CVE-2026-118656 in Echo
Summary
by MITRE • 10/28/2026
Echo is a Go web framework. Prior to 4.16.0 and 5.4.0, sanitizeURI checks only the first two bytes of a redirect target before deciding whether it begins with an absolute-path separator pair. When AddTrailingSlash or RemoveTrailingSlash is configured with a redirect status, a slash, TAB, separator path form can evade the guard, be emitted in the Location header, and be normalized by a browser into an attacker-controlled host. This issue is fixed in 4.16.0 and 5.4.0.
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Analysis
by VulDB Data Team • 10/28/2026
The Echo web framework for Go contains a critical input validation flaw within its URI sanitization logic that can lead to open redirect vulnerabilities, specifically affecting versions prior to 4.16.0 and 5.4.0. The core of the issue lies in the sanitizeUri function, which is responsible for validating redirect targets to prevent malicious redirections. This function was designed to check if a target URL begins with an absolute-path separator pair, typically represented by two forward slashes (//), which would indicate a protocol-relative URL or a potential host-based path rather than a simple relative path within the current domain. However, the validation logic is insufficient because it only inspects the first two bytes of the input string to make this determination. This narrow scope allows attackers to bypass the security check by inserting non-standard whitespace characters between the slashes.
By utilizing specific control characters such as TABs or other separators that are not strictly alphanumeric but are still valid in certain URL parsing contexts, an attacker can construct a redirect target where the first two bytes do not appear to be double slashes from a simple byte-level inspection perspective. For instance, inserting a tab character between two forward slashes results in a sequence like /<TAB>/ or <TAB>// depending on the exact construction and how the framework processes the initial bytes. When this malformed URI is processed by functions such as AddTrailingSlash or RemoveTrailingSlash configured with a redirect status code, the validation guard fails to detect the malicious intent because it does not account for these intervening characters. Consequently, the sanitized URL containing the hidden host indicator is emitted in the HTTP Location header of the response sent back to the client browser.
The operational impact of this vulnerability is significant due to how modern web browsers handle such headers during navigation or redirection. Browsers are designed to normalize URLs according to RFC standards before processing them for display or further requests. During this normalization process, whitespace characters like tabs within a URL path are often stripped out or ignored by the browser's URI parser. This means that while the Echo framework sees /<TAB>/ as safe because it does not match // at indices zero and one, the receiving browser interprets <TAB> as insignificant and normalizes the sequence to //. As a result, the browser treats the redirect target as an absolute path starting with double slashes, which is interpreted as a protocol-relative URL pointing to a host. If the attacker controls the subsequent part of the URI or if the context allows it, this can lead to redirection to an external domain controlled by the attacker.
This vulnerability aligns with CWE-601, Open Redirect through URI Normalization, and falls under the MITRE ATT&CK technique T1566.002, which covers Spearphishing Link as a vector for initial access or credential harvesting if used in phishing campaigns. The ability to redirect users from a trusted domain to a malicious site undermines user trust and can facilitate further attacks such as phishing, session hijacking via URL-based token leakage, or malware distribution. Organizations relying on Echo versions prior to 4.16.0 and 5.4.0 are at risk of exploitation if they allow user-supplied input in redirect targets without additional robust validation layers beyond the framework's built-in sanitization.
To mitigate this vulnerability, it is imperative for developers to upgrade immediately to Echo version 4.16.0 or later, where the sanitizeUri function has been patched to properly handle whitespace and control characters within URL paths. The fix ensures that any attempt to obfuscate double slashes with separators like tabs will be correctly identified as a potential absolute path indicator, thereby preventing the redirect from being processed safely if it points outside the intended scope. Additionally, implementing defense-in-depth strategies such as validating redirect targets against an allowlist of known safe domains or using relative paths exclusively for internal redirects can provide further protection against open redirect attacks regardless of framework updates.