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ex_aws_sns: Trusted-attacker `SigningCertURL` permits complete SNS signature bypass

High severity GitHub Reviewed Published May 28, 2026 in ex-aws/ex_aws_sns • Updated Jun 30, 2026

Package

erlang ex_aws_sns (Erlang)

Affected versions

>= 2.0.1, < 2.3.5

Patched versions

2.3.5

Description

Summary

ExAws.SNS.verify_message/1 fetches the signing certificate from the SigningCertURL field of the incoming SNS message without validating that the URL uses HTTPS or that its host is an AWS-owned SNS certificate domain. An unauthenticated attacker who can POST to any endpoint that calls verify_message/1 can supply an attacker-controlled SigningCertURL, sign a forged SNS message with their own RSA key, and cause the function to return :ok, completely bypassing SNS signature verification.

Details

In lib/ex_aws/sns.ex (lines 475–483), verify_message/1 performs three checks: validate_message_params/1 (confirms required fields are present), validate_signature_version/1 (confirms SignatureVersion == "1"), then signature verification. The signature step calls ExAws.SNS.PublicKeyCache.get(message["SigningCertURL"]) and passes the result to :public_key.verify/4.

Neither validate_message_params/1 nor any other step checks that SigningCertURL is an HTTPS URL or that the hostname matches the expected pattern (e.g. sns.<region>.amazonaws.com). PublicKeyCache.get/1 in lib/ex_aws/sns/public_key_cache.ex fetches whatever URL is provided and caches the certificate. The RSA signature then verifies against the attacker's own public key, and verify_message/1 returns :ok.

PoC

  1. Generate an RSA keypair and host the DER/PEM public certificate at any URL reachable from the target server (e.g. http://attacker.example/cert.pem).
  2. Build a forged Notification payload with an arbitrary TopicArn and Message, compute the canonical string-to-sign per the SNS spec, and sign it with the attacker private key.
  3. Set SigningCertURL to the attacker URL and Signature to the base64-encoded signature.
  4. POST the forged payload to any SNS webhook endpoint that calls ExAws.SNS.verify_message/1.
  5. The function returns :ok; the application treats the message as authentic.

Configurations

The application must expose an HTTP endpoint that calls ExAws.SNS.verify_message/1 on incoming request bodies (the standard SNS webhook pattern).

Impact

Complete SNS signature authentication bypass. Affects ex_aws_sns from 2.0.1 through 2.3.4. Consequences include spoofing arbitrary Notification payloads, auto-confirming attacker-controlled SubscribeURL values to hijack topic delivery, and spoofing UnsubscribeConfirmation to disrupt legitimate subscriptions. No authentication or special configuration on the attacker side is required. CVSS v4.0: 8.7 (HIGH).

Resources

References

@bernardd bernardd published to ex-aws/ex_aws_sns May 28, 2026
Published to the GitHub Advisory Database Jun 26, 2026
Reviewed Jun 26, 2026
Last updated Jun 30, 2026

Severity

High

CVSS overall score

This score calculates overall vulnerability severity from 0 to 10 and is based on the Common Vulnerability Scoring System (CVSS).
/ 10

CVSS v4 base metrics

Exploitability Metrics
Attack Vector Network
Attack Complexity Low
Attack Requirements None
Privileges Required None
User interaction None
Vulnerable System Impact Metrics
Confidentiality None
Integrity High
Availability None
Subsequent System Impact Metrics
Confidentiality None
Integrity None
Availability None

