Symfony: DataUriNormalizer Opens Data URIs With a Non-Lowercase Scheme as Local Files, Letting a Client Read Any Server File
Affects Symfony applications that deserialize client data into File, SplFileInfo or SplFileObject using the Symfony Serializer. Fixed in 5.4.56, 6.4.50, 7.4.23 and 8.1.11.
Written by anthropic/claude-haiku-5.5 from the sources below. Verify against them.
- Exploited
- No known exploitation
- Fix
- Fixed in 5.4.56, 6.4.50, 7.4.23, 8.1.11
- Who is exposed
- Default setups
Do first: Check every install: default setups are affected.
- Last modified
- A 0-100 score for how much a CVE matters across the industry: exploitation, exposure, how widely the software is used, severity and whether a fix exists.What do these mean? →
- 40/100 · provisional
- How many real-world setups can hit the bug, from the vendor's advisory and CISA data. It does not know your setup.What do these mean? →
- Default setups
- Whether working attack code is public (Exploit-DB, Metasploit, Nuclei or GitHub). Public code makes attacks easier.What do these mean? →
- None on Exploit-DB, Metasploit or GitHub
Summary
Symfony's Serializer component, which deserializes untrusted input through DataUriNormalizer, can be tricked into reading arbitrary files from the server's local disk. A remote attacker can do this without logging in by sending a single request with a data URI whose scheme is written in uppercase, such as "DATA:", which bypasses the case-sensitive check and is opened as a file path.
Read the original description
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.
cve.now Index
- Exploitation?/40Unknown. No CISA SSVC exploitation rating yet
- Exposure22/25Default setups, reachable over the network without a login
- Reach10/20Affects Packagist packages
- Severity8.2/10CVSS 8.2
- No patch0/5No signal
What would change the score
- Would score about 80 if added to CISA KEV
- Would score about 75 if CISA's SSVC rates exploitation active
What to do
Check every install: default setups are affected.
Upgrade to a fixed version: 5.4.56, 6.4.50, 7.4.23, 8.1.11.
Timeline
- CVE published
- First seen by cve.now
Affected versions
- symfony/serializerPackagist
fixed in 5.4.56; From 6.0.0 · fixed in 6.4.50; From 7.0.0 · fixed in 7.4.23; From 8.0.0 · fixed in 8.1.11
Evidence 2
- Advisory
- Patch
Corrections
None so far.
Something wrong on this page? Report an error. Every fix is listed here.
Questions about CVE-2026-121623
Is CVE-2026-121623 being exploited?
It is not in CISA's Known Exploited Vulnerabilities catalog, and CISA's SSVC assessment does not mark exploitation as active. That does not mean it has not been attacked: vendors and researchers can report attacks before CISA lists them. No public exploit code is tracked.Source: cve.now feed data
What does this mean?
A data URI with a non-lowercase scheme, such as DATA:, is opened as a local file instead of being decoded as data. A client can use this to read files the application can access on the server, such as /etc/hostname.Source: github.com
Which versions are affected by CVE-2026-121623?
symfony/serializer (Packagist): before 5.4.56; from 6.0.0 before 6.4.50; from 7.0.0 before 7.4.23; from 8.0.0 before 8.1.11.Source: cve.now feed data
Am I affected, and how do I check?
Symfony versions before 5.4.56, from 6.0.0 before 6.4.50, from 7.0.0 before 7.4.23, and from 8.0.0 before 8.1.11. The setup involved is an application that deserializes client data into File, SplFileInfo or SplFileObject with the Symfony Serializer. To check, look at the installed symfony/serializer or symfony/symfony version in composer.lock and search the code for deserialize() calls that target those types.Source: github.com
How do I fix CVE-2026-121623?
Upgrade to the fixed release for your branch: 5.4.56, 6.4.50, 7.4.23 or 8.1.11.Source: github.com