CVE-2026-35098: KTM System e-BOK Auth Bypass Vulnerability

CVE-2026-35098 Overview

CVE-2026-35098 affects the KTM System e-BOK (Electronic Customer Service Office) web application. The application does not implement any limit or timeout on consecutive login attempts. Attackers can perform unlimited authentication requests against user accounts. This absence of rate-limiting enables efficient brute-force attacks against the login endpoint. The issue is classified under [CWE-307] Improper Restriction of Excessive Authentication Attempts. The severity increases when chained with CVE-2026-35097, which restricts passwords to a six-digit numeric format. A six-digit numeric password space contains only one million combinations, making full enumeration practical within a short timeframe. KTM System addressed the flaw in a patch published in June 2026.

Critical Impact
Attackers can brute-force user credentials without throttling, gaining unauthorized access to customer accounts on the e-BOK portal.

Affected Products

KTM System e-BOK (Internetowe Biuro Obsługi Klienta)
Versions prior to the June 2026 security patch
Deployments accessible over the public network

Discovery Timeline

2026-06-30 - CVE-2026-35098 published to NVD
2026-06-30 - Last updated in NVD database

Technical Details for CVE-2026-35098

Vulnerability Analysis

The vulnerability stems from missing authentication throttling controls in the e-BOK login flow. The application accepts an unbounded number of failed authentication attempts from the same source. No account lockout, IP-based rate limit, exponential backoff, or CAPTCHA challenge intervenes between attempts. An attacker with network access to the portal can submit authentication requests as fast as the server responds.

The defect becomes more severe in combination with CVE-2026-35097. That companion flaw enforces a six-digit numeric password policy on the same platform. The reduced key space allows an attacker to enumerate every possible password for a targeted account. On a responsive endpoint, full enumeration completes within hours rather than years.

Account compromise on the e-BOK portal exposes billing records, personal data, and customer service functions tied to the affected user.

Root Cause

The root cause is the absence of server-side counters and lockout logic on the authentication endpoint. The developers did not implement per-account or per-IP request limits, and no delay is enforced after failed attempts. This pattern maps directly to [CWE-307].

Attack Vector

An attacker requires only network reachability to the e-BOK login form. The attacker enumerates or acquires valid usernames, then iterates through the numeric password space using automated tooling. Because responses distinguish valid from invalid credentials, the attacker can confirm success programmatically. No user interaction, prior privilege, or session context is required.

See the CERT Poland Security Analysis for the coordinated disclosure covering the related KTM System e-BOK issues.

Detection Methods for CVE-2026-35098

Indicators of Compromise

High volume of HTTP POST requests to the e-BOK login endpoint from a single IP or subnet within a short window
Sequential or dictionary-like patterns in submitted credential values across authentication attempts
Successful logins immediately preceded by hundreds or thousands of failed attempts for the same account
Authentication traffic originating from anonymizing proxies, hosting providers, or geographically anomalous sources

Detection Strategies

Deploy web application firewall rules that track failed login counts per account and per source IP
Correlate authentication logs to alert when failure-to-success ratios exceed baseline thresholds
Monitor for user-agent strings and TLS fingerprints consistent with credential-stuffing frameworks such as Hydra or custom scripts

Monitoring Recommendations

Ingest e-BOK application and web server logs into a centralized log platform with retention sufficient for post-incident review
Alert on burst patterns in POST requests to /login or equivalent authentication routes
Track first-time-seen source IPs authenticating against multiple accounts within short intervals

How to Mitigate CVE-2026-35098

Immediate Actions Required

Apply the KTM System e-BOK patch released in June 2026 to introduce authentication throttling
Force password rotation for all e-BOK accounts and communicate the reset to affected customers
Restrict access to the e-BOK login endpoint behind a WAF configured with brute-force protection rules
Review historical authentication logs for signs of prior brute-force activity and compromised accounts

Patch Information

KTM System published a fix in June 2026 that introduces limits and timeouts on consecutive login attempts. Administrators should confirm the deployed version incorporates the patch through the KTM System Client Support Portal. The patch also addresses the related CVE-2026-35097 password policy weakness when applied together.

Workarounds

Place the login endpoint behind a reverse proxy that enforces per-IP request rate limits until the patch is applied
Enable geo-blocking or allowlisting to restrict access to expected customer regions
Deploy CAPTCHA challenges on the authentication form to break automated request pipelines
Introduce account lockout policies at the network edge when server-side controls cannot be modified

bash

# Example nginx rate-limit configuration for the login endpoint
limit_req_zone $binary_remote_addr zone=ebok_login:10m rate=5r/m;

server {
    location /login {
        limit_req zone=ebok_login burst=3 nodelay;
        proxy_pass http://ebok_backend;
    }
}