the version in your dependency management files (Package.swift, Podfile, etc.) and apply the security patch through your normal release and deployment pipeline. If you maintain a downstream product that bundles swift-nio-http2, prioritize a security release for your customers. No workarounds are available; patching is the only mitigation.

Patch guidance

Update swift-nio-http2 to 1.44.1 or a later version. For projects using Swift Package Manager, update the package requirement in Package.swift; for CocoaPods, update the podspec. Perform functional testing on HTTP/2 traffic after patching to confirm protocol translation continues to work correctly. The fix is backward-compatible at the API level—no code changes are required in consuming applications, only the dependency version update. Verify against the official Apple swift-nio-http2 repository and release notes to confirm version 1.44.1 is available in your preferred package manager.

Detection guidance

Monitor HTTP/2 traffic for unusual pseudo-header values containing control characters (CR, LF, NUL). Intrusion detection systems should flag HTTP/2 frames where pseudo-headers (identified by the ':' prefix) contain these bytes. Log and review connection errors or protocol violations from swift-nio-http2 services, as the patched version will reject such requests with connection closure. If you run vulnerable versions, inspect access logs for any evidence of requests with anomalous header patterns, though such requests may not be logged if parsing fails early. Consider network-based detection of HTTP/2 frames with embedded control characters in the header block.

Why prioritize this

Although CVSS is MEDIUM (5.3), this vulnerability should be prioritized as HIGH for deployment if your service acts as an HTTP/2 gateway or reverse proxy. The lack of KEV designation suggests no active exploitation has been observed, but the simplicity of crafting malicious HTTP/2 frames and the potential for request smuggling warrant rapid patching. HTTP/2-to-HTTP/1.1 translation is a common architectural pattern; delaying the patch increases risk of opportunistic attacks. Patch within 30 days for internet-facing services, sooner for edge/gateway deployments.

Risk score, explained

The CVSS:3.1 score of 5.3 reflects a low barrier to exploitation (network-accessible, no authentication or user interaction required) but limited direct impact (confidentiality impact only, no integrity or availability impacts in the base case). The severity is MEDIUM. However, context matters: if the affected library is exposed to untrusted HTTP/2 clients in a gateway role, the actual risk is elevated due to the potential for request smuggling and downstream bypass attacks. The lack of assigned KEV status indicates this is not yet known to be exploited in the wild, reducing urgency slightly but not eliminating it.

Frequently asked questions