CVE-2026-53540
ADVISORY - githubSummary
Summary
parse_form() did not validate the Content-Length header before using it to bound its chunked read of the request body. A negative Content-Length turned the bounded read into a read-until-EOF, so the entire body was loaded into memory in a single read instead of in fixed-size chunks.
Details
parse_form() reads the input stream in chunks, never reading more than the remaining Content-Length at a time. The per-chunk size is computed as min(content_length - bytes_read, chunk_size). The header value was parsed to an integer without checking its sign, so a Content-Length of -1 made this expression negative, and input_stream.read(-1) reads until end of stream. The intended bounded, chunked read therefore collapsed into a single unbounded read of the whole stream. The amount read is still bounded by what the client actually sends.
Impact
This only affects code that calls parse_form() directly with a Content-Length header taken from attacker-controlled input and without normalizing a negative value first. No known package is affected:
- Starlette and FastAPI drive
MultipartParserdirectly from the ASGIreceive()stream and do not callparse_form(). - Known
parse_form()consumers either do not forwardContent-Lengthto it, recompute it from the already-read body, or run behind a layer (such as Werkzeug) that normalizes a negativeContent-Lengthto0.
The realistic exposure is limited to bespoke WSGI or http.server handlers that forward raw client headers into parse_form(). In that case a crafted request buffers the body in memory at once, degrading availability under concurrent requests rather than causing a complete denial of service.
Mitigation
Upgrade to version 0.0.31 or later, which rejects a negative Content-Length with a ValueError before reading the stream.
Common Weakness Enumeration (CWE)
Improper Validation of Specified Quantity in Input
GitHub
2.2
CVSS SCORE
3.7low| Package | Type | OS Name | OS Version | Affected Ranges | Fix Versions |
|---|---|---|---|---|---|
| python-multipart | pypi | - | - | <0.0.31 | 0.0.31 |
CVSS:3 Severity and metrics
The CVSS metrics represent different qualitative aspects of a vulnerability that impact the overall score, as defined by the CVSS Specification.
The vulnerable component is bound to the network stack, but the attack is limited at the protocol level to a logically adjacent topology. This can mean an attack must be launched from the same shared physical (e.g., Bluetooth or IEEE 802.11) or logical (e.g., local IP subnet) network, or from within a secure or otherwise limited administrative domain (e.g., MPLS, secure VPN to an administrative network zone). One example of an Adjacent attack would be an ARP (IPv4) or neighbor discovery (IPv6) flood leading to a denial of service on the local LAN segment (e.g., CVE-2013-6014).
A successful attack depends on conditions beyond the attacker's control, requiring investing a measurable amount of effort in research, preparation, or execution against the vulnerable component before a successful attack.
The attacker is unauthorized prior to attack, and therefore does not require any access to settings or files of the vulnerable system to carry out an attack.
The vulnerable system can be exploited without interaction from any user.
An exploited vulnerability can only affect resources managed by the same security authority. In this case, the vulnerable component and the impacted component are either the same, or both are managed by the same security authority.
There is no loss of confidentiality.
There is no loss of trust or accuracy within the impacted component.
Performance is reduced or there are interruptions in resource availability. Even if repeated exploitation of the vulnerability is possible, the attacker does not have the ability to completely deny service to legitimate users. The resources in the impacted component are either partially available all of the time, or fully available only some of the time, but overall there is no direct, serious consequence to the impacted component.
minimos
MINI-53xp-8722-9947
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minimos
MINI-58xh-jq4g-rrw6
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minimos
MINI-7hff-x4rc-g53c
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minimos
MINI-gwpm-ff78-8hjc
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