Because the downstream service logs whatever identity arrives, not the original user or intent behind the request. If the token was not issued for the receiving service, the log inherits a misleading identity and the system may act with broader authority than intended. That weakens per-hop authorization and makes later investigation depend on inference instead of evidence.
Why the Token Becomes a Security Boundary Problem
A bearer token is not just a transport artifact, it is the authority the next service uses to decide whether the call should be accepted. When an agent forwards that token unchanged, the receiving system cannot tell whether the request was meant for it, whether the original user intended that action, or whether the token’s original scope still fits the new hop. That is how simple pass-through turns into delegated authority without fresh authorization.
The core failure is workload identity or user identity being reused outside the boundary where it was issued. In practice, that weakens hop-by-hop control and can let one agent action inherit more reach than the workflow designer intended. It is especially risky in workflows that cross systems, because the token can become a standing shortcut to capabilities that should have been rechecked at each step.
Forwarding also collapses trust boundaries. If the downstream service accepts the upstream token at face value, then the agent is effectively acting as a conduit for authority rather than a controlled intermediary. That is a problem whether the token represents a human user, a service account, or another automation path, because the receiving service loses the chance to apply its own policy to the actual operation being requested.
Why the Audit Trail Stops Being Trustworthy
Audit systems record what identity arrived, not the intent or provenance behind a forwarded token. If the same bearer credential is reused across hops, the logs can make each downstream action look like it came directly from the original principal, even when the action was initiated, transformed, or expanded by an agent in the middle. That creates misleading attribution and makes later investigation depend on reconstruction rather than direct evidence.
This is where the distinction between evidence and inference matters. A clean audit trail should show who initiated the action, which component relayed it, what authorization was evaluated, and what scope was actually used. Unchanged forwarding hides that structure. The result is a log that may be technically correct about the token presented, but still incomplete about the decision path that led to the action.
For agent workflows, that gap is not cosmetic. It affects incident response, accountability, and control validation. If a downstream action is improper, teams need to know whether the issue was overbroad token scope, improper token reuse, missing audience restriction, or an agent making an unauthorized decision while still holding valid credentials. A log that only shows the final bearer identity cannot answer those questions on its own.
Standards & Framework Alignment
This section maps relevant standards and security frameworks to the operational risks and controls described in this guidance.
OWASP Agentic AI Top 10 address the attack and risk surface, while NIST Zero Trust (SP 800-207), NIST CSF 2.0, CIS Controls v8 and NIST SP 800-63 set the governance and control requirements practitioners need to meet.
| Framework | Control / Reference | Relevance |
|---|---|---|
| NIST Zero Trust (SP 800-207) | SA-5 — Agent Verification and Authorization | Per-hop trust and audience checks are central to forwarding tokens safely. |
| Recommendation — Enforce explicit trust decisions before an agent can reuse or relay credentials. | ||
| NIST CSF 2.0 | PR.AA-01 — Identity Management, Authentication, and Access Control | Forwarded bearer tokens affect identity assurance and access decisions at each hop. |
| Recommendation — Bind each service call to the intended identity and access context. | ||
| CIS Controls v8 | 6.3 — Access Rights Management | Overbroad token reuse creates excessive access and weakens control over privilege. |
| Recommendation — Limit and review token scope so downstream access matches the intended action. | ||
| OWASP Agentic AI Top 10 | A2 — Identity and Privilege Misuse | Unchanged token forwarding is a direct example of agent authority being reused too broadly. |
| Recommendation — Require agents to obtain narrowly scoped, audience-bound credentials for each action. | ||
| NIST SP 800-63 | 5.2 — Authentication Protocols | Audience restriction and replay-resistant protocol design help prevent token misuse across hops. |
| Recommendation — Use protocol bindings that keep bearer credentials valid only for the intended relying party. | ||
Practitioner Guidance
What to verify: Confirm that each hop has its own audience-bound authorization decision and that the downstream service can distinguish relay from origination. If a workflow can make a materially different decision with the same token, the control is too coarse for audit and too broad for security.
Common mistake: Treating successful authentication as proof that the action was properly delegated. In agent workflows, a valid token only proves possession, not that the current service was the intended recipient or that the action still fits the original consent.
What good looks like: Each service can log the initiating principal, the intermediary agent, the target audience, and the scope actually consumed. That gives investigators a direct record of decision flow instead of forcing them to infer intent from one reused credential.
Practitioner takeaway: The goal is not merely to stop token leakage, it is to preserve a verifiable chain of authority per hop so that both authorization and attribution remain accurate after the agent acts.
Related resources from NHI Mgmt Group
- How should security teams prevent bearer token replay in agent workflows?
- Why do AI-generated MCP tools and agent workflows create a different security risk than ordinary application code?
- Why do default GitHub Action token permissions create security risk in CI/CD workflows?
- Why do hybrid identity environments create more audit and security risk than single-directory setups?
Deepen Your Knowledge
Reviewed and updated by the NHIMG editorial team on September 20, 2026.
NHI Mgmt Group — the #1 independent authority on Non-Human Identity, IAM, and Agentic AI security. nhimg.org
