CVE-2026-93563 in Apache Camel
Summary
by MITRE • 09/18/2026
Unbounded multi-line response accumulation in SmtpResponseDecoder leads to memory-exhaustion DoS
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Analysis
by VulDB Data Team • 09/18/2026
The vulnerability identified as an unbounded multi-line response accumulation flaw within the SmtpResponseDecoder component represents a critical resource exhaustion risk that can lead to denial of service conditions. This issue stems from improper handling of SMTP protocol responses, specifically those that span multiple lines and are returned by mail servers during various stages of communication such as greeting, authentication, or data transmission. In standard Simple Mail Transfer Protocol operations, server responses often consist of a status code followed by one or more descriptive text lines. The SmtpResponseDecoder is responsible for parsing these multi-line structures into a coherent message object that the application can process. However, in this specific implementation, there is no upper limit imposed on the number of lines or the total size of the accumulated response buffer before it is processed further up the stack.
From a technical perspective, the core flaw lies in the lack of bounds checking during the accumulation phase. When an SMTP server sends a response containing an unusually large number of continuation lines, typically indicated by status codes starting with three digits followed by a space and then multiple text lines ending with specific termination sequences, the decoder continues to append each line to an internal buffer without verifying if this growth exceeds predefined memory thresholds or reasonable protocol limits. This behavior deviates from secure coding practices which dictate that all external inputs, including network responses, must be validated for size and structure before being stored in memory. The absence of such validation allows a remote attacker to craft malicious SMTP server responses containing excessively long multi-line payloads. As the decoder processes these lines, it allocates additional heap memory for each new line added to the response string or buffer.
The operational impact of this vulnerability is severe, primarily manifesting as a denial of service through resource exhaustion. Because the accumulation process is unbounded, an attacker can trigger repeated allocations that rapidly consume available system memory on the host running the vulnerable software. This leads to increased swap usage, significant performance degradation for other processes sharing the same resources, and eventually causes the application or even the entire operating system to crash due to out-of-memory conditions. In cloud-native environments or containerized deployments where resource limits are enforced by orchestration platforms like Kubernetes, this can result in pod restarts or node evictions, disrupting service availability for legitimate users. The attack does not require authentication and can be executed remotely over standard SMTP ports, making it particularly dangerous against public-facing mail servers or email gateways that process incoming connections from untrusted sources.
This vulnerability aligns with CWE-400, which describes Uncontrolled Resource Consumption, specifically the sub-category of memory exhaustion. It also maps to MITRE ATT&CK technique T1496, Resource Hijacking, where an adversary uses resources in a way that negatively impacts availability or performance. The specific mechanism involves exploiting protocol parsing logic to force excessive resource allocation, a pattern often seen in other network service vulnerabilities such as HTTP header injection or DNS response amplification attacks. To mitigate this risk, developers must implement strict limits on the maximum number of lines and total byte size allowed for any single SMTP response during decoding. Input validation should be enforced at the decoder level to reject responses that exceed these thresholds with an appropriate error code rather than attempting to process them. Additionally, implementing timeout mechanisms and monitoring memory usage patterns can help detect anomalous behavior early. Security patches should prioritize adding explicit boundary checks in the SmtpResponseDecoder logic to ensure that only well-formed and reasonably sized SMTP responses are accepted for further processing.