OpenTelemetry-Go: UTF-8 replacement rune bypasses attribute length truncation

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

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:

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:

=== 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:

tar -xOf /path/to/validation-artifact.tar safe_truncate_bypass/runecheck_output.txt

Expected output:

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.