Description

The API Publisher component previously used a non-cryptographic pseudorandom number generator (PRNG) to create shared secrets for Webhook HMAC validation. This PRNG lacks sufficient entropy for security-sensitive operations, allowing a sophisticated attacker to predict future secrets. This enables malicious actors to forge event payloads with valid HMAC signatures, bypassing the API Gateway's authenticity verification. Successful exploitation could allow an attacker to predict shared secrets used for Webhook HMAC validation and forge event payloads with valid signatures. This may enable bypassing API Gateway authenticity checks, leading to unauthorized event injection, data manipulation, or downstream system compromise.

Executive Summary

Technical Details

Mitigations

References

Additional Metadata

{
  "score": "5.9",
  "pubDate": "2026-09-03T13:05:37.847Z",
  "pubdate": "2026-09-03T13:05:37.847Z",
  "executiveSummary": "The API Publisher component utilizes a cryptographically insecure pseudorandom number generator (PRNG) for the generation of shared secrets utilized in Webhook HMAC signature validation. This vulnerability stems from the PRNG's inability to provide sufficient entropy required for security-sensitive cryptographic operations. Consequently, the predictability of these shared secrets presents a critical risk, allowing unauthorized actors to perform cryptographic analysis or state reconstruction to forecast future secret values.\nThe primary impact of this vulnerability is the compromise of the integrity and authenticity of Webhook event notifications. By successfully predicting the HMAC secret, an attacker can generate forged event payloads that the API Gateway will accept as legitimate. This bypasses the security boundary established by HMAC-based verification, enabling unauthorized event injection into downstream systems. Such manipulation can lead to data integrity loss, unauthorized state changes in integrated applications, and potential system compromise. The attack requires no authentication, relying solely on the observer's ability to analyze or influence the secret generation process, representing a significant risk to the supply chain and inter-service communication protocols.",
  "technicalDetails": "The vulnerability originates from the implementation of a non-cryptographic PRNG within the API Publisher's secret generation logic. In cryptographic contexts, PRNGs must satisfy strict statistical requirements for unpredictability, specifically ensuring that prior outputs cannot be used to determine future values (forward secrecy) and that the internal state cannot be recovered from observed outputs. Standard PRNGs, such as those found in generic math libraries, are designed for performance and statistical distribution rather than adversarial resistance.\nThe attack flow proceeds as follows: First, the attacker observes a sufficient number of previously generated shared secrets or interactions with the API Publisher. If the underlying PRNG implementation utilizes a linear congruential generator (LCG) or a similar algorithm with a small or predictable state space, the attacker can use these observations to reconstruct the generator's internal state. Once the state is identified, the attacker projects the PRNG algorithm forward to predict the next sequence of shared secrets assigned to newly created or rotated Webhooks.\nUpon predicting a future shared secret, the attacker crafts a malicious JSON payload. Using the predicted secret, the attacker calculates the HMAC signature (e.g., HMAC-SHA256) matching the payload contents. When this payload is transmitted to the API Gateway, the system calculates the HMAC based on its own (predictable) secret. Because the attacker's computed HMAC matches the system's generated HMAC, the API Gateway verifies the event as authentic and forwards it to the downstream consumer.\nThis exploitation technique effectively invalidates the HMAC signature mechanism, which is intended to ensure that events originate from a trusted source. Because the flaw lies in the entropy source of the secret generation process, the attack is independent of the transport layer security (TLS) or the specific HMAC algorithm employed. The downstream systems, trusting the API Gateway's validation of the HMAC, process the forged payloads as legitimate business events. Post-exploitation impact includes the injection of arbitrary commands, execution of unauthorized business logic, or the exfiltration of data handled by the Webhook processing pipeline."
}