CVE-2026-53501 (GCVE-0-2026-53501)

Vulnerability from cvelistv5 – Published: 2026-07-31 18:42 – Updated: 2026-07-31 19:58

Title

Thumbor has HMAC validation bypass via multiple .replace() calls when removing URL signature

Summary

Thumbor is an open-source photo thumbnail service by globo.com. Prior to 7.8.0, Thumbor’s HMAC validation can be bypassed due to the use of Python’s .replace() when removing the signature from the URL before validation. Since .replace() removes all occurrences of the substring, an attacker can insert the same signature multiple times in the URL and manipulate the final URL used for validation. This allows crafting URLs where the validated string differs from the actual requested resource, enabling loading images from unintended domains or paths. This issue is fixed in 7.8.0.

Severity

8.2 (High)


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

SSVC

Exploitation: none Automatable: yes Technical Impact: partial

CISA Coordinator (v2.0.3)

CWE

CWE-347 - Improper Verification of Cryptographic Signature

Assigner

GitHub_M

References

3 references

URL	Tags
https://github.com/thumbor/thumbor/security/advis…	x_refsource_CONFIRM
https://github.com/thumbor/thumbor/commit/e3ae3e2…	x_refsource_MISC
https://github.com/thumbor/thumbor/releases/tag/7.8.0	x_refsource_MISC

Impacted products

1 product

Vendor	Product	Version
thumbor	thumbor	Affected: < 7.8.0

Show details on NVD website

JSON

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GHSA-MW3H-QJXJ-6XG9

Vulnerability from github – Published: 2026-07-31 18:51 – Updated: 2026-07-31 18:51

Summary

Thumbor has HMAC validation bypass via multiple .replace() calls when removing URL signature

Details

HMAC validation bypass via multiple `.replace()` calls when removing URL signature

Summary

Thumbor’s HMAC validation can be bypassed due to the use of Python’s .replace() when removing the signature from the URL before validation. Since .replace() removes all occurrences of the substring, an attacker can insert the same signature multiple times in the URL and manipulate the final URL used for validation.

This allows crafting URLs where the validated string differs from the actual requested resource, enabling loading images from unintended domains or paths.

Details

Thumbor signs URLs using HMAC-SHA1 to prevent abuse such as loading arbitrary external images or invoking filters without authorization.

During request validation, Thumbor removes the signature from the request URL before recalculating the HMAC. The relevant code:

url_signature = self.context.request.hash
if url_signature:
    signer = self.context.modules.url_signer(
        self.context.server.security_key
    )

    try:
        quoted_hash = quote(self.context.request.hash)
    except KeyError:
        self._error(400, f"Invalid hash: {self.context.request.hash}")
        return

    url_to_validate = url.replace(
        f"/{self.context.request.hash}/", ""
    ).replace(f"/{quoted_hash}/", "")

    valid = signer.validate(
        unquote(url_signature).encode(), url_to_validate
    )

The issue is that .replace() removes every occurrence of the substring in the URL, not just the first one.

Because the signature itself appears in the URL, an attacker can inject additional copies of the signature elsewhere in the path. When Thumbor performs .replace(), these extra occurrences are also removed, resulting in a different url_to_validate than the original request.

Example

Valid request:

/ddoyYVYUbDf6Po_dzOrBhCDrXLc=/300x200/s3.glbimg.com/v1/.../image.jpg

By injecting the same hash inside the URL:

/ddoyYVYUbDf6Po_dzOrBhCDrXLc=/300x200/s3.glbimg.co/ddoyYVYUbDf6Po_dzOrBhCDrXLc=/m/v1/.../image.jpg

After .replace() removes all occurrences of the hash, the URL used for validation differs from the effective request path. This allows manipulation of the upstream host or path.

Further manipulation is possible due to the second .replace() for the URL-encoded hash (%3D), enabling more precise path manipulation.

Example:

/ddoyYVYUbDf6Po_dzOrBhCDrXLc=/300x200/s3.glbimg.com/ddoyYVYUbDf6Po/ddoyYVYUbDf6Po_dzOrBhCDrXLc%3D/ddoyYVYUbDf6Po/v1/...

