VulnerabilityPublished 31 Jul 2026

CVE-2026-53505: Thumbor proportion filter allows unbounded post-transform resize leading to remote DoS

Affects thumborSource GCVE Database

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Not known to be exploited · CVSS 7.5 High : upgrade to 7.8.0 soon.

Data as of 30 Oct 2026, 06:59 UTC

CVSS 3.1 base score

7.5High

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

EPSS: 0.6% chance of exploitation in 30 days – higher than 49% of CVEsExploit Prediction Scoring System: the likelihood attackers will use this flaw soon.
Patch: Available – fixed in 7.8.0

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 31 Jul 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 thumbor to 7.8.0.

Affected software

Product	Affected	Fixed in
PyPIthumbor	Affected <7.8.0	Fixed in 7.8.0

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

AI-suggested

A patch is available that enforces strict bounds on the 'proportion' filter parameter, limiting it to values between 0.0 and 1.0 as documented. Deployers should upgrade to Thumbor version 7.8.0 or later. If upgrading is not immediately possible, ensure that unsafe URLs are disabled and that signed URLs are tightly controlled to reduce exposure.

Description

### Summary Thumbor's `filters:proportion(<value>)` filter does not enforce an upper bound on `<value>` and runs in the post-transform phase. An attacker can trigger extremely large resizes (CPU/memory exhaustion) and cause denial of service. ### Details - Filter implementation: `thumbor/filters/proportion.py` - `value` is parsed as a float (`BaseFilter.DecimalNumber`) with no maximum. - The filter computes `new_width = source_width * value` and `new_height = source_height * value` and then calls `engine.resize(new_width, new_height)`. - Execution phase: `proportion` runs in the default POST_TRANSFORM phase (after the main transform pipeline). This means it can effectively bypass request-level size clamping that happens earlier in the request lifecycle (e.g., `MAX_WIDTH`/`MAX_HEIGHT` applied to `req.width/req.height`). Documentation states the `percentage` argument should be `0.0 to 1.0` (`docs/proportion.rst`), but the implementation does not enforce this constraint. ### PoC #### Preconditions - The `proportion` filter is enabled (it is enabled by default via `BUILTIN_FILTERS`). - Either: - `/unsafe/` URLs are allowed (`ALLOW_UNSAFE_URL=True`, common default in some deployments), OR - `/unsafe/` is disabled, and the attacker has a valid signed URL (i.e., the attacker is an authorized user/partner, or otherwise can obtain signed URLs issued by a trusted signing service). #### Example request 1 (signed URL) The following request was used to reproduce the issue and causes severe resource exhaustion: `http://<host>:<port>/<url-sign>/100x100/filters:proportion(10000)/example.jpg` #### Example request 2 (/unsafe/) If `/unsafe/` is enabled: `http://<host>:<port>/unsafe/100x100/filters:proportion(10000)/example.jpg` ### Impact - Remote Denial of Service via CPU and/or memory exhaustion (and potentially process crash / OOM kill). - Exploitability depends on deployment: - If `/unsafe/` is enabled: unauthenticated remote DoS. - If `/unsafe/` is disabled: the attacker needs a valid signed URL (i.e., the attacker can legitimately request signed URLs, or has access to signed URLs issued for other users/partners). If signed URLs are not exposed to untrusted parties, exploitability is reduced but the risk still applies to any party who can generate/use signed URLs. ### Suggested remediation - Enforce a strict bound on the `proportion` parameter (e.g., `0.0 < value <= 1.0` as documented), or define a safe maximum based on intended semantics.

Impact

This vulnerability enables remote denial of service by exhausting server CPU and memory resources through unbounded image resizing. If unsafe URLs are enabled, unauthenticated attackers can exploit this. Otherwise, exploitation requires a valid signed URL, limiting risk to authorized users or those with access to signed URLs. The attack can cause process crashes or out-of-memory kills, impacting service availability.

Summary

The Thumbor image processing service's 'filters:proportion(<value>)' filter does not enforce an upper bound on the <value> parameter, which is parsed as a float and used to compute new image dimensions during the post-transform phase. This allows an attacker to request extremely large image resizes, causing CPU and memory exhaustion and resulting in a remote denial of service. The filter runs after the main transform pipeline, bypassing earlier size clamping protections. Exploitation is possible if unsafe URLs are enabled or if the attacker has a valid signed URL. The vulnerability affects Thumbor versions before 7.8.0. A fix is available to enforce strict bounds on the proportion parameter as per documentation.

CWE-400Uncontrolled Resource Consumption (opens in a new tab)
Class weaknessLikelihood of exploit: High
The product does not properly control the allocation and maintenance of a limited resource.
MITRE discourages mapping vulnerabilities to this broad entry; the root cause is usually a more specific weakness.
Architecture and Design
Design throttling mechanisms into the system architecture. The best protection is to limit the amount of resources that an unauthorized user can cause to be expended. A strong authentication and access control model will help prevent such attacks from occurring in the first place.
Architecture and Design
Mitigation of resource exhaustion attacks requires that the target system either: recognizes the attack and denies that user further access for a given amount of time, or; uniformly throttles all requests in order to make it more difficult to consume resources more quickly than they can again be freed.
Architecture and Design
Ensure that protocols have specific limits of scale placed on them.
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
XML Ping of the Death (opens in a new tab)
Regular Expression Exponential Blowup (opens in a new tab)
Sustained Client Engagement (opens in a new tab)
From MITRE CWE, MITRE CAPEC and OWASP (Top 10, OpenCRE).

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Exploitation evidence

What to do

Affected software

Description

AI analysisMachine-generated, updated 31 Jul 2026, 21:32 UTC

WeaknessesCWE-400

Technical details

References

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