CVE-2026-69208 in Http4s
Summary
by MITRE • 09/15/2026
Http4s is a Scala interface for HTTP services. Prior to 0.23.35 and 1.0.0-M47, the DigestAuth server middleware removes fresh nonces and stops eviction at the first stale nonce because its stale-nonce comparison is inverted. On an application that protects at least one route with DigestAuth, an unauthenticated attacker can repeatedly trigger authentication challenges, causing the persistent nonce map to grow until the JVM exhausts heap memory. This issue is fixed in versions 0.23.35 and 1.0.0-M47.
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Analysis
by VulDB Data Team • 09/15/2026
The vulnerability identified within the Http4s library represents a critical resource exhaustion flaw stemming from an inverted logic condition in its Digest authentication middleware. Http4s serves as a prominent Scala interface for building HTTP services, and its security mechanisms are fundamental to protecting web applications against unauthorized access. The specific defect resides in the handling of nonce values during the digest authentication process. Nonces are random numbers used by servers to prevent replay attacks, ensuring that each authentication request is unique and time-bound. In this implementation, the middleware is responsible for managing a persistent map of these nonces, which includes logic to remove fresh nonces after use and evict stale ones based on their age or usage status.
The core technical flaw lies in an inverted comparison used to determine whether a nonce is considered stale. Due to this logical error, the system fails to correctly identify when a nonce has expired or should be removed from memory. Instead of properly cleaning up old entries, the middleware stops evicting nonces at the first instance where it encounters what it incorrectly perceives as a stale nonce. This breaks the intended lifecycle management of authentication tokens within the server's memory structure. Consequently, every time an unauthenticated attacker triggers an authentication challenge by sending invalid or repeated requests to a protected route, new nonce entries are added to this persistent map without corresponding removals for previously issued nonces that should have been discarded.
The operational impact of this vulnerability is severe, primarily manifesting as a Denial of Service (DoS) condition through heap memory exhaustion. Because the application protects at least one route with DigestAuth, any unauthenticated user can exploit this flaw by repeatedly initiating authentication challenges. Each challenge results in the generation and storage of new nonce data that remains permanently resident in the JVM's heap space due to the broken eviction logic. Over time, as an attacker floods the service with these requests, the memory footprint grows linearly or exponentially depending on request volume. Eventually, this unbounded growth leads to OutOfMemoryError conditions, causing the Java Virtual Machine to crash and rendering the entire HTTP service unavailable to legitimate users.
From a classification perspective, this vulnerability aligns with CWE-787: Out-of-bounds Write in terms of memory management consequences, though more accurately it falls under CWE-400: Uncontrolled Resource Consumption. The attack vector is classified as Remote (ATT&CK T1496) because the exploitation requires network access and does not necessitate prior authentication privileges. An attacker can initiate this resource exhaustion from a remote location by simply sending HTTP requests to the targeted endpoint, making it particularly dangerous for publicly accessible services that rely on digest authentication for security.
Mitigation strategies focus primarily on upgrading the Http4s library to patched versions where this logical inversion has been corrected. Specifically, users must upgrade to version 0.23.35 or later in the 0.23.x branch, or to version 1.0.0-M47 and later for the newer release line. These updates restore the correct logic for nonce eviction, ensuring that stale nonces are properly removed from memory and preventing unbounded growth of the authentication state map. In addition to software patching, organizations should implement rate limiting on their HTTP endpoints to mitigate the volume of requests an individual IP address can send in a given timeframe. This acts as a compensating control by slowing down the rate at which nonces are generated, thereby reducing the speed of memory exhaustion even if the underlying vulnerability is not immediately patched. Monitoring JVM heap usage and setting up alerts for abnormal spikes can also provide early warning signs of such an attack in progress.