CVE-2026-63091: ProFTPD mod_sftp Signed Integer Overflow via SCP Size-Record Parser

Vulnerability summary

CVE-2026-63091 is a high (CVSS 7.1) Buffer Over-read vulnerability in proftpd. EPSS estimates a 0.53% chance of exploitation in the next 30 days, higher than 44.5% of scored vulnerabilities.

Timeline

1. 20-07-2026
   PublishedVulnerability first disclosed.
2. 28-07-2026
   Last modifiedVulnerability information updated.

Description

ProFTPD before 1.3.9c and 1.3.10rc3 contains a signed integer overflow vulnerability in the mod_sftp module's SCP size-record parser that allows authenticated low-privilege attackers to bypass ASLR by sending a crafted file size value of UINT64_MAX, which results in a negative off_t value. Attackers can exploit the subsequent conversion to uint32_t, causing an approximately 4 GB requested read length and forcing the server to read beyond the end of the SSH channel data and write overread process memory into the uploaded file. In tested configurations, the disclosed data contains libc, libcrypto, and PIE pointers sufficient to derive their randomized base addresses, thereby bypassing ASLR and enabling reliable exploitation of memory corruption vulnerabilities in the same process.

• CWE-126Buffer Over-readThe product reads from a buffer using buffer access mechanisms such as indexes or pointers that reference memory locations after the targeted buffer.
• CWE-190Integer Overflow or WraparoundThe product performs a calculation that can produce an integer overflow or wraparound when the logic assumes that the resulting value will always be larger than the original value. This occurs when an integer value is incremented to a value that is too large to store in the associated representation. When this occurs, the value may become a very small or negative number.