VulnerabilityPublished 31 Jul 2026

CVE-2026-53502: Thumbor has path traversal via post-validation URL decoding bypass in file_loader

Affects thumborSource GCVE Database

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Not known to be exploited · CVSS 8.7 High : upgrade to 7.8.0 soon.

Data as of 30 Oct 2026, 06:59 UTC

CVSS 4.0 base score

8.7High

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

EPSS: 0.5% chance of exploitation in 30 days – higher than 44% of CVEsExploit Prediction Scoring System: the likelihood attackers will use this flaw soon.
Patch: Available – fixed in 7.8.0

CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:H/VI:N/VA:N/SC:N/SI:N/SA:N

Attack Vector: Network
Attack Complexity: Low
Attack Requirements: None
Privileges Required: None
User Interaction: None
Vulnerable system confidentiality: High
Vulnerable system integrity: None
Vulnerable system availability: None
Subsequent system confidentiality: None
Subsequent system integrity: None
Subsequent system availability: None

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

Attack Vector: Network: The vulnerable system is bound to the network stack and the set of possible attackers extends beyond the other options listed below, 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: The attacker must take no measurable action to exploit the vulnerability. The attack requires no target-specific circumvention to exploit the vulnerability. An attacker can expect repeatable success against the vulnerable system.
Attack Requirements: None: The successful attack does not depend on the deployment and execution conditions of the vulnerable system. The attacker can expect to be able to reach the vulnerability and execute the exploit under all or most instances of the vulnerability.
Privileges Required: None: 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.
User Interaction: None: The vulnerable system can be exploited without interaction from any human user, other than the attacker.
Vulnerable system confidentiality: High: There is a total loss of confidentiality, resulting in all information within the Vulnerable System being divulged to the attacker. Alternatively, access to only some restricted information is obtained, but the disclosed information presents a direct, serious impact. For example, an attacker steals the administrator's password, or private encryption keys of a web server.
Vulnerable system integrity: None: There is no loss of integrity within the Vulnerable System.
Vulnerable system availability: None: There is no impact to availability within the Vulnerable System.
Subsequent system confidentiality: None: There is no loss of confidentiality within the Subsequent System or all confidentiality impact is constrained to the Vulnerable System.
Subsequent system integrity: None: There is no loss of integrity within the Subsequent System or all integrity impact is constrained to the Vulnerable System.
Subsequent system availability: None: There is no impact to availability within the Subsequent System or all availability impact is constrained to the Vulnerable System.

Definitions: FIRST CVSS v4.0 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

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AI-suggested

A patch is available for this vulnerability. Users should upgrade Thumbor to version 7.8.0 or later where the issue is fixed. Until patched, consider disabling the watermark and frame filters or setting ALLOW_UNSAFE_URL to false to reduce exposure. Review the vendor advisory for the official fix and remediation guidance.

