CVE-2026-81869

### Summary The OpenTelemetry Go SDK trace package can fail to enforce `AttributeValueLengthLimit` for string attributes containing the valid Unicode replacement character U+FFFD. An oversized attacker-controlled attribute value that includes U+FFFD is returned untruncated, bypassing the configured memory/DoS protection and allowing increased per-span memory usage. The finding is low severity because it requires a deployment with attribute value length limits enabled and attacker-controlled data being recorded into span attributes. Introduced in commit 49a6536 ### Details String and string-slice span attributes are truncated through `safeTruncate` when `AttributeValueLengthLimit` is non-negative. The finding evidence identifies this enforcement path in `sdk/trace/span.go:303-331`, with string attributes passed to `safeTruncate` at `sdk/trace/span.go:309-310` and string-slice entries passed to `safeTruncate` in the loop beginning at `sdk/trace/span.go:312`. `safeTruncate` first calls `safeTruncateValidUTF8`; if that returns `ok=false`, it calls `strings.ToValidUTF8(input, "")` and retries. The relevant code is identified in `sdk/trace/span.go:337-355`. `safeTruncateValidUTF8` treats any `utf8.RuneError` from `utf8.DecodeRuneInString` as invalid UTF-8 and immediately returns the original input with `ok=false`. However, Go also returns `utf8.RuneError` for a valid encoded U+FFFD rune. The validation artifact confirms this behavior with output `r=U+FFFD size=3 runeError=true`. For an input such as `"AAAA" + U+FFFD + strings.Repeat("B", 20)` and a limit of 5, the first truncation attempt sees U+FFFD as `utf8.RuneError` and returns the full input with `ok=false`. `strings.ToValidUTF8` does not remove the valid U+FFFD rune, so the second attempt returns the same full input. As a result, the span attribute value remains 27 bytes long even though the configured limit is 5. ### PoC [validation-artifact.zip](https://github.com/user-attachments/files/27494267/validation-artifact.zip) The validation artifact contains a package-level Go test at `validation-artifact.tar:safe_truncate_bypass/safe_truncate_poc_test.go` and supporting output at `validation-artifact.tar:safe_truncate_bypass/runecheck_output.txt`. Reproduction configuration: - Repository: `pellared/opentelemetry-go` - Commit: `49a6536` from September 12, 2022 - Package/module path: `sdk/trace` under the `sdk` module - Attribute value length limit used by the PoC: `limit := 5` - Dependencies must be available through the network or a local module cache/vendor directory. Commands: ```sh cd /path/to/opentelemetry-go git checkout 49a6536 tar -xOf /path/to/validation-artifact.tar safe_truncate_bypass/safe_truncate_poc_test.go > sdk/trace/safe_truncate_poc_test.go cd sdk go test ./trace -run TestSafeTruncateBypass -count=1 -v ``` Expected vulnerable output includes a failing test showing that the returned value exceeds the configured limit: ```text === RUN TestSafeTruncateBypass safe_truncate_poc_test.go:14: input_len=27 got_len=27 input="AAAA�BBBBBBBBBBBBBBBBBBBB" got="AAAA�BBBBBBBBBBBBBBBBBBBB" safe_truncate_poc_test.go:16: bypass: got_len 27 > limit 5 --- FAIL: TestSafeTruncateBypass ``` The artifact also records the standalone UTF-8 behavior needed for the bypass: ```sh tar -xOf /path/to/validation-artifact.tar safe_truncate_bypass/runecheck_output.txt ``` Expected output: ```text r=U+FFFD size=3 runeError=true ``` ### Impact This is a Unicode handling and resource-limit bypass that weakens span attribute memory controls. Applications that enable `AttributeValueLengthLimit` to bound memory usage can still store oversized attacker-controlled attribute values if those values contain U+FFFD. The practical impact is increased memory use and reduced denial-of-service protection in the instrumented process; the finding does not show confidentiality or integrity impact.

OpenTelemetry-Go: UTF-8 replacement rune bypasses attribute length truncation in go.opentelemetry.io/otel/sdk