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UTF-8 replacement rune bypasses attribute length truncation

Moderate
pellared published GHSA-p9f8-wvj8-2fg8 Sep 2, 2026

Package

gomod go.opentelemetry.io/otel/sdk/trace (Go)

Affected versions

>= v1.10.0, < v1.33.0

Patched versions

>= v1.33.0

Description

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.

Severity

Moderate

CVSS overall score

This score calculates overall vulnerability severity from 0 to 10 and is based on the Common Vulnerability Scoring System (CVSS).
/ 10

CVSS v4 base metrics

Exploitability Metrics
Attack Vector Local
Attack Complexity Low
Attack Requirements None
Privileges Required None
User interaction None
Vulnerable System Impact Metrics
Confidentiality None
Integrity None
Availability Low
Subsequent System Impact Metrics
Confidentiality None
Integrity None
Availability None

CVSS v4 base metrics

Exploitability Metrics
Attack Vector: This metric reflects the context by which vulnerability exploitation is possible. This metric value (and consequently the resulting severity) will be larger the more remote (logically, and physically) an attacker can be in order to exploit the vulnerable system. The assumption is that the number of potential attackers for a vulnerability that could be exploited from across a network is larger than the number of potential attackers that could exploit a vulnerability requiring physical access to a device, and therefore warrants a greater severity.
Attack Complexity: This metric captures measurable actions that must be taken by the attacker to actively evade or circumvent existing built-in security-enhancing conditions in order to obtain a working exploit. These are conditions whose primary purpose is to increase security and/or increase exploit engineering complexity. A vulnerability exploitable without a target-specific variable has a lower complexity than a vulnerability that would require non-trivial customization. This metric is meant to capture security mechanisms utilized by the vulnerable system.
Attack Requirements: This metric captures the prerequisite deployment and execution conditions or variables of the vulnerable system that enable the attack. These differ from security-enhancing techniques/technologies (ref Attack Complexity) as the primary purpose of these conditions is not to explicitly mitigate attacks, but rather, emerge naturally as a consequence of the deployment and execution of the vulnerable system.
Privileges Required: This metric describes the level of privileges an attacker must possess prior to successfully exploiting the vulnerability. The method by which the attacker obtains privileged credentials prior to the attack (e.g., free trial accounts), is outside the scope of this metric. Generally, self-service provisioned accounts do not constitute a privilege requirement if the attacker can grant themselves privileges as part of the attack.
User interaction: This metric captures the requirement for a human user, other than the attacker, to participate in the successful compromise of the vulnerable system. This metric determines whether the vulnerability can be exploited solely at the will of the attacker, or whether a separate user (or user-initiated process) must participate in some manner.
Vulnerable System Impact Metrics
Confidentiality: This metric measures the impact to the confidentiality of the information managed by the VULNERABLE SYSTEM due to a successfully exploited vulnerability. Confidentiality refers to limiting information access and disclosure to only authorized users, as well as preventing access by, or disclosure to, unauthorized ones.
Integrity: This metric measures the impact to integrity of a successfully exploited vulnerability. Integrity refers to the trustworthiness and veracity of information. Integrity of the VULNERABLE SYSTEM is impacted when an attacker makes unauthorized modification of system data. Integrity is also impacted when a system user can repudiate critical actions taken in the context of the system (e.g. due to insufficient logging).
Availability: This metric measures the impact to the availability of the VULNERABLE SYSTEM resulting from a successfully exploited vulnerability. While the Confidentiality and Integrity impact metrics apply to the loss of confidentiality or integrity of data (e.g., information, files) used by the system, this metric refers to the loss of availability of the impacted system itself, such as a networked service (e.g., web, database, email). Since availability refers to the accessibility of information resources, attacks that consume network bandwidth, processor cycles, or disk space all impact the availability of a system.
Subsequent System Impact Metrics
Confidentiality: This metric measures the impact to the confidentiality of the information managed by the SUBSEQUENT SYSTEM due to a successfully exploited vulnerability. Confidentiality refers to limiting information access and disclosure to only authorized users, as well as preventing access by, or disclosure to, unauthorized ones.
Integrity: This metric measures the impact to integrity of a successfully exploited vulnerability. Integrity refers to the trustworthiness and veracity of information. Integrity of the SUBSEQUENT SYSTEM is impacted when an attacker makes unauthorized modification of system data. Integrity is also impacted when a system user can repudiate critical actions taken in the context of the system (e.g. due to insufficient logging).
Availability: This metric measures the impact to the availability of the SUBSEQUENT SYSTEM resulting from a successfully exploited vulnerability. While the Confidentiality and Integrity impact metrics apply to the loss of confidentiality or integrity of data (e.g., information, files) used by the system, this metric refers to the loss of availability of the impacted system itself, such as a networked service (e.g., web, database, email). Since availability refers to the accessibility of information resources, attacks that consume network bandwidth, processor cycles, or disk space all impact the availability of a system.
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

CVE ID

CVE-2026-81869

Weaknesses

Improper Handling of Unicode Encoding

The product does not properly handle when an input contains Unicode encoding. Learn more on MITRE.

Uncontrolled Resource Consumption

The product does not properly control the allocation and maintenance of a limited resource. Learn more on MITRE.

Credits