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