GHSA-443w-3rq3-5m5h: Policy Injection via Improper Input Escaping in AWS SDK for Java v2 CloudFront Utilities

The AWS SDK for Java v2 includes a dedicated module (software.amazon.awssdk:cloudfront) designed to simplify interactions with Amazon CloudFront. A core feature of this module is the generation of signed URLs and signed cookies, which enforce access control on private content delivered via the CDN. These signatures rely on a JSON-formatted Policy Document that specifies the precise conditions under which access is granted, such as allowed IP ranges, target resources, and expiration timestamps.

Versions of the SDK prior to 2.41.30 contain a flaw in the CloudFrontUtilities class related to the construction of these Policy Documents. The implementation fails to properly sanitize or escape specific control characters, most notably double quotes (") and backslashes (\), before embedding them into the JSON structure. This weakness maps to CWE-74 (Improper Neutralization of Special Elements in Output Used by a Downstream Component) and CWE-707 (Improper Neutralization).

When an application passes unsanitized, user-controllable input into the SDK's signing methods, the absence of output encoding allows the input to alter the structure of the resulting JSON document. This structural alteration permits an attacker to inject additional JSON keys and values, fundamentally modifying the constraints established by the application. The resulting signature is mathematically valid but cryptographically secures a policy that differs from the application's intent.

The vulnerability stems from the direct string concatenation or insufficient tokenization logic used within CloudFrontUtilities.java to build the policy JSON. When invoking methods such as getSignedUrlWithCustomPolicy or getSignedCookieWithCustomPolicy, the SDK accepts various input parameters, including the Resource path and IPAddress constraints. The SDK then formats these inputs into a strict JSON schema required by the AWS CloudFront service.

In the vulnerable versions, the serialization process inserts the string values provided by the user directly into the JSON string templates without applying JSON string escaping rules. A standard JSON string requires that internal double quotes be escaped as \". Because the SDK omits this escaping phase, an input containing a raw double quote will prematurely close the string value being evaluated by the JSON parser.

Following the premature string termination, any subsequent characters in the user's input are parsed as structural JSON elements (keys, objects, arrays, or boolean/numeric values). This allows an attacker to append new top-level or nested elements to the Statement array of the CloudFront Policy Document. The resulting payload is subsequently signed using the AWS cryptographic key, yielding a valid signature for the maliciously modified policy.

To understand the mechanics of the flaw, it is necessary to examine the expected JSON structure of a CloudFront custom policy. A standard policy specifies a Statement array containing objects that define the Resource and various Condition blocks. The Resource value is typically a string representing the URL path.

{
  "Statement": [
    {
      "Resource": "https://d111111abcdef8.cloudfront.net/video.mp4",
      "Condition": {
        "DateLessThan": {"AWS:EpochTime": 1600000000}
      }
    }
  ]
}

In the unpatched SDK, if an application constructs the Resource parameter dynamically using user input (e.g., https://d111111abcdef8.cloudfront.net/ + userInput), a malicious userInput can break the JSON syntax. The patch introduced in version 2.41.30 addresses this by applying strict character escaping to inputs before they are serialized into the JSON structure.

The patched implementation actively scans input strings for characters that hold syntactic meaning in JSON (", \, and control characters). These characters are replaced with their properly escaped equivalents (\", \\). This ensures that the entire user input is treated as a literal string value by the CloudFront JSON parser, completely neutralizing the injection vector.

Exploitation requires that the target application accepts user input and directly incorporates it into the parameters passed to the CloudFront signing utilities. A common scenario involves an endpoint that generates temporary download links for specific files requested by the user. If the application does not validate the requested filename, an attacker can supply a crafted payload.

Consider an application that appends user input to a base URL to form the Resource string. The attacker submits a filename containing a quote, followed by a comma, and a new JSON key-value pair. A characteristic payload resembles the following string: video.mp4", "Condition": { "DateLessThan": { "AWS:EpochTime": 2147483647 } } } //.

When the vulnerable SDK processes this payload, the resulting JSON policy document is modified. The injected quote closes the Resource string early. The injected Condition block overrides or appends to the policy conditions, setting an expiration date far into the future (e.g., the year 2038). The trailing // or similar structural characters are used to comment out or invalidate the remaining legitimate JSON structure originally intended by the SDK.

The SDK signs this manipulated JSON document and returns a signed URL to the attacker. The attacker then utilizes this signed URL to interact with CloudFront. Because the signature matches the (malicious) policy document, CloudFront grants access according to the injected conditions, effectively bypassing the application's intended temporal or network restrictions.

The vulnerability is tracked under the CVSS 4.0 vector CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:L/VA:N/SC:H/SI:N/SA:N, resulting in a base score of 4.0. While the score is mathematically categorized as Medium-Low, the practical impact depends heavily on the specific use case of the signed URLs within the affected application.

The primary consequence is a violation of Subsequent Confidentiality (SC:H). The vulnerability does not allow an attacker to read arbitrary memory from the application server or execute code. However, it entirely compromises the access control mechanism protecting the downstream CloudFront resources. An attacker can extend the validity period of a signed URL indefinitely or modify the resource path to access other files within the same CloudFront distribution.

The integrity of the policy document itself is also compromised (VI:L), as the attacker successfully manipulates the data structure generated by the application. Organizations relying on signed URLs for strict digital rights management (DRM), temporary media delivery, or IP-restricted data sharing are at highest risk, as these operational constraints can be completely bypassed by an unauthenticated attacker.

The primary and most effective remediation is to update the AWS SDK for Java v2 to version 2.41.30 or later. This release implements robust input escaping within the CloudFrontUtilities class, preventing the injection of structural JSON characters. Dependency management tools (e.g., Maven, Gradle) should be audited to ensure no transitive dependencies are pulling in vulnerable versions of software.amazon.awssdk:cloudfront.

For applications where immediate patching is not feasible, developers must implement strict input validation on all user-supplied data that influences the parameters of a signed URL or cookie. Input should be sanitized to reject any requests containing double quotes ("), backslashes (\), or other non-alphanumeric control characters in filename or path segments. Utilizing allow-lists for expected input patterns is highly recommended.

Security teams can detect potential exploitation attempts by analyzing Amazon CloudFront access logs. Anomalous log entries characterized by excessively long requested URIs, the presence of JSON syntax within URL parameters, or unexpected Condition blocks in the query string indicate possible attacks. Additionally, application-side logging should be configured to flag requests containing double quotes in fields designated for file paths or identifiers.