fix: sanitize control bytes in x-amzn-{request,lambda}-context headers#734
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Strip C0 control bytes (< 0x20 except \t) and DEL (0x7F) from the JSON serialized into the x-amzn-request-context and x-amzn-lambda-context headers before constructing HeaderValue. RFC 7230 forbids these bytes in header field values, and the request context echoes the original request path verbatim — so a Lambda Function URL probed by a security scanner with crafted paths (e.g. nuclei) would fail every invocation with InvalidHeaderValue, tripping alarms on otherwise-fine traffic. Fixes #732
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Can we add a test for a parallel assertion for x-amzn-lambda-context, since both call sites changed. And a test for the all-clean fast path (worth covering once the signature is Cow-based). And maybe we can assert that the stripped JSON still deserializes back into the original RequestContext type, not just into serde_json::Value. |
seshubaws
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May 27, 2026
Helps diagnose mis-configured AWS_LWA_REMOVE_BASE_PATH: only fires when the prefix actually matched (stripped length differs from original), so unmatched requests stay quiet.
- Switch strip_forbidden_header_bytes to Cow<'_, [u8]>: header-safe input (the common case) skips allocation, and the fast path becomes observable in tests via Cow::Borrowed. - New test_strip_forbidden_header_bytes_all_clean asserts the borrowed fast path for clean input. - Existing unit test now asserts Cow::Owned on the stripping branch. - Regression test parses the sanitized x-amzn-request-context back into the typed RequestContext (not just serde_json::Value), and adds a parallel assertion for x-amzn-lambda-context — both call sites now exercise strip_forbidden_header_bytes and round-trip through JSON.
seshubaws
approved these changes
May 27, 2026
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Summary
< 0x20except\t) and DEL (0x7F) from the JSON serialized intox-amzn-request-contextandx-amzn-lambda-contextbefore constructingHeaderValue. RFC 7230 forbids these bytes in header field values.ApiGatewayV2event with control bytes (0x04,0x7F,0x18) inrequestContext.http.pathand asserts the forwarded request carries a header-safe, JSON-parseablex-amzn-request-context.Fixes #732.
Why
The reporter's Lambda (behind CloudFront → Function URL) returns 5xx every time
nucleiprobes it with paths like/%04%7F%18;{curl,...};because the adapter copiesrequestContext.http.pathverbatim into a JSON header value.http::HeaderValue::from_bytesthen rejects the bytes and the whole invocation fails — tripping alarms on traffic that is otherwise harmless and well-formed at the HTTP layer.Test plan
cargo test --lib— 23 passed (was 21, +2 new)cargo clippy --lib --tests -- -D warningscleanInvalidHeaderValue, then restored it and confirmed it passes