CVE-2026-47783: Memcached Timing Side Channel Vulnerability

CVE-2026-47783 Overview

CVE-2026-47783 is a timing side channel vulnerability in memcached versions before 1.6.42. The flaw resides in the Simple Authentication and Security Layer (SASL) password database authentication path. The sasl_server_userdb_checkpass function exits its lookup loop as soon as a valid username is matched, producing measurable timing differences between valid and invalid usernames. Attackers can remotely enumerate accounts by observing response latency. This issue is classified under [CWE-208: Observable Timing Discrepancy].

Critical Impact

Remote attackers can enumerate valid SASL usernames on memcached deployments by measuring authentication response times, enabling targeted credential attacks against confidentiality, integrity, and availability.

Affected Products

• memcached versions prior to 1.6.42
• Deployments configured with SASL password database authentication
• Applications relying on sasl_server_userdb_checkpass for user lookup

Discovery Timeline

• 2026-05-20 - CVE-2026-47783 published to NVD
• 2026-05-20 - Last updated in NVD database

Technical Details for CVE-2026-47783

Vulnerability Analysis

The vulnerability is a classic observable timing discrepancy in the SASL authentication code path of memcached. When a client submits credentials, the server iterates through the SASL user database to locate the supplied username. The loop terminates immediately on a successful username match, before the password comparison completes the full code path. This creates a difference in response latency between submitted usernames that exist and those that do not. Remote attackers can measure these timing differences across many authentication attempts to enumerate valid accounts without ever submitting correct passwords.

Root Cause

The root cause is non-constant-time username lookup logic inside sasl_server_userdb_checkpass. The loop uses early termination on a positive match instead of completing a constant-time scan over all entries. Combined with the absence of timing equalization on the password verification branch, the function leaks information about the contents of the SASL user database through measurable execution time.

Attack Vector

Attackers exploit the flaw remotely over the network by sending repeated SASL authentication requests with candidate usernames. By statistically analyzing response latency, attackers distinguish valid usernames from invalid ones. The high attack complexity reflects the need to filter network jitter through repeated sampling. Once usernames are enumerated, attackers can pivot to brute force or credential stuffing attacks targeting the confirmed accounts, increasing the likelihood of full account compromise.

No public proof-of-concept exploit code is currently available. See the memcached commit fixing the issue and the version comparison between 1.6.41 and 1.6.42 for technical details.

Detection Methods for CVE-2026-47783

Indicators of Compromise

• High volumes of SASL authentication failures from a single source or distributed sources targeting one memcached instance
• Repetitive authentication requests cycling through enumerated username lists
• Unusual patterns of short-interval connection establishment to memcached SASL ports

Detection Strategies

• Monitor memcached logs for rapid sequences of failed AUTH attempts with varying usernames
• Establish baselines for SASL authentication latency and alert on probing patterns consistent with timing analysis
• Correlate authentication failures against network telemetry to identify enumeration campaigns

Monitoring Recommendations

• Enable verbose logging for SASL authentication events on memcached servers
• Forward authentication telemetry to a centralized analytics platform for behavioral analysis
• Track running versions of memcached across the environment and flag any instance below 1.6.42

How to Mitigate CVE-2026-47783

Immediate Actions Required

• Upgrade memcached to version 1.6.42 or later on all affected hosts
• Restrict network access to memcached instances using firewall rules and internal-only segmentation
• Rotate any SASL credentials that may have been enumerated prior to patching
• Audit SASL user databases and remove unused or stale accounts to shrink the attack surface

Patch Information

The upstream fix is included in memcached 1.6.42. The corrective change is published in the memcached commit d13f282b. Refer to the memcached 1.6.42 release notes for full details on the security update and any operational changes.

Workarounds

• Disable SASL authentication where feasible and rely on network-level access controls
• Place memcached behind authenticated proxies or service meshes that enforce mutual TLS
• Apply rate limiting on authentication endpoints to reduce the practicality of timing measurement

# Configuration example: restrict memcached to localhost and rate limit external probes
memcached -l 127.0.0.1 -p 11211 -U 0
iptables -A INPUT -p tcp --dport 11211 -m conntrack --ctstate NEW -m limit --limit 10/second -j ACCEPT
iptables -A INPUT -p tcp --dport 11211 -j DROP