What is CVE-2026-45077?

CVE-2026-45077 is a high-severity insecure deserialization vulnerability in symfony/monolog-bridge (composer), affecting versions < 5.4.52. It is fixed in 5.4.52, 6.4.40, 7.4.12, 8.0.12. Untrusted serialized data is processed by a deserializer that can instantiate arbitrary objects or execute code as a side effect.

Which packages are affected by CVE-2026-45077?

symfony/monolog-bridge (composer) versions < 5.4.52; symfony/symfony (composer) versions < 5.4.52.

Is there a fix for CVE-2026-45077?

Yes. CVE-2026-45077 is fixed in 5.4.52, 6.4.40, 7.4.12, 8.0.12. Upgrade to this version or later.

Is CVE-2026-45077 exploitable, and should I be worried?

Whether CVE-2026-45077 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk.

What actually determines whether CVE-2026-45077 is exploitable, and how bad it is?

Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.

CVE-2026-45077 in symfony/monolog-bridge

Q: Which packages are affected by CVE-2026-45077?

symfony/monolog-bridge (composer) versions < 5.4.52; symfony/symfony (composer) versions < 5.4.52.

Q: Is there a fix for CVE-2026-45077?

Yes. CVE-2026-45077 is fixed in 5.4.52, 6.4.40, 7.4.12, 8.0.12. Upgrade to this version or later.

Q: Is CVE-2026-45077 exploitable, and should I be worried?

Whether CVE-2026-45077 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk.

Q: What actually determines whether CVE-2026-45077 is exploitable, and how bad it is?

Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.

Q: How do I fix CVE-2026-45077?

Upgrade: symfony/monolog-bridge to 5.4.52 or later, symfony/symfony to 5.4.52 or later, symfony/monolog-bridge to 6.4.40 or later, symfony/monolog-bridge to 7.4.12 or later, symfony/monolog-bridge to 8.0.12 or later, symfony/symfony to 6.4.40 or later, symfony/symfony to 7.4.12 or later, symfony/symfony to 8.0.12 or later.

Summary

Symfony has Unauthenticated PHP Object Deserialization in MonologBridge server:log Listener

Description

Symfony\Bridge\Monolog\Command\ServerLogCommand (the server:log console command) is a development-time helper that opens a TCP listener and displays log records pushed to it by the application's logging pipeline. Two unsafe defaults combine into a remotely reachable PHP object-deserialization sink:

The listener binds to 0.0.0.0:9911 by default; it accepts connections on every interface, not only loopback.
Each received frame is processed as unserialize(base64_decode($message)) without an allowed_classes allowlist, without authentication, and without any integrity check. The decoded value is then passed to displayLog(..., array $record) which assumes (without validating) that the result is an array.

Any host that can reach TCP port 9911 on a machine running server:log can therefore submit attacker-chosen serialized PHP payloads. The minimum impact is an unauthenticated denial of service (sending a non-array, e.g. serialize(new stdClass()), crashes the listener with a type error). Object injection with magic-method side effects (__wakeup() / __destruct() / etc.) is reachable before the array type-check fires; full remote code execution is environment-dependent and contingent on usable gadget chains in the autoload set of the target process.

Resolution

The server:log command no longer binds to all interfaces by default: the default --host is now 127.0.0.1:9911, requiring explicit opt-in to accept off-host traffic. Message decoding is gated by an unserialize() allowlist restricted to the Symfony\Component\VarDumper\Caster\* and Symfony\Component\VarDumper\Cloner\* classes that legitimately appear inside dumped log records; any other class is rejected and the record discarded.

The patch for this issue is available here for branch 5.4.

Credits

Symfony would like to thank Toàn Thắng and Sam Sanoop for reporting the issue and Nicolas Grekas for fixing it.

Impact

Untrusted serialized data is processed by a deserializer that can instantiate arbitrary objects or execute code as a side effect. Typical impact: arbitrary code execution or logic abuse.

Affected versions

symfony/monolog-bridge (< 5.4.52) symfony/symfony (< 5.4.52) symfony/monolog-bridge (>= 6.0.0, < 6.4.40) symfony/monolog-bridge (>= 7.0.0, < 7.4.12) symfony/monolog-bridge (>= 8.0.0, < 8.0.12) symfony/symfony (>= 6.0.0, < 6.4.40) symfony/symfony (>= 7.0.0, < 7.4.12) symfony/symfony (>= 8.0.0, < 8.0.12)

Security releases

symfony/monolog-bridge → 5.4.52 (composer) symfony/symfony → 5.4.52 (composer) symfony/monolog-bridge → 6.4.40 (composer) symfony/monolog-bridge → 7.4.12 (composer) symfony/monolog-bridge → 8.0.12 (composer) symfony/symfony → 6.4.40 (composer) symfony/symfony → 7.4.12 (composer) symfony/symfony → 8.0.12 (composer)

Kodem intelligence

Severity tells you how bad this could be in the worst case. It does not tell you whether you are exposed. Exploitability and impact are functions of runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A vulnerable package can sit in your dependency tree and never run.

Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter. Kodem's runtime-powered SCA identifies whether this CVE is reachable in your applications.

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Remediation advice

Upgrade the following packages to resolve this vulnerability:

symfony/monolog-bridge to 5.4.52 or later; symfony/symfony to 5.4.52 or later; symfony/monolog-bridge to 6.4.40 or later; symfony/monolog-bridge to 7.4.12 or later; symfony/monolog-bridge to 8.0.12 or later; symfony/symfony to 6.4.40 or later; symfony/symfony to 7.4.12 or later; symfony/symfony to 8.0.12 or later

Kodem Kai can prioritize this vulnerability in your dependency tree and generate a fix recommendation.

Frequently Asked Questions

What is CVE-2026-45077? CVE-2026-45077 is a high-severity insecure deserialization vulnerability in symfony/monolog-bridge (composer), affecting versions < 5.4.52. It is fixed in 5.4.52, 6.4.40, 7.4.12, 8.0.12. Untrusted serialized data is processed by a deserializer that can instantiate arbitrary objects or execute code as a side effect.
Which packages are affected by CVE-2026-45077?
- symfony/monolog-bridge (composer) (versions < 5.4.52)
- symfony/symfony (composer) (versions < 5.4.52)
Is there a fix for CVE-2026-45077? Yes. CVE-2026-45077 is fixed in 5.4.52, 6.4.40, 7.4.12, 8.0.12. Upgrade to this version or later.
Is CVE-2026-45077 exploitable, and should I be worried? Whether CVE-2026-45077 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
What actually determines whether CVE-2026-45077 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
How do I fix CVE-2026-45077?
- Upgrade symfony/monolog-bridge to 5.4.52 or later
- Upgrade symfony/symfony to 5.4.52 or later
- Upgrade symfony/monolog-bridge to 6.4.40 or later
- Upgrade symfony/monolog-bridge to 7.4.12 or later
- Upgrade symfony/monolog-bridge to 8.0.12 or later
- Upgrade symfony/symfony to 6.4.40 or later
- Upgrade symfony/symfony to 7.4.12 or later
- Upgrade symfony/symfony to 8.0.12 or later