CVE-2026-42191

The OTLP disk retry feature in OpenTelemetry.Exporter.OpenTelemetryProtocol silently fell back to Path.GetTempPath() when OTEL_DOTNET_EXPERIMENTAL_OTLP_RETRY=disk was set but OTEL_DOTNET_EXPERIMENTAL_OTLP_DISK_RETRY_DIRECTORY_PATH was not configured.

The exporter stored and loaded *.blob files under fixed, signal-named subdirectories (traces, metrics, logs) beneath that shared temporary root path.

On multi-user systems where the temporary directory is accessible to other local accounts, this exposed three attack surfaces:

• Blob injection (integrity): an attacker could write crafted *.blob files into the predictable path; the exporter picks them up on the next retry cycle and forwards them to the configured OTLP endpoint under the application's identity.
• Telemetry disclosure (confidentiality): an attacker reads *.blob files written by the application between export failures, recovering encoded telemetry payloads (spans, metric data points, log records).
• Resource exhaustion (availability): an attacker deposits numerous or oversized blob files, degrading retry-loop performance or consuming disk space.

Details

Preconditions

1. OTEL_DOTNET_EXPERIMENTAL_OTLP_RETRY is set to disk.
2. OTEL_DOTNET_EXPERIMENTAL_OTLP_DISK_RETRY_DIRECTORY_PATH is not set, causing the exporter to resolve the blob storage root using the System.IO.Path.GetTempPath() API.
3. A local attacker has read or write access to the process' temporary directory (e.g., /tmp on Linux, or %TEMP% on a multi-user Windows installation).

Exploit path

1. A target application starts with OTEL_DOTNET_EXPERIMENTAL_OTLP_RETRY=disk and no explicit blob directory. The exporter resolves the storage root to Path.GetTempPath(), producing paths such as %TEMP%\traces, %TEMP%\metrics, and %TEMP%\logs (or /tmp/traces etc. on Linux).
2. Injection scenario: before or during the application's retry window, an attacker writes crafted *.blob files into one of those signal subdirectories. On the next retry interval (by default every 60 seconds), OtlpExporterPersistentStorageTransmissionHandler scans the directory, loads the attacker-supplied blobs, and forwards them to the configured OTLP endpoint using the application's identity and transport credentials.
3. Disclosure scenario: the attacker reads *.blob files that the application wrote after a transient export failure, recovering the full serialized telemetry payloads (spans, metric data points, or log records in Protobuf encoding).
4. DoS scenario: the attacker deposits a large number of oversized blob files in the temporary subdirectories, causing the retry loop to consume excess CPU/IO processing them, potentially exhausting available disk space.

Mitigations

If an immediate upgrade to a patched version is not possible:

1. Avoid enabling disk retry in shared environments.
2. Configure a dedicated directory with strict ACL/ownership and least privilege.
3. Ensure the directory is not shared across tenants/users.
4. Monitor for unexpected *.blob files or abnormal retry backlog growth.

Resources

• #7106

CVE-2026-42191 has a CVSS score of 6.5 (Medium). The vector is requires local access, low privileges required, and no user interaction. A CVSS score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether this affects your application depends on whether the vulnerable code is present and reachable in your environment. A fixed version is available (1.15.3); upgrading removes the vulnerable code path.

Kodem intelligence

1. What is CVE-2026-42191? CVE-2026-42191 is a medium-severity security vulnerability in OpenTelemetry.Exporter.OpenTelemetryProtocol (nuget), affecting versions >= 1.8.0, <= 1.15.2. It is fixed in 1.15.3.
2. How severe is CVE-2026-42191? CVE-2026-42191 has a CVSS score of 6.5 (Medium). This score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether it represents real risk in your environment depends on whether the vulnerable code is present and reachable.
3. Which versions of OpenTelemetry.Exporter.OpenTelemetryProtocol are affected by CVE-2026-42191? OpenTelemetry.Exporter.OpenTelemetryProtocol (nuget) versions >= 1.8.0, <= 1.15.2 is affected.
4. Is there a fix for CVE-2026-42191? Yes. CVE-2026-42191 is fixed in 1.15.3. Upgrade to this version or later.
5. Is CVE-2026-42191 exploitable, and should I be worried? Whether CVE-2026-42191 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
6. What actually determines whether CVE-2026-42191 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
7. How do I fix CVE-2026-42191? Upgrade OpenTelemetry.Exporter.OpenTelemetryProtocol to 1.15.3 or later.