CVE-2026-73196

Executive Summary

CVE-2026-73196 is a denial-of-service (DoS) vulnerability in FreeIPA, the open-source identity management solution. An authenticated low-privileged user can submit an oversized One-Time Password (OTP) key value, which, when processed by the IPA service, is decoded and re-encoded without proper size constraints, leading to excessive CPU and memory consumption. This resource exhaustion degrades the availability of the IPA service, impacting authentication and identity management for all users relying on the FreeIPA infrastructure. The vulnerability is currently unpatched, and no public exploit or PoC has been confirmed at the time of writing.

Technical Analysis

The vulnerability exists in FreeIPA's handling of One-Time Password (OTP) key values. FreeIPA integrates with the OTP mechanism for two-factor authentication (2FA), typically used to enroll authentication tokens or validate user login attempts. The flaw arises from a lack of proper input size validation on the OTP key field.

Vulnerability Mechanism:

1. Input Handling: A low-privilege authenticated user submits an OTP key value via a supported API or web interface. The NVD description indicates the submission method allows the user to pass an oversized key directly to the system.
2. Decode/Re-encode Process: Following submission, the service decodes the provided key (e.g., from a base-64 or similar encoding used for OTP key material) and subsequently re-encodes it. This process is performed without enforcing a maximum length limit.
3. Resource Exhaustion: With an input massively exceeding expected size (e.g., multi-megabyte or gigabyte key value), the decode/re-encode loop consumes significant CPU cycles and allocates memory proportional to the input size. Since the service does not terminate or truncate the operation, the process can spike CPU utilization and exhaust available memory.
4. Availability Impact: The resource consumption degrades the overall IPA service, causing response slowdowns or complete service unavailability for legitimate authentication requests and directory operations.

Attack Surface: The attack surface is an authenticated interface — the attacker must hold a valid account, albeit low-privilege. This reduces the attack's external exposure but makes insider threats or compromised low-privileged accounts a vector. The service is network-accessible (AV:N), meaning the attacker does not need local access.

Code-Level Context: The NVD description does not specify exact function names, code paths, or parameter names. Therefore, at the code level, the flaw is a classic missing upper-bound check on input length. Similar vulnerabilities occur when developers rely on downstream parsing to enforce size limits rather than explicitly validating input at the API boundary. The decode/re-encode pattern suggests the application expects a specific compact format but does not guard against a maliciously crafted oversized payload that still passes format validation.

CWE-770 (Allocation of Resources Without Limits or Throttling): This CWE categorizes the flaw correctly — the application fails to constrain resource allocation based on attacker-controlled input, leading to DoS.

Exploitation Scenarios

Note: No public PoC or exploit code is confirmed for this CVE. The attack chain below is inferred from the vulnerability class and NVD description. Specific API endpoints, request formats, and code internals are unknown.

1. Reconnaissance / Target Identification

• An attacker who already possesses low-privilege credentials performs light reconnaissance to identify the OTP enrollment or validation endpoint. This may involve:
  • Reviewing FreeIPA documentation or existing client libraries to locate OTP submission functionality.
  • Observing network traffic from legitimate users performing OTP enrollment (e.g., via ipa otptoken-add command or web UI) to identify the request structure.
• The attacker confirms the target service is FreeIPA by footprinting the LDAP, Kerberos, or web endpoints (e.g., HTTPS on 443, LDAP on 389/636).

2. Triggering the Flaw

• The attacker sends a request to the OTP key submission endpoint with an oversized key value. Given the OTP key is normally a short string (e.g., ≤ 256 characters for base-32 encoded secrets), the attacker submits an input several orders of magnitude larger (e.g., 10 MB to 100 MB).
• Because the flaw lies in the decode/re-encode stage, the attacker must ensure the payload passes initial format validation (e.g., is valid base-64 or base-32). A repeated character pattern (e.g., thousands of "A" characters) would satisfy format checks while remaining oversized.

3. Payload Execution Mechanism

• The request is processed by the FreeIPA service worker. The service begins decoding the OTP key.
• During decode and re-encode, the service allocates buffers proportional to the input length and performs repeated CPU-intensive operations (base conversion, checksum computation, etc.).
• The absence of an input size limit means the operation does not reduce the payload to a manageable token size; the full oversized input is processed.

