---
id: "CVE-2026-81869"
kind: "vulnerability"
aliases: ["CVE-2026-81869", "GHSA-p9f8-wvj8-2fg8", "GO-2026-6616"]
title: "OpenTelemetry-Go: UTF-8 replacement rune bypasses attribute length truncation in go.opentelemetry.io/otel/sdk"
severity: "medium"
cvss: 5.1
epss: 0.0018
exploited: false
status: "open"
category: "package"
products: [{"name": "go.opentelemetry.io/otel/sdk", "fixed": ["1.33.0"], "affected": ">= 1.10.0, < 1.33.0", "ecosystem": "go"}]
ecosystems: ["go"]
tags: ["go", "observability", "opentelemetry", "package", "self-hosted"]
not_affected_if: ["attributes-not-attacker-controlled", "version-1.33.0-or-later"]
published_at: "2026-09-16T21:17:22+00:00"
first_seen_at: "2026-10-23T19:13:08+00:00"
updated_at: "2026-10-23T21:49:04+00:00"
sources: [{"source": "govuln", "url": "https://pkg.go.dev/vuln/GO-2026-6616", "first_seen_at": "2026-10-23T19:13:08+00:00"}, {"source": "epss", "url": "https://api.first.org/data/v1/epss?cve=CVE-2026-81869", "first_seen_at": "2026-10-23T19:13:43+00:00"}, {"source": "nvd", "url": "https://nvd.nist.gov/vuln/detail/CVE-2026-81869", "first_seen_at": "2026-10-23T19:35:23+00:00"}]
---

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

OpenTelemetry-Go's SDK fails to enforce the attribute length limit for string attributes that contain the Unicode replacement character U+FFFD, so attacker-controlled span attributes can exceed the configured size. An attacker who controls span attribute content can inflate per-span memory use in the instrumented process, weakening denial-of-service protection.

_Summary, check and fix written by a model from the sources; labelled as such._

## Affected

- go:go.opentelemetry.io/otel/sdk affected >= 1.10.0, < 1.33.0 fixed 1.33.0

## How to check

Run `go list -m go.opentelemetry.io/otel/sdk` in the module and check whether the version is >= 1.10.0 and < 1.33.0.

## Fix

Upgrade go.opentelemetry.io/otel/sdk to 1.33.0 or later.

## Timeline

- 2026-10-23T19:13:08+00:00 govuln https://pkg.go.dev/vuln/GO-2026-6616
- 2026-10-23T19:13:43+00:00 epss https://api.first.org/data/v1/epss?cve=CVE-2026-81869
- 2026-10-23T19:35:23+00:00 nvd https://nvd.nist.gov/vuln/detail/CVE-2026-81869

## From the sources

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.

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_Free to read at a human pace. An API with keys and an MCP server are in preparation; early access: hello@infrastack.ai_