VulnerabilityPublished 31 Jul 2026

CVE-2026-53503: Thumbor Convolution Filter Allows Divide-by-zero

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 31 Oct 2026, 08:20 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 54% 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 30 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

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AI-suggested

A patch is available for Thumbor to reject zero values for the 'columns' parameter in the convolution filter. Users should upgrade to version 7.8.0 or later where this validation is implemented. The fix includes checks in both the Python filter and the C extension to prevent divide-by-zero errors. Until patched, restricting access to signed URLs and disabling unsafe URLs can reduce exposure.

Description

### Summary Thumbor's `filters:convolution(<matrix>, <columns>, <should_normalize>)` filter passes the user-controlled `<columns>` value to a C extension (`thumbor/ext/filters/_convolution.c`) where it is used as a divisor (for `%` and `/`) without validating `columns > 0`. When `columns=0`, the C code triggers undefined behavior; on x86_64 this reliably results in a fatal divide-by-zero trap (SIGFPE) and crashes the Thumbor process (confirmed on Linux x86_64 and macOS Intel x86_64), causing a remote denial of service. ### Details #### Root cause The Python filter accepts `columns=0`, and the native C extension uses `columns_count` as a divisor without validating it. 1) Python filter entry point allows `columns=0` (`thumbor/filters/convolution.py`): ```python @filter_method( r"(?:[-]?[\d]+\.?[\d]*[;])*(?:[-]?[\d]+\.?[\d]*)", BaseFilter.PositiveNumber, # accepts 0 BaseFilter.Boolean, ) async def convolution(self, matrix, columns, should_normalize=True): ... imgdata = _convolution.apply(..., matrix, columns, should_normalize) ``` `BaseFilter.PositiveNumber` matches `"0"` (`thumbor/filters/__init__.py`): ```python class BaseFilter: PositiveNumber = {"regex": r"[\d]+", "parse": int} # matches "0" PositiveNonZeroNumber = {"regex": r"[\d]*[1-9][\d]*", "parse": int} ``` 2) C extension divides/modulos by `columns_count` without a zero check (`thumbor/ext/filters/_convolution.c`): ```c kernel_size = PyTuple_Size(kernel_tuple); if ((kernel_size % columns_count != 0) || (kernel_size % 2 == 0) || ((kernel_size / columns_count) % 2) == 0) { // TODO: error, not a valid kernel return NULL; } ``` ### PoC #### Test environment - Linux x86_64 #### Preconditions - The `convolution` filter is enabled (it is enabled by default via `BUILTIN_FILTERS`). - Either: - `/unsafe/` URLs are allowed (`ALLOW_UNSAFE_URL=True`), OR - `/unsafe/` is disabled, and the attacker has a valid signed URL (i.e., the attacker is an authorized user/partner, or can obtain signed URLs from a trusted signing service). #### Example request (signed URL) `http://<host>:<port>/<url-sign>/400x400/filters:convolution(1;2;1;2;4;2;1;2;1,0,true)/example.jpg` #### Example request (/unsafe/) `http://<host>:<port>/unsafe/400x400/filters:convolution(1;2;1;2;4;2;1;2;1,0,true)/example.jpg` ### Impact - Remote Denial of Service via process crash (SIGFPE) on x86_64 (confirmed on Linux x86_64 and macOS Intel x86_64). - 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 is an authorized user/partner, or can obtain signed URLs from a trusted signing service) ### Suggested remediation - In the C extension (`thumbor/ext/filters/_convolution.c`): - Reject `columns_count <= 0` before any `%` or `/`. - In the Python filter (`thumbor/filters/convolution.py`): - Require `columns` to be non-zero (e.g., use `BaseFilter.PositiveNonZeroNumber`).

Impact

Successful exploitation causes a remote denial of service by crashing the Thumbor process with a fatal divide-by-zero error. There is no impact on confidentiality or integrity. The exploitability depends on whether unsafe URLs are enabled or if the attacker has access to valid signed URLs.

Summary

Thumbor's convolution filter accepts a user-controlled 'columns' parameter that is passed to a native C extension without validation for zero. The C extension performs division and modulo operations using this parameter as a divisor, leading to a divide-by-zero trap (SIGFPE) when 'columns' is zero. This causes the Thumbor process to crash, resulting in a remote denial of service. The vulnerability affects Thumbor versions before 7.8.0. Exploitation depends on deployment configuration: unauthenticated if unsafe URLs are allowed, or authenticated if only signed URLs are permitted. The recommended remediation is to validate and reject zero values for 'columns' in both the Python filter and the C extension.

CWE-20Improper Input Validation (opens in a new tab)
Class weaknessLikelihood of exploit: HighCWE Top 25 (2025): #18OWASP Top 10 A05:2025: Injection (opens in a new tab)
The product receives input or data, but it does not validate or incorrectly validates that the input has the properties that are required to process the data safely and correctly.
MITRE discourages mapping vulnerabilities to this broad entry; the root cause is usually a more specific weakness.
Architecture and Design · Attack Surface Reduction
Consider using language-theoretic security (LangSec) techniques that characterize inputs using a formal language and build "recognizers" for that language. This effectively requires parsing to be a distinct layer that effectively enforces a boundary between raw input and internal data representations, instead of…
Architecture and Design · Libraries or Frameworks
Use an input validation framework such as Struts or the OWASP ESAPI Validation API. Note that using a framework does not automatically address all input validation problems; be mindful of weaknesses that could arise from misusing the framework itself (CWE-1173).
Architecture and Design · Implementation · Attack Surface Reduction
Understand all the potential areas where untrusted inputs can enter the product, including but not limited to: parameters or arguments, cookies, anything read from the network, environment variables, reverse DNS lookups, query results, request headers, URL components, e-mail, files, filenames, databases, and any…
OWASP Cheat Sheets
XML Security (opens in a new tab)
Mass Assignment (opens in a new tab)
Injection Prevention (opens in a new tab)
SQL Injection Prevention (opens in a new tab)
Injection Prevention in Java (opens in a new tab)
OWASP ASVS 4.0.3
V13.3.1 (opens in a new tab)
V5.1.4 (opens in a new tab)
V5.1.3 (opens in a new tab)
Attack patterns
Buffer Overflow via Environment Variables (opens in a new tab)
Server Side Include (SSI) Injection (opens in a new tab)
Cross Zone Scripting (opens in a new tab)
Command Line Execution through SQL Injection (opens in a new tab)
Object Relational Mapping Injection (opens in a new tab)
From MITRE CWE, MITRE CAPEC and OWASP (Top 10, OpenCRE).
CWE-369Divide By Zero (opens in a new tab)
Base weaknessLikelihood of exploit: MediumOWASP Top 10 A10:2025: Mishandling of Exceptional Conditions (opens in a new tab)
The product divides a value by zero.
OWASP Cheat Sheets
Logging (opens in a new tab)
Error Handling (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 31 Jul 2026, 20:33 UTC

WeaknessesCWE-20, CWE-369

Technical details

References

Community reviews

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