SwiftNIO NIOHTTP1: HTTPDecoder accepts unbounded HTTP/1 header blocks, enabling remote DoS
Summary
The HTTPDecoder in NIOHTTP1 enforces no limit on the total size of an HTTP/1 message's header block or on the number of header fields per message. A remote peer can submit an arbitrary number of small, valid headers in a single request and have them all accumulated into the resulting HTTPHeaders value before any application code runs. This can be used to exhaust memory, or — for consumers that subsequently convert headers into swift-http-types' HTTPFields — to crash the process.
Details
HTTPDecoder previously enforced only a single hardcoded parsing limit: 80 KB per individual header field (name + value). There was no cap on the cumulative size of the header block, nor on the number of header fields per message. Because each individual field can remain well below the 80 KB threshold, a peer can submit hundreds of thousands of valid headers in a single request, all of which are appended to the decoded HTTPHeaders without bound.
The headers are then visible to user code through the standard HTTPServerRequestPart.head / HTTPClientResponsePart.head events. Two observed downstream effects:
- Hummingbird 2 (and other consumers that bridge
HTTPHeadersintoswift-http-types'HTTPFields) crashes via a precondition failure insideHTTPFieldsonce the configured field count is exceeded. - Vapor 4 does not crash, but the per-request memory footprint scales linearly with the number of headers received, allowing a single connection to inflate server memory use substantially.
Impact
A single unauthenticated remote peer can trigger a denial of service against any HTTP/1 server (or, in the response direction, any HTTP/1 client) built on NIOHTTP1 — either by crashing the process, depending on the downstream framework, or by driving the process's resident memory to arbitrary sizes.
Patches
This issue is addressed in swift-nio 2.100.0 and later.
The HTTPDecoder now applies three parsing limits with conservative defaults, exposed through the new NIOHTTPDecoderLimitConfiguration type:
| Limit | Default |
| --- | --- |
| maxHeaderFieldSize | 80 KB |
| maxHeaderListSize | 2 MB |
| maxHeaderFieldCount | 256 |
Exceeding any of these limits causes the decoder to fail with HTTPParserError.headerOverflow. The configuration can be supplied directly to HTTPRequestDecoder / HTTPResponseDecoder, or via the decoderConfiguration property on NIOUpgradableHTTPServerPipelineConfiguration and NIOUpgradableHTTPClientPipelineConfiguration.
Users who require larger limits — for example, applications that legitimately exchange very large header blocks — can opt into them explicitly by constructing a custom NIOHTTPDecoderLimitConfiguration.
Workarounds
Users unable to upgrade can mitigate by placing a reverse proxy in front of the service that enforces equivalent limits on request header count and total header size.
Credit
This issue was reported by @Joannis. SwiftNIO thanks @Joannis for the report and the support in landing the fix.