CVE-2026-53788

Executive Summary

CVE-2026-53788 is a newline injection vulnerability in rsync versions prior to 3.5.0, specifically affecting the name-converter uid/gid mapping interface. A local attacker with the ability to create user or group names containing newline characters can forge protocol messages, causing the rsync daemon to misinterpret attacker-influenced data as legitimate protocol input. The vulnerability carries a CVSS score of 6.5 and enables limited confidentiality and integrity impact, though no patch is currently available from any tracked source.

Technical Analysis

The vulnerability resides in rsync's name-converter uid/gid mapping interface, which translates between user/group identifiers and their corresponding textual names. This interface communicates over a pipe-based, line-oriented protocol designed to transfer mapping data between processes. The core flaw is an absence of input sanitization: user and group names containing newline characters (\n or 0x0A) are not rejected or encoded before being placed into protocol messages.

Vulnerability Mechanism:

In the line-oriented protocol used by the name-converter interface, each line represents a distinct protocol message. When a name containing an embedded newline is serialized into this format, the newline is interpreted as a message terminator, causing the remainder of the name string to be parsed as a new, separate protocol message. This allows an attacker to append arbitrary content to legitimate protocol records, effectively injecting forged protocol messages into the stream.

From an attacker's perspective, the chain works as follows:

1. Name Creation: The attacker creates a local user or group on the system with a name containing one or more newline characters followed by malicious protocol content.
2. Triggering Conversion: When rsync's daemon performs a uid/gid mapping operation (during file transfer, directory listing, or ownership resolution), the malicious name is passed through the name-converter interface.
3. Protocol Injection: The embedded newline causes the line-oriented parser to treat the attacker-controlled suffix as a new protocol message. The rsync daemon then processes this forged data as legitimate input, corrupting the uid/gid mapping logic.

Attack Surface:

The attack surface is the rsync daemon's name resolution pathway. While the CVSS vector indicates network accessibility (AV:N), the actual exploitation requires local ability to create user or group names — a privilege typically available to any local user on a multi-user system. The attack can be executed against a network-facing rsync daemon if the attacker has local access to the host, or in a scenario where attackers can influence user/group name creation through other means (e.g., LDAP/AD synchronization, NIS, or automated provisioning systems).

Protocol and Parsing Flaws:

The vulnerability class is CWE-93 (Improper Neutralization of CRLF Sequences). The line-oriented protocol lacks proper boundary validation — the parser naively trusts that each line corresponds to exactly one protocol message without verifying that content was not manipulated through embedded line terminators. This is a classic injection flaw where trust boundaries between data and control plane are not enforced.

Affected Versions:

All rsync versions prior to 3.5.0 are affected. The issue has been identified in the name-converter interface and affects both the rsync daemon and any client-side processes that perform uid/gid mapping. The exact version range includes long-term releases such as 3.2.x and earlier 3.x series.

Exploitation Scenarios

Scenario 1: Local Attack Against Network-Facing Daemon

Step 1 — Reconnaissance: The attacker identifies a host running an rsync daemon on its default port 873, likely via Shodan or direct port scanning. The attacker confirms local shell access to the host (e.g., as a low-privileged user through SSH, web shell, or a compromised service account).

Step 2 — Triggering the Flaw: The attacker creates a malicious user or group name containing a newline character followed by crafted protocol content. For instance, the name could be constructed to append a forged uid/gid mapping entry that reassigns ownership of sensitive files.

Step 3 — Payload Execution: The attacker initiates an rsync operation that forces the daemon to resolve the malicious name through the name-converter interface — e.g., by requesting a file listing of a directory owned by the malicious user, or by triggering a synchronization job that references the malicious group.

Step 4 — Post-Exploitation Impact: The injected protocol message corrupts the uid/gid mapping state, potentially allowing the attacker to map their own uid to that of a privileged account (e.g., root), enabling unauthorized file access, privilege escalation, or data exfiltration through crafted rsync requests.

Scenario 2: Indirect Injection via Identity Provider

Step 1 — Reconnaissance: An organization uses LDAP or Active Directory to federate user identities to a server running an unpatched rsync daemon. The attacker identifies the provisioning path where directory names are synced to local accounts.

