{"id":"CVE-2026-81869","aliases":["GHSA-p9f8-wvj8-2fg8","GO-2026-6616"],"url":"https://o3.security/vulnerability/CVE-2026-81869","summary":"OpenTelemetry-Go: UTF-8 replacement rune bypasses attribute length truncation","details":"### Summary\n\nThe 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.\n\nIntroduced in commit 49a6536\n\n### Details\n\nString 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`.\n\n`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`.\n\nFor 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.\n\n### PoC\n\n[validation-artifact.zip](https://github.com/user-attachments/files/27494267/validation-artifact.zip)\n\n\nThe 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`.\n\nReproduction configuration:\n- Repository: `pellared/opentelemetry-go`\n- Commit: `49a6536` from September 12, 2022\n- Package/module path: `sdk/trace` under the `sdk` module\n- Attribute value length limit used by the PoC: `limit := 5`\n- Dependencies must be available through the network or a local module cache/vendor directory.\n\nCommands:\n```sh\ncd /path/to/opentelemetry-go\ngit checkout 49a6536\ntar -xOf /path/to/validation-artifact.tar safe_truncate_bypass/safe_truncate_poc_test.go > sdk/trace/safe_truncate_poc_test.go\ncd sdk\ngo test ./trace -run TestSafeTruncateBypass -count=1 -v\n```\n\nExpected vulnerable output includes a failing test showing that the returned value exceeds the configured limit:\n```text\n=== RUN   TestSafeTruncateBypass\n    safe_truncate_poc_test.go:14: input_len=27 got_len=27 input=\"AAAA�BBBBBBBBBBBBBBBBBBBB\" got=\"AAAA�BBBBBBBBBBBBBBBBBBBB\"\n    safe_truncate_poc_test.go:16: bypass: got_len 27 > limit 5\n--- FAIL: TestSafeTruncateBypass\n```\n\nThe artifact also records the standalone UTF-8 behavior needed for the bypass:\n```sh\ntar -xOf /path/to/validation-artifact.tar safe_truncate_bypass/runecheck_output.txt\n```\n\nExpected output:\n```text\nr=U+FFFD size=3 runeError=true\n```\n\n### Impact\n\nThis 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.","published":"2026-09-16T20:41:15.806Z","modified":"2026-10-21T03:00:00.667053497Z","cvss":null,"epss":null,"cisaKev":null,"exploitsKnown":null,"affectedPackages":[{"ecosystem":"Go","name":"go.opentelemetry.io/otel/sdk","fixedVersion":"1.33.0"}],"fix":{"url":"https://github.com/open-telemetry/opentelemetry-go/commit/e016a78c9f5b24a1c2beeaad47686c2f2213f49a","label":"open-telemetry/opentelemetry-go@e016a78"},"references":[{"type":"WEB","url":"https://github.com/open-telemetry/opentelemetry-go/releases/tag/sdk/v1.33.0"},{"type":"ADVISORY","url":"https://github.com/CVEProject/cvelistV5/tree/main/cves/2026/81xxx/CVE-2026-81869.json"},{"type":"ADVISORY","url":"https://github.com/open-telemetry/opentelemetry-go/security/advisories/GHSA-p9f8-wvj8-2fg8"},{"type":"ADVISORY","url":"https://nvd.nist.gov/vuln/detail/CVE-2026-81869"},{"type":"REPORT","url":"https://github.com/open-telemetry/opentelemetry-go/issues/5996"},{"type":"FIX","url":"https://github.com/open-telemetry/opentelemetry-go/commit/e016a78c9f5b24a1c2beeaad47686c2f2213f49a"},{"type":"FIX","url":"https://github.com/open-telemetry/opentelemetry-go/pull/5997"},{"type":"PACKAGE","url":"https://github.com/open-telemetry/opentelemetry-go"}],"provenance":{"sources":["OSV.dev","FIRST.org (EPSS)"],"lastVerified":"2026-10-21T03:00:00.667053497Z"}}