Parse Lambda AppSec request bodies according to their content type

[claponcet]

What Does This Do

Dispatches Lambda AppSec request bodies on their declared Content-Type instead of handing every body to a JSON parser:

Declared type	Reported to the WAF as
JSON-ish (application/json, any +json suffix, application/x-amz-json-1.1, application/javascript) or no usable type at all	best-effort JSON parse
application/x-www-form-urlencoded	multimap, the same shape already used for query parameters
multipart/*	form fields, each part dispatched on its own Content-Type; file parts contribute their filename, not their content
text/* and everything else	raw string

A body is never dropped. Any type we cannot structure, and any parse failure, degrades to the raw string, which the WAF can still match string rules against. In particular a text/plain body of 12345 must reach the WAF as a String and not as the Double a JSON parse would produce.

The same rule decides what happens to a malformed multipart body. A part whose headers run into the next delimiter is not reported part-by-part: no conforming parser accepts such a body — Commons FileUpload and Netty both reject it outright — so reporting the parts that happen to survive would show the WAF less than the application receives. The whole body degrades to the raw string instead.

Parsing is bounded by three allowances, shared across nesting levels rather than re-satisfied at each one: 1 MiB of characters read, 256 parts or parameters, and 20 levels of nesting. The depth and part limits mirror the WAF's own (WAFModule.MAX_DEPTH / MAX_ELEMENTS) — structure beyond them is discarded by ObjectIntrospection before the WAF sees it, so producing it would be wasted work. Exceeding an allowance degrades the body to a raw string rather than truncating it, since a truncated map would hide the dropped parameter from every rule.

Behavior changes worth a reviewer's eye. The JSON-ish decision is now made in one place, ContentTypeBodyParser.isJsonOrUntyped, which the response path in LambdaEventParser shares with the request path. It matches on the subtype alone, where the response path previously looked for json or javascript anywhere in the raw header. Three consequences:

• A Content-Type holding nothing but parameters — ; charset=utf-8 — counts as declaring no type, and so gets the same best-effort JSON parse an absent header gets. Pinned by a case in dispatchesOnContentType.
• A response body with a blank content-type now gets that best-effort JSON parse too. Previously only an absent header did, since "" does not contain json.
• A response body whose type carries the word in a parameter — multipart/form-data; boundary=--json — no longer reaches the JSON parser. This is why the gate matches on the subtype only: a client-chosen boundary must not decide how the body is read.

Motivation

Peer tracers already structure urlencoded and multipart Lambda bodies. Without it, a form-encoded or multipart request reaching a Lambda is visible to the WAF only as one opaque string, so any rule addressing server.request.body by key cannot match — the same request routed through a non-Lambda entrypoint would be inspected properly.

Additional Notes

MultipartSplitter no longer collects a HashMap of every header per part. Only Content-Disposition and Content-Type are ever read, so the others are matched and dropped as the scan passes them: a body that is nothing but header lines would otherwise allocate a map entry per line, which is why the change is here at all. The naive version of the fix — matching header names inline with a case-insensitive regionMatches — measured slower on well-formed many-part forms, so the canonical spelling is tried with a case-sensitive compare first and the matching lives in its own small method rather than inlined into split.

Two known flaky failures were seen in dd-trace-core while validating, both unrelated to this diff and both passing on re-run in isolation: PendingTraceBufferTest.testingTracerFlareDumpWithMultipleTraces and DDAgentWriterCombinedTest.statsdCommFailure. Neither is annotated @Flaky.

File parts populate server.request.body.filenames, which comes almost free once the parts are split. The other two file addresses are left empty on purpose: the Lambda event delivers the body as one UTF-8-decoded string, so a file's bytes are already corrupted by the time we see them, and files_content would report that damage as if it were the upload. files_field_names waits with it.

The repeated callback-registration boilerplate in LambdaAppSecHandler is left for its own change.

Contributor Checklist

• Format the title according to the contribution guidelines
• Assign the type: and (comp: or inst:) labels in addition to any other useful labels
• Avoid using close, fix, or any linking keywords when referencing an issue
  Use solves instead, and assign the PR milestone to the issue
• Update the CODEOWNERS file on source file addition, migration, or deletion
• Update public documentation with any new configuration flags or behaviors
• Once approved, use merge queue to merge the PR

Jira ticket: [PROJ-IDENT]

🤖 Generated with Claude Code

[claponcet]

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[dd-octo-sts]

🟢 Java Benchmark SLOs — All performance SLOs passed

Suite	Status
Startup	🟢 pass

SLO thresholds are defined here based on automatically generated metrics. A warning is raised when results are within 5% of the threshold.

PR vs. master results

Scenario	Candidate	master	Δ (95% CI of mean)
startup:insecure-bank:iast:Agent	14.77 s	14.59 s	[+0.2%; +2.4%] (maybe worse)
startup:insecure-bank:tracing:Agent	13.57 s	13.69 s	[-1.7%; -0.1%] (maybe better)
startup:petclinic:appsec:Agent	17.66 s	17.48 s	[+0.1%; +1.9%] (maybe worse)
startup:petclinic:iast:Agent	17.52 s	17.62 s	[-1.5%; +0.3%] (no difference)
startup:petclinic:profiling:Agent	17.42 s	17.48 s	[-1.5%; +0.8%] (no difference)
startup:petclinic:sca:Agent	17.53 s	17.45 s	[-0.3%; +1.2%] (no difference)
startup:petclinic:tracing:Agent	16.53 s	16.73 s	[-2.3%; -0.1%] (maybe better)

Commit: 92168741 · CI Pipeline · Benchmarking Platform UI

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Load and DaCapo benchmarks can be triggered manually in the GitLab pipeline. Results will appear in the Benchmarking Platform UI after completion.

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[datadog-prod-us1-5]

The new body limit lets the WAF drop later form fields without a raw-body fallback. The multipart parser also treats a valid preamble near match as a close delimiter and misses later fields.

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[joeyzhao2018]

Suggested change

	headers.put(key, values.stream().map(String::valueOf).collect(Collectors.joining(", ")));
	String separator = "cookie".equals(key) ? "; " : ", ";
	headers.put(key, values.stream().map(String::valueOf).collect(joining(separator)));

to take care of 'cookie' header since it uses ';'

[claponcet]

Fixed!

[joeyzhao2018]

can we also use the MultipartSplitter here? like

String declared = MultipartSplitter.parameter(contentType, "charset");

[claponcet]

Yes that simplifies things, thank you! Fixed