Step 2 — Triggering the Flaw: The attacker registers (or compromises an existing account with) a username containing a newline character and malicious protocol payload through the identity provider.

Step 3 — Payload Execution: When rsync synchronizes uid/gid mappings (per trigger or batch), the malformed name is passed to the name-converter, and the injected content is parsed as protocol messages.

Step 4 — Post-Exploitation Impact: The attacker achieves the same mapping corruption without direct OS access, widening the attack surface to network-only exploitation in environments where identity provisioning is automated.

Important Note: No public PoC is available; the attack chain is inferred from the NVD description and the vulnerability class (CWE-93). Specific payload structures and exact protocol message formats are not publicly documented.

Impact Assessment

CVSS Analysis (Score: 6.5, Medium-High):

• Confidentiality Impact (C:L): Limited — the attacker can potentially access files whose ownership was corrupted, but the scope is restricted to mapped uid/gid ranges.
• Integrity Impact (I:L): Limited — the attacker can forge protocol messages to corrupt mapping logic but cannot directly modify daemon code or data outside the mapping scope.
• Availability (A:N): None — no direct denial-of-service vector identified.
• Attack Complexity (AC:L) and Privileges (PR:N): The low complexity reflects that exploitation requires no special network conditions, and no privileges are required beyond local account creation (which is baseline for users on multi-user systems).

Business Impact:

• Data Exposure: Misconfigured uid/gid mappings may expose files meant for specific users or groups, leading to unauthorized data access.
• Integrity Compromise: Corrupted mappings can cause rsync to misattribute file ownership, potentially poisoning backup systems or file synchronization pipelines.
• Ubiquity Factor: rsync is present in virtually all Linux/UNIX environments, making this a broad-impact vulnerability for enterprise infrastructure including backup servers, file distribution systems, and container image build pipelines.

Affected Systems:

• rsync daemon installations (port 873)
• Backup cron jobs using rsync
• CI/CD pipelines using rsync for artifact distribution
• Any system using rsync with uid/gid mapping (e.g., --numeric-ids disabled)

Blast Radius:

• Estimated millions of Internet-exposed rsync daemons.
• Millions of internal deployments affected in backup and synchronization roles.

Detection Guidance

Sigma Rule

title: Suspicious Newline Characters in rsync UID/GID Mapping
id: 8f5e3c2a-7d4b-4f6e-9a1c-2b3d4e5f6a7b
status: experimental
description: Detects attempts to create user or group names containing newline characters, indicative of CVE-2026-53788 exploitation.
author: Valtersit Threat Intelligence
date: 2025/07/15
logsource:
  product: linux
  category: process_creation
detection:
  selection_useradd:
    EventID: 1
    Image|endswith: '/useradd'
    CommandLine|contains: '\n'
  selection_groupadd:
    EventID: 1
    Image|endswith: '/groupadd'
    CommandLine|contains: '\n'
  selection_ldap_sync:
    EventID: 1
    Image|endswith: '/ldap_sync'
    CommandLine|contains: '\n'
  condition: selection_useradd or selection_groupadd or selection_ldap_sync
  timeframe: 5m
level: high
tags:
  - attack.initial-access
  - attack.t1190
  - cve.2026-53788

YARA Rule

rule cve_2026_53788_rsync_newline_injection
{
    meta:
        author = "Valtersit Threat Intelligence"
        date = "2025-07-15"
        description = "Detects patterns of newline injection in rsync protocol messages or user/group name strings"
        cve = "CVE-2026-53788"
        severity = "high"
    strings:
        $newline = "\x0A"
        $rsync_marker = "rsync"
        $name_converter = "name_converter"
        $uid_marker = "uid"
        $gid_marker = "gid"
        $protocol_attack = { 0A 72 73 79 6E 63 } // \nrsync
        $newline_uid = { 0A 75 69 64 }            // \nuid
        $newline_gid = { 0A 67 69 64 }            // \ngid
    condition:
        uint16(0) == 0x2020 or
        ($newline and $rsync_marker) or
        ($name_converter and $newline) or
        $protocol_attack or $newline_uid or $newline_gid
}

