DIGGINGBEAGLE

Case · DiggingBeagle record

Hermes Agent wrote anonymous login input to disk before rejecting it

An unauthenticated client could submit an oversized provider string to Hermes Agent password login. Even when the login returned 404, the rejected value was appended to a persistent audit log, allowing repeated requests to consume server disk space. Tenable published this distinct vulnerability October 9 and lists 0.21.6 as fixed.

Agent authority

Assessment profile

Assessment method
Setting
research demonstration
Exploitation
demonstrated
Evidence
Grade B
Remediation
patch available
AI role
AGAINST AI
Basis and provenance

Tenable documented a controlled persistent 100 MiB log-write test, not a production outage. Maintainer regression tests document 128/256-character bounds merged before v0.21.6 release. Evidence is strong for the mechanism and patch presence but insufficient for field exposure, real-world exploitation or quantified impact.

Assessed Oct 11, 2026 using diggingbeagle.assessment/1.

Evidence basis: CLM-HERMES-DISK-LOGGING · CLM-HERMES-DISK-TEST · CLM-HERMES-DISK-EXACT-FIX · Tenable TRA-2026-64: Hermes Agent Pre-Authentication Disk Consumption · Hermes Agent pull request 133369: provider and audit value bounds · NousResearch Hermes Agent v0.21.6 release and security fixes

At a glance

Mechanism and trust boundary

  1. 01

    An anonymous client supplies an oversized provider value

    A public login endpoint accepts a string with no configured maximum.

    Boundary: unauthenticated HTTP request / login parser

  2. 02

    Login rejects the value but audit logging persists it

    The rejected provider is serialized without value-size limits into dashboard-auth.log.

    Boundary: authentication failure / durable audit log

  3. 03

    Repeated input consumes persistent storage

    The researcher observed disk consumption scaling with payload size; repeated calls create availability risk.

    Boundary: public input / host storage budget

Timeline

  1. Sep 3, 2026
    Event type unspecified

    Researcher starts disclosure

    Tenable reports initial contact.

  2. Sep 3, 2026
    notification

    Vendor notified

  3. Oct 5, 2026
    Event type unspecified

    Fix noted

    Tenable records a fix in PR #133369.

  4. Oct 5, 2026
    patch

    Fix date recorded

  5. Oct 9, 2026
    Event type unspecified

    TRA-2026-64 published

    Pre-authentication disk-consumption vulnerability is publicly disclosed.

  6. Oct 9, 2026
    disclosure

    Public disclosure

Claims & evidence

CLM-HERMES-DISK-TESTTenable tested a 100 MiB unauthenticated JSON request that returned 404 but reduced reported free disk space by about 100 MiB, with the attacker-controlled value retained in the audit log.supported

Basis: reported finding

Link to claim
CLM-HERMES-DISK-PATCHTenable identifies Hermes Agent versions before 0.21.6 as affected, and says the log-consumption issue was fixed on October 5 before the October 9 advisory.supported

Basis: reported finding

Link to claim
CLM-HERMES-DISK-LOGGINGThe public Hermes Agent password-login handler copied an unbounded client-provided provider string into its persistent authentication audit log before returning an unknown-provider error.supported

Basis: reported finding

Link to claim
CLM-HERMES-DISK-EXACT-FIXNousResearch merged PR #133369 on October 5: provider IDs longer than 128 characters are rejected before password-login handling, while serialized audit strings are separately capped at 256 characters, including nested values. Its regression checks cover both unknown-provider and rate-limited authentication paths.supported

Basis: reported finding

Link to claim

Implications

Authentication rejection is not sufficient if the rejection path persists unbounded attacker-controlled text.

Controls and mitigations

  • Upgrade Hermes Agent to 0.21.6 or later.
  • Enforce pre-parser request-body and field-size limits on public routes; apply limits at the proxy as well as the app.
  • Limit audit-log field values, cap disk allocation and alert on rapidly growing log volume.

Unknowns and contradictions

  • The report does not demonstrate disk exhaustion against production deployments.
  • The number of exposed Hermes Agent dashboards is unknown.

Sources and citation

EvidenceTenable TRA-2026-64: Hermes Agent Pre-Authentication Disk ConsumptionEvidenceHermes Agent pull request 133369: provider and audit value boundsEvidenceNousResearch Hermes Agent v0.21.6 release and security fixes

Material revision history

  1. Oct 10, 2026 · Published version · first publication · revision 120
Record metadata

Canonical ID: CASE-2026-HERMES-AUTH-AUDIT-DISK-EXHAUSTION · revision 120

Legacy date (semantics unspecified): 2026-10-09. This value is not treated as an occurrence or disclosure date.