VulnerabilityPublished 12 Mar 2026Updated 24 Aug 2026

CVE-2026-28356: defnull multipart · ReDoS

Affects defnull multipartSource CVE Database V5

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Not known to be exploited · CVSS 7.5 High : upgrade to 1.3.1 or 1.2.2 soon.

Data as of 29 Oct 2026, 19:04 UTC

CVSS 3.1 base score

7.5High

How severe the flaw is on the 0–10 CVSS scale.

EPSS: 0.7% chance of exploitation in 30 days – higher than 55% of CVEsExploit Prediction Scoring System: the likelihood attackers will use this flaw soon.
Patch: Available – fixed in 1.3.1, 1.2.2

CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H

Attack Vector: Network
Attack Complexity: Low
Privileges Required: None
User Interaction: None
Scope: Unchanged
Confidentiality: None
Integrity: None
Availability: High

Bold values are the worst case: easiest to exploit or largest impact.

Attack Vector: Network: The vulnerable component is bound to the network stack and the set of possible attackers extends beyond the other options listed, up to and including the entire Internet. Such a vulnerability is often termed 'remotely exploitable' and can be thought of as an attack being exploitable at the protocol level one or more network hops away (e.g., across one or more routers).
Attack Complexity: Low: Specialized access conditions or extenuating circumstances do not exist. An attacker can expect repeatable success against the vulnerable component.
Privileges Required: None: The attacker is unauthorized prior to attack, and therefore does not require any access to settings or files to carry out an attack.
User Interaction: None: The vulnerable system can be exploited without any interaction from any user.
Scope: Unchanged: 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.
Confidentiality: None: There is no loss of confidentiality within the impacted component.
Integrity: None: There is no loss of integrity within the impacted component.
Availability: High: There is total loss of availability, resulting in the attacker being able to fully deny access to resources in the impacted component; this loss is either sustained (while the attacker continues to deliver the attack) or persistent (the condition persists even after the attack has completed). Alternatively, the attacker has the ability to deny some availability, but the loss of availability presents a direct, serious consequence to the impacted component (e.g., the attacker cannot disrupt existing connections, but can prevent new connections; the attacker can repeatedly exploit a vulnerability that, in each instance of a successful attack, leaks a only small amount of memory, but after repeated exploitation causes a service to become completely unavailable).

Definitions: FIRST CVSS v3.1 specification.

Exploitation evidence

Disclosed: evidence found.
CVE published 12 Mar 2026
Detectable: no evidence on record.
No public nuclei template indexed
Weaponized: no evidence on record.
No public exploit code indexed in Metasploit or Exploit-DB
Exploited in the wildExploited: no evidence on record.
Not in CISA KEV and no source reports exploitation
Observed by OffSeq MirageSeen by Mirage: no evidence on record.
No attack traffic recorded by OffSeq Mirage sensors

A missing step is not evidence of safety. Sources: cve.org (opens in a new tab), the Metasploit, Exploit-DB and nuclei-templates index as of 29 Oct 2026.

Disclosed: The CVE record is public on cve.org. Its publication is t = 0.
Detectable: A public nuclei template can find exposed instances, for defenders and attackers alike.
Weaponized: A Metasploit exploit module or Exploit-DB entry exists (Metasploit scanner modules do not count), or CISA SSVC or the CVSS exploit maturity reports a public proof of concept.
Exploited in the wild: CISA KEV lists it, CISA SSVC rates exploitation active, or a source reports attacks.
Observed by OffSeq Mirage: OffSeq Mirage honeypot sensors recorded attack traffic for this CVE, or found exposed hosts without attack traffic.

Moments are spaced evenly, not to scale; a dashed line marks a gap of six months or more. Deltas count from t = 0. Sources that give only a date (CISA KEV, Exploit-DB, or a time of exactly 00:00Z) read “same day” rather than hours. Metasploit modules carry the flaw’s disclosure date, not their release date, and nuclei templates carry no date, so both appear above but not on the timeline.