CVSS v4 base metrics

Exploitability Metrics
Attack Vector: This metric reflects the context by which vulnerability exploitation is possible. This metric value (and consequently the resulting severity) will be larger the more remote (logically, and physically) an attacker can be in order to exploit the vulnerable system. The assumption is that the number of potential attackers for a vulnerability that could be exploited from across a network is larger than the number of potential attackers that could exploit a vulnerability requiring physical access to a device, and therefore warrants a greater severity.
Attack Complexity: This metric captures measurable actions that must be taken by the attacker to actively evade or circumvent existing built-in security-enhancing conditions in order to obtain a working exploit. These are conditions whose primary purpose is to increase security and/or increase exploit engineering complexity. A vulnerability exploitable without a target-specific variable has a lower complexity than a vulnerability that would require non-trivial customization. This metric is meant to capture security mechanisms utilized by the vulnerable system.
Attack Requirements: This metric captures the prerequisite deployment and execution conditions or variables of the vulnerable system that enable the attack. These differ from security-enhancing techniques/technologies (ref Attack Complexity) as the primary purpose of these conditions is not to explicitly mitigate attacks, but rather, emerge naturally as a consequence of the deployment and execution of the vulnerable system.
Privileges Required: This metric describes the level of privileges an attacker must possess prior to successfully exploiting the vulnerability. The method by which the attacker obtains privileged credentials prior to the attack (e.g., free trial accounts), is outside the scope of this metric. Generally, self-service provisioned accounts do not constitute a privilege requirement if the attacker can grant themselves privileges as part of the attack.
User interaction: This metric captures the requirement for a human user, other than the attacker, to participate in the successful compromise of the vulnerable system. This metric determines whether the vulnerability can be exploited solely at the will of the attacker, or whether a separate user (or user-initiated process) must participate in some manner.
Vulnerable System Impact Metrics
Confidentiality: This metric measures the impact to the confidentiality of the information managed by the VULNERABLE SYSTEM due to a successfully exploited vulnerability. Confidentiality refers to limiting information access and disclosure to only authorized users, as well as preventing access by, or disclosure to, unauthorized ones.
Integrity: This metric measures the impact to integrity of a successfully exploited vulnerability. Integrity refers to the trustworthiness and veracity of information. Integrity of the VULNERABLE SYSTEM is impacted when an attacker makes unauthorized modification of system data. Integrity is also impacted when a system user can repudiate critical actions taken in the context of the system (e.g. due to insufficient logging).
Availability: This metric measures the impact to the availability of the VULNERABLE SYSTEM resulting from a successfully exploited vulnerability. While the Confidentiality and Integrity impact metrics apply to the loss of confidentiality or integrity of data (e.g., information, files) used by the system, this metric refers to the loss of availability of the impacted system itself, such as a networked service (e.g., web, database, email). Since availability refers to the accessibility of information resources, attacks that consume network bandwidth, processor cycles, or disk space all impact the availability of a system.
Subsequent System Impact Metrics
Confidentiality: This metric measures the impact to the confidentiality of the information managed by the SUBSEQUENT SYSTEM due to a successfully exploited vulnerability. Confidentiality refers to limiting information access and disclosure to only authorized users, as well as preventing access by, or disclosure to, unauthorized ones.
Integrity: This metric measures the impact to integrity of a successfully exploited vulnerability. Integrity refers to the trustworthiness and veracity of information. Integrity of the SUBSEQUENT SYSTEM is impacted when an attacker makes unauthorized modification of system data. Integrity is also impacted when a system user can repudiate critical actions taken in the context of the system (e.g. due to insufficient logging).
Availability: This metric measures the impact to the availability of the SUBSEQUENT SYSTEM resulting from a successfully exploited vulnerability. While the Confidentiality and Integrity impact metrics apply to the loss of confidentiality or integrity of data (e.g., information, files) used by the system, this metric refers to the loss of availability of the impacted system itself, such as a networked service (e.g., web, database, email). Since availability refers to the accessibility of information resources, attacks that consume network bandwidth, processor cycles, or disk space all impact the availability of a system.
CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:H/VA:N/SC:N/SI:N/SA:N

EPSS score

Exploit Prediction Scoring System (EPSS)

This score estimates the probability of this vulnerability being exploited within the next 30 days. Data provided by FIRST.
(13th percentile)

Weaknesses

Improper Certificate Validation

The product does not validate, or incorrectly validates, a certificate. Learn more on MITRE.

CVE ID

CVE-2026-47074

GHSA ID

GHSA-8jgf-23q5-x7xx

Source code

Credits

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