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CVE-2026-39858 is a high-severity authentication bypass vulnerability in Traefik’s ForwardAuth and snippet-based authentication middleware affecting versions prior to 2.11.43, 3.6.14, and 3.7.0-rc.2. The flaw arises because Traefik sanitizes only canonical hyphenated forwarded-header names, such as X-Forwarded-Proto, X-Forwarded-Host, and X-Forwarded-For, but fails to strip or normalize alias variants that use underscores instead of dashes. These underscore-form headers are forwarded to the authentication backend intact. If the downstream authentication service or framework normalizes underscore and hyphen variants equivalently, an attacker can inject spoofed trusted request context, including scheme, host, client identity, or related forwarding metadata, causing the backend to make authorization decisions based on attacker-controlled values and allowing access to protected routes without valid credentials.
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What an attacker gets, and what they’ve been doing with it.
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No public exploit code observed for this vulnerability.
Products and vendors Mallory has correlated with this vulnerability. Open in Mallory to drill down to specific CPE configurations and version ranges.
Vendor-confirmed product mapping. Mallory continuously reconciles this list against your asset inventory.
6 sources tracked across advisories, community write-ups, and news. New activity surfaces here as Mallory finds it.
Query your assets running an affected version, and investigate the blast radius.
Every observed campaign linking this CVE to a named adversary.
Malware families riding this exploit, with evidence and IOCs.
YARA, Sigma, Snort, and vendor rules, auto-deployed to your SIEM.
Cross-references every affected SKU, including bundled OEM variants.
Community discussion across Reddit, Mastodon, and other social sources.