CVE-2026-53994 ProFTPD mod_sftp Heap Buffer Overflow via Unsigned Integer Underflow and Size Truncation

High 7.7 Published 18 Jul 2026 Updated 28 Jul 2026

An authenticated SFTP user can trigger a heap buffer overflow in ProFTPD, causing a denial of service via a malformed packet.

Exploited in the wild: Not known
Login needed: Any signed-in user
Attackable: Over the internet
Victim action needed: Not needed
Fix: Available

What is CVE-2026-53994?

CVE-2026-53994 is a high-severity memory corruption vulnerability in ProFTPD, published on 18 Jul 2026. Any signed-in user can exploit it over the network, with no action from the victim.

Official description

ProFTPD mod_sftp contains a heap-based buffer overflow reachable by an authenticated SFTP user. The fxp_packet_read() function accepts the attacker-supplied 32-bit big-endian SFTP packet length without a minimum sanity check. A value of 0 causes an unsigned subtraction elsewhere in the read path to underflow to approximately 4 GB. That oversized request reaches the core memory allocator, where the rounded size is computed in size_t but passed to new_block() as a 32-bit int; the low 32 bits of 0x100000000 are 0, so new_block() returns a small (~512-byte) block while the caller is told it received ~4 GB. The subsequent fill loop then streams attacker-controlled bytes past the end of the 544-byte allocation, producing an attacker-controlled heap buffer overflow. An authenticated user can crash the per-connection ProFTPD session child on demand with a single malformed SFTP packet (packet_len=0 followed by a body greater than approximately 544 bytes), producing reliable authenticated remote denial of service. Depending on heap layout and adjacent allocations, heap metadata corruption and further consequences beyond denial of service may be possible, though only denial of service is demonstrated by the supplied proof of concept.

Weakness: CWE-122

Which versions are affected by CVE-2026-53994?

It affects 1 product.

Products include Proftpd Proftpd.

How do I fix CVE-2026-53994?

Apply the vendor's patch.

Upgrade to the fixed version. Memory-safe languages, compiler hardening and fuzzing prevent most new cases. More about Memory corruption.

Is CVE-2026-53994 being exploited?

There is no known exploitation of CVE-2026-53994 in the wild and no public exploit code.

Sources

Also known as DEBIAN-CVE-2026-53994, GHSA-435f-wcrw-9x3x, UBUNTU-CVE-2026-53994.