Description

Coder: PKCS#7 signature bypass in Azure instance identity allows unauthenticated agent token theft

Summary

azureidentity.Validate() verifies that the PKCS#7 signer certificate chains to a trusted Azure CA but never verifies the PKCS#7 signature itself. An attacker can embed a legitimate Azure certificate alongside arbitrary content e.g. {"vmId":"<target>"} and the forged vmId will be accepted returning the victim workspace agent's session token.

No authentication is required. The attacker only needs to know a target VM's vmId which is a UUIDv4.

that's a practical limitation which would typically require prior access to be exploited

Root Cause

In unpatched Coder releases the signature over the PKCS#7 content is not validated - only the signing certificate is checked.

Impact

An attacker on any Azure VM or with access to a publicly available Azure IMDS certificate from CT logs can:

Steal an agent session token by sending a forged PKCS#7 envelope to POST /api/v2/workspaceagents/azure-instance-identity which is unauthenticated.

With the stolen token access:

Git SSH private key via GET /workspaceagents/me/gitsshkey: push to repositories and impersonate the workspace owner.

OAuth access tokens via GET /workspaceagents/me/external-auth: GitHub, GitLab, and Bitbucket tokens in plaintext.

Workspace secrets via the agent manifest: environment variables, file paths, and API keys.

Attack Path Diagram

Affected Versions

All versions of Coder v2 are affected.

Patches

Fixed in #25286

The fix was backported to all supported release lines:

Workarounds

If unable to patch we recommend immediately reconfiguring any Azure templates to use token authentication rather than azure-instance-identity until the patch is released and you are fully upgraded.

Modify the coder_agent.auth value to be token.

Add CODER_AGENT_TOKEN=${coder_agent.main.token} to the set of environment variables for the Coder Workspace Agent initialization script.

Recognition

We'd like to thank Ben Tran of calif.io and Anthropic’s Security Team (ANT-2026-22445) for independently disclosing this issue!

Mitigation

Aliases

References