multi-value baggage: header extraction parses each header field-value independently and aggregates members across values. this allows an attacker to amplify cpu and allocations by sending many baggage: header lines, even when each individual value is within the 8192-byte per-value parse limit.
severity
HIGH (availability / remote request amplification)
relevant links
- repository: https://github.com/open-telemetry/opentelemetry-go - pinned callsite: https://github.com/open-telemetry/opentelemetry-go/blob/1ee4a4126dbdd1bc79e9fae072fa488beffac52a/propagation/baggage.go#L58
vulnerability details
pins: open-telemetry/opentelemetry-go@1ee4a4126dbdd1bc79e9fae072fa488beffac52a as-of: 2026-02-04 policy: direct (no program scope provided)
callsite: propagation/baggage.go:58 (extractMultiBaggage) attacker control: inbound HTTP request headers (many baggage field-values) → propagation.HeaderCarrier.Values("baggage") → repeated baggage.Parse + member aggregation
root cause
extractMultiBaggage iterates over all baggage header field-values and parses each one independently, then appends members into a shared slice. the 8192-byte parsing cap applies per header value, but the multi-value path repeats that work once per header line (bounded only by the server/proxy header byte limit).
impact
in a default net/http configuration (max header bytes 1mb), a single request with many baggage: header field-values can cause large per-request allocations and increased latency.
example from the attached PoC harness (darwin/arm64; 80 values; 40 requests):
- canonical: perreqallocbytes=10315458 and p95ms=7 - control: perreqallocbytes=133429 and p95ms=0
proof of concept
canonical:
bash mkdir -p poc unzip poc.zip -d poc cd poc make test
output (excerpt):
[CALLSITEHIT]: propagation/baggage.go:58 extractMultiBaggage [PROOFMARKER]: baggagemultivalueamplification p95ms=7 perreqallocbytes=10315458 perreqallocs=16165
control:
bash cd poc make control
control output (excerpt):
[NCMARKER]: baggagesinglevaluebaseline p95ms=0 perreqallocbytes=133429 perreqallocs=480
expected: multiple baggage header field-values should be semantically equivalent to a single comma-joined baggage value and should not multiply parsing/alloc work within the effective header byte budget. actual: multiple baggage header field-values trigger repeated parsing and member aggregation, causing high per-request allocations and increased latency even when each individual value is within 8192 bytes.
fix recommendation
avoid repeated parsing across multi-values by enforcing a global budget and/or normalizing multi-values into a single value before parsing. one mitigation approach is to treat multi-values as a single comma-joined string and cap total parsed bytes (for example 8192 bytes total).
fix accepted when: under the default PoC harness settings, canonical stays within 2x of control for perreqallocbytes and perreqallocs, and p95ms stays below 2ms.
poc.zip PRDESCRIPTION.md