CVE-2026-28980
CVE-2026-28980 is a remote code execution vulnerability in github.com/apple/swift-nio. O3 Security confirms whether CVE-2026-28980 is actually reachable in your code before you act, and blocks exploitation at runtime until you patch.
SwiftNIO NIOHTTP1: HTTPDecoder accepts unbounded HTTP/1 header blocks, enabling remote DoS
Real-World Exposure
github.com/apple/swift-nioReal-time download stats are indexed for npm and PyPI packages. This vulnerability affects SwiftURL packages — download data is not available via public APIs for these ecosystems.
Description
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.
Affected Packages
| Ecosystem | Package | Vulnerable range | Fix |
|---|---|---|---|
| 📦SwiftURL | github.com/apple/swift-nio | all versions | 2.100.0 |
Detection & mitigation playbook
Open-source dependencyDetect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for github.com/apple/swift-nio. O3's reachability analysis confirms whether the vulnerable code path is actually invoked in your application, so you act on real exposure instead of every transitive match.
Fix
Update github.com/apple/swift-nio to 2.100.0 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2026-28980 is resolved across your whole dependency graph.
Workarounds
If you can't upgrade right away: gate or disable the affected feature, validate untrusted input at the boundary, and avoid passing attacker-controlled data into the vulnerable path. O3's runtime protection blocks exploitation in production as an interim safeguard until the upgrade lands.
How O3 protects you
O3 pinpoints whether CVE-2026-28980 is reachable in your code and exactly where to fix it, then blocks exploitation in production at runtime until the patched version is deployed.
Tailored to CVE-2026-28980. Runtime protection reduces exposure until a permanent patch is applied and verified — it complements patching, it doesn't replace it.
Frequently Asked Questions
Is CVE-2026-28980 in your dependencies?
O3 detects CVE-2026-28980 across SwiftURL dependencies and uses function-level reachability to confirm whether the vulnerable code path is actually reachable — not just present. No false positives.