CVE-2026-93649 in Adoptium Temurin
Summary
by MITRE • 10/15/2026
In the Eclipse Adoptium website (adoptium.net), in every deployment built from commit 0b24ea1 (2025-06-18) up to the fix in commit cbb53cf (2026-09-24), the post-download page /[locale]/download takes a download URL from the "link" query parameter and automatically navigates to it with a meta http-equiv="refresh". The URL is checked against a list of trusted prefixes using String.startsWith() on the raw string, while the browser applies RFC 3986 dot-segment normalisation before navigating, so a value such as https://github.com/adoptium/temurin/../../attacker/malware/releases/download/v1/jdk.exe passes the check but resolves to an attacker-controlled repository. A victim who follows a crafted adoptium.net link is shown the genuine "Thank you for downloading Eclipse Temurin" page while their browser downloads a binary of the attacker's choosing
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Analysis
by VulDB Data Team • 10/15/2026
The vulnerability identified in the Eclipse Adoptium website represents a classic Open Redirect flaw rooted in insufficient validation of user-supplied input during URL redirection processes. Specifically, this issue affects deployments built between commit 0b24ea1 on June 18, 2025, and the remediation introduced in commit cbb53cf on September 24, 2026. The core technical flaw lies in how the application handles the link query parameter within the post-download page at /[locale]/download. When a user accesses this endpoint with a specified download URL, the server-side logic employs String.startsWith() to verify that the provided URL begins with one of several trusted prefixes. This approach assumes that string prefix matching is sufficient to guarantee safety, but it fails to account for how web browsers interpret and normalize URLs before initiating navigation.
The operational impact stems from the discrepancy between server-side validation and client-side browser behavior regarding RFC 3986 dot-segment normalization. While the application checks if the raw input string starts with a trusted domain such as github.com, the browser applies path segment resolution rules defined in RFC 3986 before performing the actual navigation via meta http-equiv="refresh". This means that an attacker can craft a URL containing relative path traversal sequences like ../../ to escape the intended directory structure. For instance, providing https://github.com/adoptium/temurin/../../attacker/malware/releases/download/v1/jdk.exe will pass the server-side check because it technically starts with github.com. However, once normalized by the browser, this URL resolves to an entirely different host controlled by the attacker. This discrepancy allows malicious actors to bypass security controls that rely solely on prefix matching without considering path normalization semantics.
From a user experience and trust perspective, this vulnerability is particularly dangerous due to its stealthy nature. Victims who click on crafted links are presented with the legitimate Eclipse Temurin thank you page, which maintains the visual appearance of authenticity. This social engineering aspect lowers the victim's suspicion significantly, as they believe they are interacting with the official download portal. Meanwhile, in the background, the browser initiates a redirect to the attacker-controlled domain where malicious binaries can be downloaded. This technique effectively weaponizes trust in the Eclipse Adoptium brand to facilitate malware distribution or phishing attacks without triggering typical security warnings associated with obvious external redirects.
This vulnerability aligns closely with CWE-601, which describes URL Redirection to Untrusted Site via User-supplied Input. It also maps to MITRE ATT&CK technique T1583.002, specifically Domain Fronting or Spoofed Domains in the context of initial access campaigns where attackers use trusted domains to mask malicious infrastructure. The failure here is not just a logic error but a fundamental misunderstanding of how web standards govern URL resolution across different contexts. To mitigate this issue effectively, developers must implement robust URL parsing and validation mechanisms rather than relying on simple string operations. Best practices include using dedicated URI libraries that handle normalization consistently with browser behavior or explicitly rejecting URLs containing dot-segments such as .. before performing any checks. Additionally implementing Content Security Policy headers to restrict navigation targets can provide an additional layer of defense against unauthorized redirects. Organizations should also consider adopting strict allow-listing strategies where only specific, pre-approved paths are permitted rather than broad domain prefixes that are susceptible to traversal attacks.