# CVE-2023-51764: Security feature bypass in n/a > Remote attackers can bypass SPF protection and spoof email senders in Postfix if specific SMTP smuggling mitigations are not configured. - **Severity:** Medium 5.3 (`CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:L/A:N`) - **Exploited in the wild:** Not known - **Login needed:** Not needed - **Attackable:** Over the internet - **Victim action needed:** Not needed - **Fix available:** Yes - **Published:** 24 Dec 2023 - **Updated:** 8 Oct 2026 - **Also known as:** ALSA-2024:9243, AZL-32296, AZL-35110, CLSA-2025-1738267482, CLSA-2026-1778834899, CLSA-2026-1778834936, DEBIAN-CVE-2023-51764, DLA-3725-1, GHSA-j5jm-hg4x-w8rx, MGASA-2024-0029, RHSA-2024:9243, RLSA-2024:9243, SUSE-SU-2023:4981-1, SUSE-SU-2024:0012-1, SUSE-SU-2024:1149-1, UBUNTU-CVE-2023-51764, USN-6591-1, USN-6591-2, openSUSE-SU-2024:13538-1 ## What is CVE-2023-51764? CVE-2023-51764 is a medium-severity vulnerability in Fedoraproject Fedora, published on 24 Dec 2023. An attacker can exploit it over the internet without logging in, with no action from the victim. ### Official description Postfix through 3.8.5 allows SMTP smuggling unless configured with smtpd_data_restrictions=reject_unauth_pipelining and smtpd_discard_ehlo_keywords=chunking (or certain other options that exist in recent versions). Remote attackers can use a published exploitation technique to inject e-mail messages with a spoofed MAIL FROM address, allowing bypass of an SPF protection mechanism. This occurs because Postfix supports . but some other popular e-mail servers do not. To prevent attack variants (by always disallowing without ), a different solution is required, such as the smtpd_forbid_bare_newline=yes option with a Postfix minimum version of 3.5.23, 3.6.13, 3.7.9, 3.8.4, or 3.9. Weakness: CWE-345 (Insufficient Verification of Data Authenticity) ## Which versions are affected by CVE-2023-51764? It affects 3 products. | Package | Ecosystem | Affected | Fixed in | | --- | --- | --- | --- | | `postfix` | AlmaLinux:9 | All versions | 2:3.5.25-1.el9 | | `postfix-cdb` | AlmaLinux:9 | All versions | 2:3.5.25-1.el9 | | `postfix-ldap` | AlmaLinux:9 | All versions | 2:3.5.25-1.el9 | | `postfix-lmdb` | AlmaLinux:9 | All versions | 2:3.5.25-1.el9 | | `postfix-mysql` | AlmaLinux:9 | All versions | 2:3.5.25-1.el9 | | `postfix-pcre` | AlmaLinux:9 | All versions | 2:3.5.25-1.el9 | | `postfix-perl-scripts` | AlmaLinux:9 | All versions | 2:3.5.25-1.el9 | | `postfix-pgsql` | AlmaLinux:9 | All versions | 2:3.5.25-1.el9 | | `postfix-sqlite` | AlmaLinux:9 | All versions | 2:3.5.25-1.el9 | | `postfix` | Red Hat:enterprise_linux:9::appstream | All versions | 2:3.5.25-1.el9 | | `postfix-cdb` | Red Hat:enterprise_linux:9::appstream | All versions | 2:3.5.25-1.el9 | | `postfix-cdb-debuginfo` | Red Hat:enterprise_linux:9::appstream | All versions | 2:3.5.25-1.el9 | | `postfix-debuginfo` | Red Hat:enterprise_linux:9::appstream | All versions | 2:3.5.25-1.el9 | | `postfix-debugsource` | Red Hat:enterprise_linux:9::appstream | All versions | 2:3.5.25-1.el9 | | `postfix-ldap` | Red Hat:enterprise_linux:9::appstream | All versions | 2:3.5.25-1.el9 | | `postfix-ldap-debuginfo` | Red Hat:enterprise_linux:9::appstream | All versions | 2:3.5.25-1.el9 | | `postfix-lmdb` | Red Hat:enterprise_linux:9::appstream | All versions | 2:3.5.25-1.el9 | | `postfix-lmdb-debuginfo` | Red Hat:enterprise_linux:9::appstream | All versions | 2:3.5.25-1.el9 | | `postfix-mysql` | Red Hat:enterprise_linux:9::appstream | All versions | 2:3.5.25-1.el9 | | `postfix-mysql-debuginfo` | Red Hat:enterprise_linux:9::appstream | All versions | 2:3.5.25-1.el9 | | `postfix-pcre` | Red Hat:enterprise_linux:9::appstream | All versions | 2:3.5.25-1.el9 | | `postfix-pcre-debuginfo` | Red Hat:enterprise_linux:9::appstream | All versions | 2:3.5.25-1.el9 | | `postfix-perl-scripts` | Red Hat:enterprise_linux:9::appstream | All versions | 2:3.5.25-1.el9 | | `postfix-pgsql` | Red Hat:enterprise_linux:9::appstream | All versions | 2:3.5.25-1.el9 | | `postfix-pgsql-debuginfo` | Red Hat:enterprise_linux:9::appstream | All versions | 2:3.5.25-1.el9 | | `postfix-sqlite` | Red Hat:enterprise_linux:9::appstream | All versions | 