---
id: "CVE-2026-121623"
page: "https://infrastack.ai/live/CVE-2026-121623"
kind: "vulnerability"
aliases: ["CVE-2026-121623"]
title: "Symfony Serializer: uppercase data: scheme lets requests read local files"
severity: "high"
cvss: 8.2
epss: null
exploited: false
status: "open"
category: "package"
products: []
ecosystems: []
tags: ["package", "php", "symfony", "symfony-serializer"]
not_affected_if: ["no-file-type-denormalization", "no-untrusted-deserialization"]
published_at: "2026-11-06T16:17:52+00:00"
first_seen_at: "2026-11-06T16:42:41+00:00"
updated_at: "2026-11-06T16:44:35+00:00"
sources: [{"source": "nvd", "url": "https://nvd.nist.gov/vuln/detail/CVE-2026-121623", "first_seen_at": "2026-11-06T16:42:41+00:00"}]
exploit_sources: []
---

# Symfony Serializer: uppercase data: scheme lets requests read local files

Link for people: https://infrastack.ai/live/CVE-2026-121623 (this .md is for agents)

The Symfony Serializer's DataUriNormalizer checks for the data: scheme case-sensitively, so a value like DATA:,/../../../etc/passwd is treated as a file path instead of inline data. An attacker who can reach a deserializing endpoint can read arbitrary local files the application can access.

_Summary, check and fix written by a model from the sources; labelled as such._

## How to check

- Check the installed version with `composer show symfony/serializer` and compare it to the fixed ranges below.
- Look for endpoints that deserialize untrusted input into File, SplFileInfo or SplFileObject types.

## Fix

- Upgrade symfony/serializer to `5.4.56`, `6.4.50`, `7.4.23` or `8.1.11` depending on the major line in use.

## Timeline

- 2026-11-06T16:42:41+00:00 nvd https://nvd.nist.gov/vuln/detail/CVE-2026-121623

## Documents

- [advisory] Vendor advisory: github.com https://github.com/symfony/symfony/security/advisories/GHSA-pxj9-jh84-vhm5
- [fix] Patch: github.com https://github.com/symfony/symfony/commit/a0c551861ac23f888dfe6d1157ca78f4e220f3b7

## From the sources

The Symfony Serializer component DataUriNormalizer::denormalize() matches the data: scheme with a case-sensitive regular expression, allowing a value with an uppercase scheme, such as DATA:,/../../../etc/passwd, to bypass it, resulting in Arbitrary Local File Read through Path Traversal in Symfony versions prior to 5.4.56, 6.x prior to 6.4.50, 7.x prior to 7.4.23, and 8.x prior to 8.1.11.

A value whose scheme is not lowercase data: are treated as a local file path and opened instead of being decoded as inline data, so the resulting File, \SplFileInfo, or \SplFileObject points at a file on the server, which the application then handles as the uploaded content.

Any endpoint that deserializes untrusted input into one of these types is affected. FrameworkBundle registers DataUriNormalizer ahead of ObjectNormalizer by default. A single request with a crafted DATA: value is enough to read arbitrary local files that the application can access, with no authentication needed if the endpoint is public.

---

_Free to read, with fair use of about 60 pages a minute. For higher limits or API access: sales@infrastack.ai_