What to do

Upgrade multipart to 1.3.1 or 1.2.2; multipart to 1.2.2.

Affected software

Product	Affected	Fixed in
defnullmultipart	Affected >=1.3.0 <1.3.1	Fixed in 1.3.1
defnullmultipart	Affected <1.2.2	Fixed in 1.2.2
PyPImultipart`pkg:pypi/multipart`Other threats	Affected <1.2.2	Fixed in 1.2.2

Check your own hosts for these packages with threat-finder (opens in a new tab), our free open-source scanner.

AI-suggested

A fix is available in multipart versions 1.2.2, 1.3.1, and 1.4.0-dev. Users and administrators should upgrade to one of these fixed versions to remediate the vulnerability. There are no vendor advisories indicating that no action is required or that the issue is already mitigated by other means. Therefore, upgrading the multipart library to a fixed version is the recommended mitigation.

Description

## Summary The `parse_options_header()` function in `multipart.py` uses a regular expression with an *ambiguous alternation*, which can cause *exponential backtracking (ReDoS)* when parsing maliciously crafted HTTP or multipart segment headers. This can be abused for **denial of service (DoS)** attacks against web applications using this library to parse request headers or `multipart/form-data` streams. ## Impact Any WSGI or ASGI application using `multipart.parse_form_data()` directly or indirectly (e.g. while parsing `multipart/form-data` streams) is vulnerable. The slow-down is significant enough to block request handling threads for multiple seconds per request. ## Affected versions All versions up to and including `1.3.0` are affected. The issue is fixed in `1.2.2`, `1.3.1` and `1.4.0-dev`.

Impact

Exploitation of this vulnerability can lead to denial of service (DoS) conditions in web applications that use the affected versions of the multipart library to parse HTTP or multipart/form-data headers. The attack causes excessive CPU consumption due to exponential backtracking in the vulnerable regular expression, potentially making the application unresponsive or degraded in availability. There is no impact on confidentiality or integrity reported.

Summary

The multipart library for Python, maintained by defnull, contained a vulnerability (CWE-1333) in its parse_options_header() function prior to versions 1.2.2, 1.3.1, and 1.4.0-dev. The vulnerability arises from the use of a regular expression with ambiguous alternation, which can trigger exponential backtracking when parsing specially crafted HTTP or multipart headers. This behavior can be exploited to cause denial of service (DoS) by exhausting CPU resources during parsing. The vulnerability is tracked as CVE-2026-28356 with a CVSS 3.1 base score of 7.5 (high severity), indicating network attack vector, low attack complexity, no privileges or user interaction required, and impact limited to availability. The issue is fixed in the specified versions. Red Hat has published advisories confirming the vulnerability and providing updates.

CWE-1333Inefficient Regular Expression Complexity (opens in a new tab)
Base weaknessLikelihood of exploit: High
The product uses a regular expression with a worst-case computational complexity that is inefficient and possibly exponential.
Architecture and Design
Use regular expressions that do not support backtracking, e.g. by removing nested quantifiers.
System Configuration
Set backtracking limits in the configuration of the regular expression implementation, such as PHP's pcre.backtrack_limit. Also consider limits on execution time for the process.
Implementation
Do not use regular expressions with untrusted input. If regular expressions must be used, avoid using backtracking in the expression.
OWASP Cheat Sheets
File Upload (opens in a new tab)
OWASP ASVS 4.0.3
V12.1.1 (opens in a new tab)
Attack patterns
Regular Expression Exponential Blowup (opens in a new tab)
From MITRE CWE, MITRE CAPEC and OWASP (Top 10, OpenCRE).

References

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Exploitation evidence

What to do

Affected software

Description

AI analysisMachine-generated, updated 22 Jul 2026, 22:31 UTC

WeaknessesCWE-1333

Technical details

References

Community reviews

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