2:3.5.25-1.el9 | | `postfix-sqlite-debuginfo` | Red Hat:enterprise_linux:9::appstream | All versions | 2:3.5.25-1.el9 | | `postfix` | Rocky Linux:9 | All versions | 2:3.5.25-1.el9 | | `postfix` | SUSE:Enterprise Storage 7.1 | All versions | 3.5.9-150300.5.15.1 | | `postfix-bdb` | SUSE:Enterprise Storage 7.1 | All versions | 3.5.9-150300.5.15.1 | | `postfix` | SUSE:Linux Enterprise High Performance Computing 15 SP3-ESPOS | All versions | 3.5.9-150300.5.15.1 | | `postfix-bdb` | SUSE:Linux Enterprise High Performance Computing 15 SP3-ESPOS | All versions | 3.5.9-150300.5.15.1 | | `postfix` | SUSE:Linux Enterprise High Performance Computing 15 SP3-LTSS | All versions | 3.5.9-150300.5.15.1 | | `postfix-bdb` | SUSE:Linux Enterprise High Performance Computing 15 SP3-LTSS | All versions | 3.5.9-150300.5.15.1 | | `postfix` | SUSE:Linux Enterprise High Performance Computing 15 SP4-ESPOS | All versions | 3.5.9-150300.5.15.1 | | `postfix-bdb` | SUSE:Linux Enterprise High Performance Computing 15 SP4-ESPOS | All versions | 3.5.9-150300.5.15.1 | | `postfix` | SUSE:Linux Enterprise High Performance Computing 15 SP4-LTSS | All versions | 3.5.9-150300.5.15.1 | | `postfix-bdb` | SUSE:Linux Enterprise High Performance Computing 15 SP4-LTSS | All versions | 3.5.9-150300.5.15.1 | | `postfix` | SUSE:Linux Enterprise Module for Basesystem 15 SP4 | All versions | 3.5.9-150300.5.15.1 | | `postfix` | SUSE:Linux Enterprise Module for Basesystem 15 SP5 | All versions | 3.7.3-150500.3.11.1 | Products: Fedoraproject Fedora, Postfix Postfix, Redhat Enterprise Linux. ## How do I fix CVE-2023-51764? Change a configuration setting. ## Is CVE-2023-51764 being exploited? There is no known exploitation of CVE-2023-51764 in the wild and no public exploit code. Public exploit code: - Postfix SMTP Smuggling - Expect Script POC (github_poc): https://github.com/duy-31/CVE-2023-51764 - PoC CVE-2023-51764 (github_poc): https://github.com/eeenvik1/CVE-2023-51764 - just idea, no cp pls (github_poc): https://github.com/d4op/CVE-2023-51764-POC - CVE-2023-51764 poc (github_poc): https://github.com/Double-q1015/CVE-2023-51764 ## Other vulnerabilities in Fedoraproject Fedora - [CVE-2024-4577](https://insights.sbomify.com/CVE-2024-4577/): PHP-CGI OS Command Injection (critical 9.8, exploited in the wild) - [CVE-2021-39226](https://insights.sbomify.com/CVE-2021-39226/): Grafana Authentication Bypass (critical 9.8, exploited in the wild) - [CVE-2019-5544](https://insights.sbomify.com/CVE-2019-5544/): VMware ESXi and Horizon DaaS OpenSLP Heap-Based Buffer Overflow (critical 9.8, exploited in the wild) - [CVE-2024-4947](https://insights.sbomify.com/CVE-2024-4947/): Google Chromium V8 Type Confusion (critical 9.6, exploited in the wild) - [CVE-2024-4671](https://insights.sbomify.com/CVE-2024-4671/): Google Chromium Visuals Use-After-Free (critical 9.6, exploited in the wild) ## Fix prompt for AI coding agents ```text Check this codebase for CVE-2023-51764 and fix it if it is affected. What it is: CVE-2023-51764 is a medium-severity vulnerability in Fedoraproject Fedora, published on 24 Dec 2023. An attacker can exploit it over the internet without logging in, with no action from the victim. Exploitation: There is no known exploitation of CVE-2023-51764 in the wild and no public exploit code. Affected product: Fedoraproject Fedora. No package-level version data is available, so check the vendor advisory linked from the reference page. Task: 1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies. 2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it. 3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead. 4. Run the tests and summarise what changed and why. 5. If nothing in the project is affected, say so and change nothing. Reference: https://insights.sbomify.com/CVE-2023-51764/ (Markdown: https://insights.sbomify.com/CVE-2023-51764/index.md) ``` ## Sources - CVE record: https://www.cve.org/CVERecord?id=CVE-2023-51764 - NVD: https://nvd.nist.gov/vuln/detail/CVE-2023-51764 - OSV: https://osv.dev/vulnerability/CVE-2023-51764 --- From sbomify Insights: https://insights.sbomify.com/CVE-2023-51764/ Summaries are generated and may contain mistakes; the official description above is the record.