Description

The file_loader performs `unquote()` on the file path AFTER the `abspath() + startswith()` security check. An attacker can use percent-encoded path traversal sequences (`%2e%2e` for `..`) that pass the security check as literal directory names, but are then decoded to actual `..` traversal sequences. ## Affected code `thumbor/loaders/file_loader.py` lines 28-49: ```python async def load(context, path): file_path = join( context.config.FILE_LOADER_ROOT_PATH.rstrip("/"), path.lstrip("/") ) file_path = abspath(file_path) inside_root_path = file_path.startswith( # Security check abspath(context.config.FILE_LOADER_ROOT_PATH) ) result = LoaderResult() if not inside_root_path: result.error = LoaderResult.ERROR_NOT_FOUND result.successful = False return result if not exists(file_path): file_path = unquote(file_path) # unquote AFTER check! if exists(file_path) and isfile(file_path): with open(file_path, "rb") as source_file: ... ``` The watermark and frame filters pass their URL parameters directly to the loader without re-encoding (unlike the main image URL flow which applies `quote()` in `imaging.py` line 37). ## PoC ```bash # Read /etc/passwd via watermark filter path traversal # %252e is double-encoded: Tornado decodes to %2e, filter passes to file_loader, # file_loader unquote decodes %2e to . curl 'http://thumbor-host:8888/unsafe/filters:watermark(%252e%252e/%252e%252e/%252e%252e/%252e%252e/etc/passwd,0,0,100)/some-valid-image.jpg' ``` Flow: 1. `%252e%252e` arrives at Tornado, decoded to `%2e%2e` 2. Watermark filter passes `%2e%2e/%2e%2e/.../etc/passwd` to file_loader 3. `join(ROOT, '%2e%2e/...')` = `ROOT/%2e%2e/...` 4. `abspath()` sees `%2e%2e` as literal dir name (no dots to resolve) 5. `startswith(ROOT)` = True (passes check) 6. `exists()` returns False (literal `%2e%2e` dir doesn't exist) 7. `unquote()` converts `%2e%2e` to `..` 8. OS resolves `..` → reads files outside ROOT ## Prerequisites - `LOADER = thumbor.loaders.file_loader` (or `file_loader_http_fallback`) - `ALLOW_UNSAFE_URL = True` (this is the default) - watermark/frame filters are enabled by default (in BUILTIN_FILTERS) ## Impact Arbitrary file read on the thumbor server. When the file is a valid image format, its content is returned in the response overlaid as a watermark. Can be used to read configuration files, secrets, private keys, etc.

Impact

An attacker can read arbitrary files on the Thumbor server by exploiting the path traversal vulnerability. When the accessed file is a valid image, its content can be returned in the response overlaid as a watermark. This can expose sensitive information such as configuration files, secrets, and private keys, leading to potential compromise of the server or associated systems.

Summary

Thumbor's file_loader performs an abspath and startswith check to ensure file paths are within a configured root directory. However, it applies URL decoding (unquote) only after this check. Attackers can exploit this by sending double-encoded path traversal sequences (e.g., %252e%252e) that decode to '..' after the check, allowing traversal outside the root directory. This flaw affects the watermark and frame filters that pass parameters directly to the loader without re-encoding. The vulnerability allows arbitrary file reads on the Thumbor server, potentially exposing sensitive files. The vulnerability is tracked as CVE-2026-53502 and affects Thumbor versions before 7.8.0.

CWE-22Improper Limitation of a Pathname to a Restricted Directory (Path Traversal) (opens in a new tab)
Base weaknessLikelihood of exploit: HighCWE Top 25 (2025): #6OWASP Top 10 A01:2025: Broken Access Control (opens in a new tab)
The product uses external input to construct a pathname that is intended to identify a file or directory that is located underneath a restricted parent directory, but the product does not properly neutralize special elements within the pathname that can cause the pathname to resolve to a location that is outside of the restricted directory.
Implementation · Input Validation
Assume all input is malicious. Use an "accept known good" input validation strategy, i.e., use a list of acceptable inputs that strictly conform to specifications. Reject any input that does not strictly conform to specifications, or transform it into something that does.
Architecture and Design
For any security checks that are performed on the client side, ensure that these checks are duplicated on the server side, in order to avoid CWE-602. Attackers can bypass the client-side checks by modifying values after the checks have been performed, or by changing the client to remove the client-side checks…
Implementation · Input Validation
Inputs should be decoded and canonicalized to the application's current internal representation before being validated (CWE-180). Make sure that the application does not decode the same input twice (CWE-174).
OWASP Cheat Sheets
Authorization (opens in a new tab)
OWASP ASVS 4.0.3
V12.3.1 (opens in a new tab)
Attack patterns
Path Traversal (opens in a new tab)
Using Slashes and URL Encoding Combined to Bypass Validation Logic (opens in a new tab)
Manipulating Web Input to File System Calls (opens in a new tab)
Using Escaped Slashes in Alternate Encoding (opens in a new tab)
Using Slashes in Alternate Encoding (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, 21:33 UTC

WeaknessesCWE-22

Technical details

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