CVE-2026-18355

A heap buffer overflow flaw was found in the SASL I/O layer of 389 Directory Server (389-ds-base). In sasl_io_start_packet(), the wrapped-record length read from the wire is validated only against an upper bound. A small wire length (0, 1, or 2) produces an encrypted_buffer_count below the already-consumed encrypted_buffer_offset, causing an unsigned subtraction underflow in sasl_io_read_packet(). PR_Recv is then requested to read approximately 4 GiB into a 1024-byte heap buffer, resulting in a heap buffer overflow with attacker-controlled content. After a successful SASL bind with integrity protection (SSF > 0), a remote authenticated attacker can cause a denial of service or potentially achieve remote code execution. This flaw is distinct from CVE-2026-11774, whose fix only guards against upper-bound overflow.

Red Hat rates this issue as Important. After a successful SASL bind with integrity protection (SSF > 0), an authenticated attacker can send a SASL wrapped record whose length prefix is unusually small. The SASL I/O layer validates that length only against an upper size limit, so when it computes how many more bytes to read it can underflow and write past the end of its receive buffer with attacker-controlled data. This flaw is independent of CVE-2026-11774, which only rejected near-maximum length values; that fix does not cover the lower-bound case. Deployments that do not negotiate a SASL security layer after bind are not exposed on this path.

The CVSS score(s) listed for this vulnerability do not reflect the associated product's status, and are included for informational purposes to better understand the severity of this vulnerability.

Red Hat has investigated whether a possible mitigation exists for this issue, and has not been able to identify a practical example. Please update the affected package as soon as possible.