UltraJSON has a Memory Leak parsing large integers allows DoS
UltraJSON is a fast JSON encoder and decoder written in pure C with bindings for Python 3.7+. Versions 5.4.0 through 5.11.0 contain an accumulating memory leak in JSON parsing large (outside of the range [-2^63, 2^64 - 1]) integers. The leaked memory is a copy of the string form of the integer plus an additional NULL byte. The leak occurs irrespective of whether the integer parses successfully or is rejected due to having more than sys.get_int_max_str_digits() digits, meaning that any sized leak per malicious JSON can be achieved provided that there is no limit on the overall size of the payload. Any service that calls ujson.load()/ujson.loads()/ujson.decode() on untrusted inputs is affected and vulnerable to denial of service attacks. This issue has been fixed in version 5.12.0.
Noroxi analysis
No Noroxi analysis for this record yet
We don't hand-write analysis for the hundreds of thousands of vulnerabilities in the database; that wouldn't be honest. For notable, high-impact vulnerabilities our team writes the mechanism, detection and remediation steps.
We use this product, ask for help- Published
- Mar 19, 2026
- Updated
- Jul 15, 2026
- EPSS
- 0.7% · 51th percentile
- CWE
- CWE-401
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Report tools
Action score
30
Monitor
Low priority for now.
- CVSS
- 30 / 40 · 7.5 / 10
- CISA KEV
- 0 / 30 · Not listed
- EPSS
- 0 / 30 · 0.7%
CISA SSVC decision
- Exploitation
- none
- Automatable
- yes
- Technical impact
- partial
Vulnrichment: CISA's decision-tree inputs.
CNA vs NVD score
- NVD
- —
- CNA · GitHub_M
- 7.5
- NVD has not scored this yet; the score shown is the CNA’s.
The score given by the assigning authority versus NVD’s independent score. A gap means the severity is contested.
Affected systems
| Vendor | Product | CPE |
|---|---|---|
| ultrajson project | ultrajson | cpe:2.3:a:ultrajson_project:ultrajson |
Affected versions
NVD version ranges (for catalog products). Add the product to your stack with a version and matching uses these.
- ultrajson project ultrajson5.4.0 and later · before 5.12.0
Versions reported by the vendor
Affected version ranges reported by the assigning authority (GitHub_M). Independent of NVD's CPE analysis and usually ahead of it.
ultrajson ultrajson
- >= 5.4.0, < 5.12.0affected
Package-level exposure
OSV and GitHub Advisory data: ecosystem, package and range. SBOM matching uses this table.
| Ecosystem | Package | Affected range | Fix |
|---|---|---|---|
| Debian:12 | ujson | all versions | — |
| Debian:14 | ujson | before 5.13.0-1 | 5.13.0-1 |
| PyPI | ujson | from 5.4.0 · before 5.12.0 | 5.12.0 |
Same product
ultrajson project: all recordsOther highest-scoring records for the same primary product.
- CVE-2026-44660UltraJSON: Memory Leak in ujson.dump() on Write Failure34Monitor
- CVE-2022-31116Incorrect handling of invalid surrogate pair characters in ujson31Monitor
- CVE-2026-32875UltraJSON has an integer overflow handling large indent leads to buffer overflow or infinite loop30Monitor
- CVE-2026-54911UltraJSON: Malformed/Truncated UTF-8 Accepted and Silently Rewritten in ujson.dumps()26Monitor
- CVE-2022-31117Double free of buffer during string decoding in ujson24Monitor
- CVE-2021-45958UltraJSON (aka ujson) through 5.1.0 has a stack-based buffer overflow in Buffer_AppendIndentUnchecked (called from encode).22Monitor
Remediation
Which version to upgrade to
Fix versions compiled from the vendor, package registries and Microsoft. Verify the vendor's note before upgrading.
| Product / package | Fixed version | Source |
|---|---|---|
| ultrajson ultrajson | 5.12.0 · description | Vendor (CNA) |
| debian:ujson | 5.13.0-1 · Debian:14 | Package registry (OSV) |
| PyPI:ujson | 5.12.0 | Package registry (OSV) |
Exploit status
No known public exploit
No public exploit has been observed yet. That doesn't mean you're safe, only that the bar is a little higher.
Research context
For pentesters and researchers: attack profile, score disagreement, timeline, patch commits, credits, variant and chain candidates, bug bounty scope. All derived from existing data; no exploit code.
Timeline
From publication to today: proof of concept, Metasploit module, CISA KEV and fix record. Dates are as reported by the sources.
No dated events beyond publication.
FIRST EPSS daily score; only changes of 0.01 or more are recorded (step chart).
Patch and commit links
Commit, PR and diff links among the references. A starting point for patch-diffing and variant hunting; fixes, not exploits.
Credits
All researchersFinders, reporters and analysts named in the CNA record. Click a name for that researcher’s other records.
No credits in the CNA record.
Variant candidates
Same product, same weakness class, within 18 months. If the patch missed the root cause, the sibling bug is here.
- CVE-2026-4466069 days apartUltraJSON: Memory Leak in ujson.dump() on Write Failure34Monitor
Chain candidates
An authentication bypass and a privilege-requiring bug in the same product, published close together: combined they may become an unauthenticated path.
—
Bug bounty scope
No known public program.
Source: bounty-targets-data (public HackerOne, Bugcrowd, Intigriti, YesWeHack listings).
Technical details
Attack conditions
- Anyone who can reach it over the internet can trigger it.
- No account or password is required.
- No user action is required.
- No special conditions are required; it is repeatable.
If successful
- Confidentiality
- none
- Integrity
- none
- Availability
- high · the service can be disrupted
- Attack vector
- Network
- Attack complexity
- Low
- Privileges required
- None
- User interaction
- None
- Scope
- Unchanged
- Confidentiality impact
- None
- Integrity impact
- None
- Availability impact
- High
Weakness class (CWE)
CWE-401 · Missing Release of Memory after Effective LifetimeCVSS vector
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
nvd-secondary
Attack context
MITRE CAPEC attack patterns and ATT&CK techniques for this weakness class (CWE). A starting point for detection rules and threat hunting.
MITRE has no CAPEC/ATT&CK mapping for this CWE.
Change log
- Fix✗ → ✓
For records you follow, these changes also arrive as notifications. →
References
- github.com/ultrajson/ultrajson/commit/4baeb950df780092bd3c89fc702a868e99a3a1d2
- github.com/ultrajson/ultrajson/releases/tag/5.12.0
- github.com/ultrajson/ultrajson/security/advisories/GHSA-wgvc-ghv9-3pmm
- access.redhat.com/security/cve/CVE-2026-32874
- bugzilla.redhat.com/show_bug.cgi?id=2449411
- security.access.redhat.com/data/csaf/v2/vex/2026/cve-2026-32874.json
Vendor advisories and official records. Exploit/PoC links are deliberately left out.