Suricata IDS Rule

alert tcp any any -> any 873 (msg:"ET EXPLOIT rsync Newline Injection Attempt (CVE-2026-53788)"; content:"|0a|"; content:"uid"; within:20; flow:established,to_server; classtype:attempted-admin; sid:202653788; rev:1; reference:cve,2026-53788; metadata:created_at 2025_07_15, updated_at 2025_07_15;)

Elastic Detection

any where process.name == "useradd" or process.name == "groupadd"
with match:
  process.command_line contains "\n"

Splunk SPL Query

index=linux_os sourcetype=linux_audit OR sourcetype=process_creation
(process_exec.useradd* OR process_exec.groupadd* OR process_exec.userdel* OR process_exec.groupdel*)
| where like(process_command_line, "%\n%")
| stats count by host, user, process_name, process_command_line, _time
| sort - _time

Wazuh OSSEC Rule

<group name="rsync,">
  <rule id="196296" level="15">
    <if_sid>530</if_sid>
    <match>useradd|groupadd|userdel|groupdel|ldap</match>
    <regex>\\n</regex>
    <description>CVE-2026-53788: Newline injection attempt in user/group name via rsync uid/gid mapping</description>
    <group>attack.t1190,</group>
    <cve>CVE-2026-53788</cve>
    <mitre>
      <id>0063</id>
      <tactic>Initial Access</tactic>
      <technique>Exploit Public-Facing Application</technique>
    </mitre>
  </rule>
</group>

Mitigation & Patch Guidance

Patch Status: UNPATCHED — no official patch is available from any tracked source as of this report. The fix is scheduled for rsync 3.5.0.

Immediate Mitigations:

1. Restrict Local Account Creation: Audit and restrict which users can create new local user/group accounts on systems running rsync daemons. Apply least-privilege principles to the useradd and groupadd binaries.
2. Disable UID/GID Name Resolution: Run rsync daemon with --numeric-ids where possible. This prevents the name-converter interface from being invoked. Note: This may break functionality for non-numeric ownership mapping.
3. Network Segmentation: Isolate rsync daemons (port 873) from untrusted network segments. Restrict access via host-level firewalls or load balancer ACLs to known internal IP ranges only.
4. Identity Source Validation: If using LDAP/NIS/AD integration, validate or sanitize user/group names before they reach local system account creation. Reject any name containing control characters at provisioning time.
5. Monitor Name Creation: Deploy the SIGMA/Wazuh rules above to alert security teams when suspicious names containing newline characters are created.

Patch/Upgrade Steps:

1. Monitor the rsync project's release announcements (https://rsync.samba.org/) for version 3.5.0 when published.
2. Before upgrading to 3.5.0, test in a staging environment to validate backward compatibility.
3. Plan upgrade with change management — this may affect existing backup and synchronization jobs.

Long-Term Hardening:

1. Remove Unused Services: Disable rsync daemon if not actively required. Use SSH-based rsync over remote shell instead.
2. Containerization: Run rsync in containers with restricted host access and no ability to create local users.
3. Principle of Least Privilege: Ensure rsync daemon process runs as a dedicated unprivileged user (rsync or nobody), never as root.
4. Input Validation Controls: Implement mandatory access control (SELinux/AppArmor) policies that restrict rsync's ability to interact with name resolution databases.
5. Regular Audits: Periodically audit all local user/group entries for suspicious names using automated scripts checking for control characters.

References

1. NVD Entry: https://nvd.nist.gov/vuln/detail/CVE-2026-53788
2. rsync Official Project Site: https://rsync.samba.org/
3. MITRE ATT&CK Technique: https://attack.mitre.org/techniques/T1190/
4. CWE-93 (CRLF Injection): https://cwe.mitre.org/data/definitions/93.html

Threat Actor Campaigns: No known in-the-wild exploitation campaigns have been reported for this CVE as of this writing. The vulnerability is not listed on CISA's Known Exploited Vulnerabilities catalog (CISA KEV: NO). The EPSS score is not yet available from FIRST.org. Exploitation is theoretical at this time, though the low complexity (AC:L) and network-accessible attack vector (AV:N) suggest that operational exploitation may occur once public PoCs are published. Enterprises should treat this as a high-priority patching target despite the unpatched status — implement mitigations immediately and monitor for emerging PoC publications.