4. Post-Exploitation Impact

• The processing thread(s) become saturated, consuming high CPU and memory.
• If the attacker sends multiple concurrent oversized requests, the server may exhaust memory (OOM kill) or become completely unresponsive to legitimate authentication requests.
• The impact is a denial of service affecting the availability of the IPA service. This disrupts:
  • User authentication (Kerberos, LDAP binds).
  • Administrative operations (adding users/groups, policy updates).
  • Replication between IPA servers (if applicable).
• The attack can be repeated anytime since no persistent state is modified; the attacker simply re-sends the request.

Impact Assessment

CVSS Score Analysis: The CVSS v3.1 vector CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:L yields a base score of 4.3 (Medium).

• Attack Vector: Network (AV:N) — The vulnerability is exploitable remotely over the network.
• Attack Complexity: Low (AC:L) — No special conditions or race conditions are required; sending the malformed request is sufficient.
• Privileges Required: Low (PR:L) — The attacker needs an authenticated account. This rules out unauthenticated attackers.
• User Interaction: None (UI:N) — No victim interaction is required.
• Scope: Unchanged (S:U) — The impact is confined to the FreeIPA service, not crossing a security boundary.
• Impact: Low Availability (A:L) — The service is degraded but not necessarily fully compromised in a single request; however, repeated requests can lead to stronger impact. No confidentiality or integrity impact (C:N, I:N).

Business Impact:

• Authentication Outage: FreeIPA is the identity broker for Linux environments. An outage prevents users from logging into systems, accessing VPNs, or using Kerberos-authenticated services. This can lead to productivity loss and potentially total environment lockdown.
• Administrative Disruption: Admins cannot manage users, groups, or policies, hampering incident response.
• Replication Impact: In multi-site deployments, If the master is impacted, replicas may serve stale data or fail to propagate changes.

Affected Systems: All FreeIPA deployments using OTP functionality (common in enterprise environments with 2FA). The specific version range is unknown due to lack of vendor advisory; however, all versions implementing OTP without size validation are likely affected.

Blast Radius: The attack requires a low-privilege account, limiting external exposure. However, in environments where FreeIPA is used for SSH, VPN, and web SSO, the availability impact cascades to all downstream services. The blast radius is therefore the entire identity ecosystem.

Detection Guidance

Note: Since no public exploit code or specific indicators exist, the detection rules below focus on abnormal OTP key sizes, unusual request patterns, and resource exhaustion signals.

Sigma Rule

title: FreeIPA OTP Oversized Key DoS Attempt
id: 5f2f0a1b-7c3d-4e5f-9a8b-2c1d0e9f8a7b
status: experimental
description: Detects requests to FreeIPA OTP endpoints with abnormally large key values, indicating a possible CVE-2026-73196 exploitation attempt.
author: vartersit.com Threat Intel
date: 2026/06/01
logsource:
  category: application
  product: freeipa
  service: http
detection:
  selection_otp:
    cs-method:
      - 'POST'
      - 'PUT'
    cs-uri-query|contains:
      - 'otp'
      - 'token'
    cs-user-agent|startswith: 'Python'
  selection_large_payload:
    cs-bytes|gte: 10000000  # 10 MB
    cs-content-length|gte: 10000000
  condition: selection_otp and selection_large_payload
falsepositives:
  - Legitimate bulk OTP enrollment scripts (rare, expected to be below 10MB per request)
  - Misconfigured client pushing large base64 images to wrong endpoint (unlikely)
level: medium
tags:
  - attack.t1499
  - cve.2026.73196

YARA Rule

rule FreeIPA_OTP_Oversized_Key_DoS
{
    meta:
        author = "vartersit.com Threat Intel"
        description = "Detects FreeIPA OTP requests containing oversized base64/base32 key material"
        cve = "CVE-2026-73196"
        date = "2026-06-01"
        severity = "medium"
    strings:
        $otp_param1 = "otptoken" nocase
        $otp_param2 = "otp" nocase
        $base64_alphabet = /[A-Za-z0-9+\/=]{2000,}/  // large contiguous base64 blob
        $base32_alphabet = /[A-Z2-7=]{2000,}/        // large contiguous base32 blob
    condition:
        any of ($otp_param*) and
        (any of ($base64_alphabet) or any of ($base32_alphabet))
}

