CVE-2026-18259: Drupal Token Content Access Auth Bypass

CVE-2026-18259 Overview

CVE-2026-18259 is an Observable Timing Discrepancy vulnerability [CWE-208] in the Drupal Token Content Access contributed module. The flaw affects Token Content Access versions 0.0.0 through 3.1.2 and enables brute-force attacks against protected content. An unauthenticated attacker can measure response-time differences to infer valid access tokens over the network. Successful exploitation exposes content that the module is meant to gate behind tokenized URLs.

Critical Impact
Remote attackers can brute-force valid access tokens by observing timing side channels, leading to unauthorized disclosure of token-protected Drupal content.

Affected Products

Drupal Token Content Access module versions 0.0.0 to 3.1.2
Drupal sites using tokenized content-access URLs supplied by this module
Any deployment permitting unauthenticated network requests to token-validated endpoints

Discovery Timeline

2026-08-25 - CVE-2026-18259 published to NVD
2026-08-26 - Last updated in NVD database

Technical Details for CVE-2026-18259

Vulnerability Analysis

The Token Content Access module grants access to Drupal content when a request presents a valid token in the URL. The module compares the submitted token against the expected value using a non-constant-time comparison. Response times therefore correlate with how many leading characters of the token match. An attacker who can send many requests and measure server response latency can recover the token one character at a time. Recovering the token yields the same access as a legitimate recipient of the tokenized link.

Root Cause

The root cause is an observable timing discrepancy during token verification. Standard string comparison routines short-circuit on the first mismatched byte, producing measurable timing differences between near-match and far-mismatch inputs. Secret comparisons must instead use constant-time equality routines such as PHP's hash_equals() to eliminate this signal.

Attack Vector

Exploitation requires only network access to a Drupal site running an affected version of the module. No authentication, privileges, or user interaction are needed. The attacker issues repeated requests to a token-protected URL with candidate tokens, records per-request timing, and iteratively selects the character producing the slowest response. Detailed remediation guidance is available in the Drupal Security Advisory.

No public proof-of-concept exploit code is available. The vulnerability mechanism is described in prose because verified exploit code has not been released.

Detection Methods for CVE-2026-18259

Indicators of Compromise

High-volume requests from a single source to Drupal URLs that contain a token query parameter or path segment used by the Token Content Access module.
Sequences of requests where each token value differs from the prior by a single character position.
Successful HTTP 200 responses to token-protected content immediately following long runs of HTTP 403 or 404 responses from the same client.

Detection Strategies

Baseline normal request rates to token-gated URLs and alert on sustained deviations from a single IP or user agent.
Inspect web server and reverse-proxy logs for repeated requests to the same base path with varying token parameters.
Correlate request timing measurements at the edge; brute-force campaigns often produce evenly spaced request bursts.

Monitoring Recommendations

Forward Drupal, web server, and WAF logs into a centralized analytics platform for correlation across sources.
Enable rate-limiting metrics on token-protected endpoints and alert when thresholds are exceeded.
Track authentication and access-decision events for token-gated content to identify anomalous success patterns.

How to Mitigate CVE-2026-18259

Immediate Actions Required

Upgrade the Token Content Access module to a fixed release published after version 3.1.2 per the Drupal Security Advisory.
Rotate any tokens that were in use prior to patching, since previously issued tokens may have been observed or partially recovered.
Apply rate limiting to token-validated URLs at the web server, reverse proxy, or WAF layer.

Patch Information

Drupal has published guidance and a fixed release under advisory SA-CONTRIB-2026-090. Site operators should update the module using Composer or the Drupal admin update workflow and then run database updates. Review the Drupal Security Advisory for the specific fixed version and upgrade steps.

Workarounds

Restrict access to token-protected URLs to known IP ranges until the patch is applied.
Enforce short token lifetimes and single-use semantics to reduce the window available for brute-force attempts.
Place the affected Drupal site behind a WAF rule that blocks clients issuing high-frequency requests to token-gated paths.

bash

# Update the Drupal contributed module using Composer
composer update drupal/token_content_access --with-dependencies
drush updatedb
drush cache:rebuild