CWE-174
Double Decoding of the Same Data
Common consequences1
- Access ControlConfidentialityAvailabilityIntegrityOtherBypass Protection MechanismExecute Unauthorized Code or CommandsVaries by Context
Potential mitigations4
- Architecture and Design
Avoid making decisions based on names of resources (e.g. files) if those resources can have alternate names.
- Implementation
Assume all input is malicious. Use an "accept known good" input validation strategy, i.e., use a list of acceptable inputs that strictly conform to specifications. Reject any input that does not strictly conform to specifications, or transform it into something that does. When performing input validation, consider all potentially relevant properties, including length, type of input, the full range of acceptable values, missing or extra inputs, syntax, consistency across related fields, and conformance to business rules. As an example of business rule logic, "boat" may be syntactically valid because it only contains alphanumeric characters, but it is not valid if the input is only expected to contain colors such as "red" or "blue." Do not rely exclusively on looking for malicious or malformed inputs. This is likely to miss at least one undesirable input, especially if the code's environment changes. This can give attackers enough room to bypass the intended validation. However, denylists can be useful for detecting potential attacks or determining which inputs are so malformed that they should be rejected outright.
- Implementation
Use and specify an output encoding that can be handled by the downstream component that is reading the output. Common encodings include ISO-8859-1, UTF-7, and UTF-8. When an encoding is not specified, a downstream component may choose a different encoding, either by assuming a default encoding or automatically inferring which encoding is being used, which can be erroneous. When the encodings are inconsistent, the downstream component might treat some character or byte sequences as special, even if they are not special in the original encoding. Attackers might then be able to exploit this discrepancy and conduct injection attacks; they even might be able to bypass protection mechanisms that assume the original encoding is also being used by the downstream component.
- Implementation
Inputs should be decoded and canonicalized to the application's current internal representation before being validated (CWE-180). Make sure that the application does not decode the same input twice (CWE-174). Such errors could be used to bypass allowlist validation schemes by introducing dangerous inputs after they have been checked.
CVEs referencing this CWE7
| CVE | Description | Severity | EPSS | Flags | Modified |
|---|---|---|---|---|---|
| CVE-2026-118663 | @hono/node-server allows running the Hono application on Node.js. From 1.19.10 until 2.1.3, serveStatic decodes a request path a second time after Hono has already decoded it for routing, so a malformed percent-encoded sequence can make route middleware evaluate an encoded path while static file resolution uses a different directory. An unauthenticated attacker can bypass middleware mounted on a protected static prefix and read a file inside the configured static root, although the flaw does not permit traversal outside that root. This issue is fixed in version 2.1.3. | MEDIUM5.3 | 0.35%p29 | 2026-10-29 | |
| CVE-2026-109955 | Hono from 4.12.4 before 4.13.11 contains a middleware bypass vulnerability in serveStatic that allows unauthenticated attackers to skip prefix middleware by exploiting double path decoding. Attackers can send malformed percent-encoded paths that the router matches in encoded form, reaching static files without authentication checks running. | MEDIUM5.3 | 0.32%p24 | 2026-10-13 | |
| CVE-2026-111757 | Hono is a Web application framework that provides support for any JavaScript runtime. From 4.12.4 until 4.13.11, serveStatic can interpret a malformed percent-encoded path differently from upstream routing middleware and bypass protection for a static prefix. The root cause is serveStatic decodes a routed request path a second time before filesystem lookup. The attack mechanism is malformed percent encoding survives the first routing interpretation and becomes a protected path only during the later static-file decode. The affected component is serveStatic. The trigger conditions are an application protects a static prefix with upstream middleware and serveStatic receives an attacker-controlled malformed percent-encoded path. The impact is unauthorized disclosure of a static file that upstream middleware was intended to protect. The issue does not permit traversal outside the configured static root. This issue is fixed in version 4.13.11. | MEDIUM5.3 | 0.31%p24 | 2026-10-20 | |
| CVE-2026-118655 | Echo is a Go web framework. Prior to 4.15.3 and 5.2.0, middleware.Static can decode an already decoded Request.URL.Path a second time. Double-encoded separators can consequently be reconstructed before filesystem resolution. This can reach a file under a guarded route when the static middleware is registered globally. This issue is fixed in 4.15.3 and 5.2.0. | MEDIUM5.9 | 0.30%p23 | 2026-10-29 | |
| CVE-2026-92839 | Canva Desktop before v1.125.0 performed double decoding in the deeplink handler. A threat actor could cause the application to load arbitrary same-origin content under the user’s session. | MEDIUM4.3 | 0.28%p20 | 2026-09-18 | |
| CVE-2026-93903 | LiteSpeed Web Server (LSWS) before 6.3.7 build 1 mishandles internal redirect URL validation in a certain "corner case." | NONE | 0.26%p17 | PoC | 2026-10-29 |
| CVE-2026-75899 | fast-uri is a URI parser for Node.js. It decodes percent escapes in a hostname during parsing and then decodes the parsed hostname a second time during authority recomposition, so a single call to normalize or resolve can turn nested percent-encoded input into a different network destination such as a loopback hostname or address. For example, a doubly encoded host that spells out a loopback name decodes to that live host in one operation, which contradicts RFC 3986 section 2.4 that an implementation must not decode the same string more than once. An application that normalizes or resolves an untrusted HTTP-family URI before outbound routing, redirect validation, or a host-policy check can receive a destination different from the one the original encoded host represented, giving a server-side request forgery and host-policy bypass primitive. This is an incomplete-fix variant of CVE-2026-6322. The affected versions are 2.4.1 up to but not including 2.4.5, 3.1.2 up to but not including 3.1.6, and 4.0.0 up to but not including 4.1.3. The issue is fixed in 2.4.5, 3.1.6, and 4.1.3, which normalize percent escapes once and preserve encoded percent signs. Users should upgrade to a patched version. | HIGH7.5 | 0.22%p13 | 2026-10-14 |