CVE-2026-55490 in OpenWrt
Summary
by MITRE • 07/08/2026
OpenWrt is a Linux operating system targeting embedded devices. Before v25.12.5, an integer underflow in handle_send_a() of the Emergency Access Daemon allows any unauthenticated attacker on the local network to crash the daemon by sending a single crafted UDP packet. The message length underflows before a bounds check and is then passed to memcpy as a very large size. This issue is fixed v25.12.5.
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Analysis
by VulDB Data Team • 07/08/2026
The vulnerability exists within the Emergency Access Daemon component of OpenWrt operating systems prior to version 25.12.5, representing a critical security flaw that undermines the system's stability and availability. This daemon serves as a crucial network service responsible for emergency access functionality, making it an attractive target for attackers seeking to disrupt network operations. The flaw manifests in the handle_send_a() function where improper input validation leads to a specific type of integer underflow condition that can be exploited through network-based attacks.
The technical implementation of this vulnerability involves a precise manipulation of packet data that triggers an integer underflow during message length processing. When an unauthenticated attacker sends a specially crafted UDP packet to the affected daemon, the system processes the message length field in a manner that causes it to wrap around to an extremely large positive value due to the underflow condition. This corrupted length value then bypasses normal bounds checking mechanisms and is subsequently passed directly to the memcpy function as a parameter specifying the number of bytes to copy. The consequence is that memcpy attempts to allocate and copy an enormous amount of memory, typically resulting in a segmentation fault or crash of the daemon process.
The operational impact of this vulnerability extends beyond simple service disruption, creating potential for broader network instability within embedded device environments where the Emergency Access Daemon may be critical for system recovery operations. Attackers can exploit this weakness without requiring any authentication credentials, making it particularly dangerous in local network environments where such devices are commonly deployed. The crash effect can potentially lead to denial of service conditions that prevent legitimate users from accessing emergency access services, which may be essential during critical situations or system failures.
This vulnerability aligns with CWE-191, which specifically addresses integer underflow conditions, and demonstrates characteristics consistent with the ATT&CK technique T1498 related to network disruption and service availability attacks. The issue represents a classic buffer overflow exploitation vector through integer arithmetic manipulation, where improper validation of unsigned integer operations creates a path for malicious input to corrupt system memory management functions. Organizations should immediately update their OpenWrt systems to version 25.12.5 or later to remediate this vulnerability and prevent potential exploitation by unauthorized network entities.
The fix implemented in version 25.12.5 addresses the root cause by adding proper bounds checking and input validation before integer operations that could lead to underflow conditions. This patch ensures that message length parameters are validated against expected maximum values before being processed, preventing the overflow condition from occurring and eliminating the subsequent memory corruption scenario. Network administrators should verify their system configurations and ensure all embedded devices running OpenWrt are updated to maintain system integrity and prevent potential exploitation through local network attacks targeting this specific daemon implementation.