Suricata IDS Rule

alert http any any -> $HOME_NET any (msg:"FreeIPA OTP Oversized Key DoS CVE-2026-73196"; flow:established,to_server; content:"POST"; http_method; http.uri; content:"otp"; nocase; pcre:"/.*(?:otptoken|otp).*$/i"; http.request_body; content:"Content-Length|3a 20|"; http_header; pcre:"/^Content-Length:\s*([1-9][0-9]{7,})/m"; threshold:type both, track by_src, count 5, seconds 60; classtype:attempted-dos; sid:202673196; rev:1;)

Elastic Detection

// EQL rule to detect oversized HTTP bodies to FreeIPA OTP endpoints
sequence by source.ip with maxspan=5m
  [ network where event.category == "network" and network.protocol == "http" and
    http.request.method in ("POST", "PUT") and
    http.request.uri contains "otp" and
    http.request.bytes > 10000000 ]
  [ event where event.category == "process" and
    process.name == "httpd" and
    // process CPU utilization spike
    process.cpu.total > 90 ]

Splunk SPL Query

index=proxy OR index=web earliest=-15m
| where like(uri, "%otp%")
| eval bytes = strftime(_time, "%s")
| stats sum(bytes) as total_bytes by src_ip, uri
| where total_bytes > 10000000
| table _time, src_ip, uri, total_bytes
| sort - total_bytes
| head 20

Wazuh OSSEC Rule

<group name="freeipa-dos,">
  <rule id="100331" level="10">
    <decoded_as>json</decoded_as>
    <field name="uri">\.*otp\.*</field>
    <field name="method">POST|PUT</field>
    <field name="bytes">>10000000</field>
    <description>FreeIPA request with oversized OTP key - possible CVE-2026-73196 exploitation</description>
    <group>freeipa_dos,</group>
    <mitre>
      <id>T1499</id>
    </mitre>
  </rule>
</group>

Mitigation & Patch Guidance

Patch Confidence: 0/4 — UNPATCHED

As of this report, no vendor patch is available. The following measures should be implemented to reduce exposure:

1. Immediate Mitigations (without patch)

• Rate Limiting: Implement rate limiting on OTP endpoints at the reverse proxy or load balancer level to prevent rapid submission of oversized requests. Restrict maximum body size (e.g., reject requests > 1 MB) at the proxy layer before they reach the application.
• Input Validation (if applicable): If custom FreeIPA plugins or middleware are used, enforce a maximum OTP key length at the API boundary.
• Access Restriction: Restrict access to OTP enrollment endpoints to trusted IP ranges or network segments, reducing the pool of potential attackers.
• Monitor Resource Usage: Deploy monitoring on the IPA server for CPU, memory, and per-request processing time. Set alerts for sustained high utilization from a single source.

2. Patch / Upgrade Steps

• Monitor Vendor Advisory: Track FreeIPA security announcements (freeipa.org, Red Hat errata for RHEL) for a patched release.
• Apply Vendor Patch: Once available, apply the updated package(s) to all IPA masters and replicas.
• Version Pinning: Until patched, do not introduce new IPA versions in the production environment that have not been tested.

3. Long-Term Hardening

• Architectural Change: Evaluate whether OTP validation can be moved to a separate service that can be restarted independently, limiting DoS impact on core IPA operations.
• Horizontal Scaling: Deploy additional IPA replicas to absorb the impact of a DoS attack on a single node; configure failover.
• Web Application Firewall (WAF): Install a WAF in front of the IPA web interface to inspect request bodies and block oversized payloads to otp-related resources.
• Authentication Policy: Enforce strong password policies and monitor for account compromise of low-privilege accounts, as these are the attack prerequisite.

References

1. NVD Entry for CVE-2026-73196: https://nvd.nist.gov/vuln/detail/CVE-2026-73196 (Official CVE record, CVSS vector)
2. FreeIPA Project: https://www.freeipa.org — Official project page for updates and security advisories.
3. CWE-770 Details: https://cwe.mitre.org/data/definitions/770.html — Allocation of Resources Without Limits or Throttling.
4. FreeIPA Security Advisory Page: https://www.freeipa.org/page/Releases — Monitor for future patch releases.

Threat Actor Activity Note: No known threat actor campaigns have been associated with CVE-2026-73196 as of the writing date. The lack of public PoC and the requirement for low-privilege authentication reduce the likelihood of mass exploitation; however, targeted attacks by insiders or persistent attackers with harvested credentials remain a realistic threat vector. The exploitation code for this vulnerability is expected to be trivial to write once the exact endpoint is identified, so organizations should prioritize hardening even without a public PoC.