CVE-2026-29181: OpenTelemetry-Go multi-value `baggage` header extraction causes excessive allocations (remote dos amplification)

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