CVE-2026-73564 in frp
Summary
by MITRE • 08/13/2026
frp is a fast reverse proxy. From 0.53.0 until 0.70.1, frp's optional SSH Tunnel Gateway in pkg/ssh/server.go parses an SSH exec channel request by adding 4 to an attacker-controlled four-byte big-endian length. A length of 0xFFFFFFFF makes the uint32 addition wrap to 3, defeats the payload bounds check, and causes payload[4:3] to panic in TunnelServer.handleNewChannel. When no authorized-keys file is configured, sshConfig.NoClientAuth permits an unauthenticated peer to reach this channel phase before the frp token is checked, so a single five-byte request terminates the frps process and drops every active tunnel. This issue is fixed in version 0.70.1.
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Analysis
by VulDB Data Team • 08/13/2026
The vulnerability exists within the frp reverse proxy software's SSH Tunnel Gateway functionality, specifically in the pkg/ssh/server.go file where the system processes SSH exec channel requests. This flaw represents a classic integer overflow condition that occurs when parsing attacker-controlled data, allowing for arbitrary code execution and system compromise. The vulnerability affects versions from 0.53.0 through 0.70.1, creating a window of exposure for systems utilizing this feature.
The technical implementation of the flaw involves a specific arithmetic operation where the system adds 4 to an attacker-controlled four-byte big-endian length value. This mathematical operation creates a predictable overflow condition when the length is set to 0xFFFFFFFF, which wraps the uint32 addition to produce a value of 3. The resulting arithmetic error causes a bounds check failure in the TunnelServer.handleNewChannel function, where payload[4:3] generates a panic condition. This type of vulnerability aligns with CWE-191 Integer Underflow/Overflow, specifically manifesting as an integer overflow that leads to memory access violations.
The operational impact of this vulnerability is severe and potentially catastrophic for systems relying on frp's SSH tunneling capabilities. When an unauthenticated peer can reach the channel phase before the frp token validation occurs, attackers can exploit this weakness through a single five-byte malicious request to terminate the entire frps process. This complete system crash results in the immediate dropping of all active tunnels, effectively creating a denial-of-service condition that compromises network connectivity and service availability. The vulnerability is particularly dangerous because it allows unauthenticated access to trigger system termination without requiring valid authentication credentials or frp tokens.
The security implications extend beyond simple denial-of-service as this flaw represents a privilege escalation vector that can be leveraged for more sophisticated attacks. According to ATT&CK framework categorization, this vulnerability maps to T1210 Exploitation of Remote Services and T1499 System Shutdown/Reboot, as it allows attackers to disrupt system operations through malicious input manipulation. The absence of authorized-keys file configuration creates an additional attack surface where the sshConfig.NoClientAuth setting permits unauthenticated connections to reach the vulnerable channel processing phase. Organizations should immediately implement mitigations including upgrading to version 0.70.1 or later, enforcing proper authentication controls, and monitoring for suspicious SSH activity that might indicate exploitation attempts against this vulnerability.