Executive brief
ProFTPD is a widely used file transfer server. A vulnerability in its SFTP module allows a logged-in user to crash the server session by sending a specially crafted data packet. While primarily resulting in a service outage for that user, this flaw could potentially lead to more severe system compromise depending on how the server's memory is organized.
Technical details
A heap-based buffer overflow exists in the mod_sftp module of ProFTPD within the fxp_packet_read() function. The vulnerability is triggered when the function accepts an attacker-supplied 32-bit SFTP packet length of 0, leading to an unsigned integer underflow during size calculations. This results in a request for approximately 4GB of memory; however, due to an integer truncation when passing the size to the new_block() allocator as a 32-bit int, a small (~512-byte) buffer is actually allocated. Subsequent data reads then overflow this small buffer with up to 4GB of attacker-controlled data. While the primary impact is a reliable remote denial of service (crashing the session child process), heap metadata corruption may allow for more advanced exploitation. The issue is fixed in version 1.3.10.
Affected products
- ProFTPD Project ProFTPD < 1.3.10
Timeline
- 2026-07-18: disclosed
- 2026-07-18: advisory