Impact

This behavior may allow attackers to:

Bypass HMAC URL validation
Load images from arbitrary domains
Abuse Thumbor deployments as an open proxy / image fetcher
Circumvent domain restrictions intended by the signed URL mechanism

Root Cause

Use of .replace() without limiting the number of replacements when removing the signature from the URL.

Since the signature is part of the request path, using a global replacement allows attackers to place additional occurrences of the same substring to influence the validated string.

Suggested Fix

Remove only the first occurrence of the signature or explicitly parse the URL components instead of performing global string replacement.

Example:

url.replace(f"/{self.context.request.hash}/", "", 1)

Alternatively, reconstruct the unsigned URL deterministically from the parsed request components.

Severity

8.2 (High)


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

Show details on source website

JSON

To clipboard

{
  "affected": [
    {
      "database_specific": {
        "last_known_affected_version_range": "\u003c= 7.7.7"
      },
      "package": {
        "ecosystem": "PyPI",
        "name": "thumbor"
      },
      "ranges": [
        {
          "events": [
            {
              "introduced": "0"
            },
            {
              "fixed": "7.8.0"
            }
          ],
          "type": "ECOSYSTEM"
        }
      ]
    }
  ],
  "aliases": [
    "CVE-2026-53501"
  ],
  "database_specific": {
    "cwe_ids": [
      "CWE-347"
    ],
    "github_reviewed": true,
    "github_reviewed_at": "2026-07-31T18:51:54Z",
    "nvd_published_at": null,
    "severity": "HIGH"
  },
  "details": "# HMAC validation bypass via multiple `.replace()` calls when removing URL signature\n\n## Summary\n\nThumbor\u2019s HMAC validation can be bypassed due to the use of Python\u2019s `.replace()` when removing the signature from the URL before validation. Since `.replace()` removes **all occurrences** of the substring, an attacker can insert the same signature multiple times in the URL and manipulate the final URL used for validation.\n\nThis allows crafting URLs where the validated string differs from the actual requested resource, enabling loading images from unintended domains or paths.\n\n## Details\n\nThumbor signs URLs using **HMAC-SHA1** to prevent abuse such as loading arbitrary external images or invoking filters without authorization.\n\nDuring request validation, Thumbor removes the signature from the request URL before recalculating the HMAC. The relevant code:\n\n```python\nurl_signature = self.context.request.hash\nif url_signature:\n    signer = self.context.modules.url_signer(\n        self.context.server.security_key\n    )\n\n    try:\n        quoted_hash = quote(self.context.request.hash)\n    except KeyError:\n        self._error(400, f\"Invalid hash: {self.context.request.hash}\")\n        return\n\n    url_to_validate = url.replace(\n        f\"/{self.context.request.hash}/\", \"\"\n    ).replace(f\"/{quoted_hash}/\", \"\")\n\n    valid = signer.validate(\n        unquote(url_signature).encode(), url_to_validate\n    )\n```\n\nThe issue is that `.replace()` removes **every occurrence** of the substring in the URL, not just the first one.\n\nBecause the signature itself appears in the URL, an attacker can **inject additional copies of the signature** elsewhere in the path. When Thumbor performs `.replace()`, these extra occurrences are also removed, resulting in a different `url_to_validate` than the original request.\n\n## Example\n\nValid request:\n\n```\n/ddoyYVYUbDf6Po_dzOrBhCDrXLc=/300x200/s3.glbimg.com/v1/.../image.jpg\n```\n\nBy injecting the same hash inside the URL:\n\n```\n/ddoyYVYUbDf6Po_dzOrBhCDrXLc=/300x200/s3.glbimg.co/ddoyYVYUbDf6Po_dzOrBhCDrXLc=/m/v1/.../image.jpg\n```\n\nAfter `.replace()` removes all occurrences of the hash, the URL used for validation differs from the effective request path. This allows manipulation of the upstream host or path.\n\nFurther manipulation is possible due to the **second `.replace()` for the URL-encoded hash (`%3D`)**, enabling more precise path manipulation.\n\nExample:\n\n```\n/ddoyYVYUbDf6Po_dzOrBhCDrXLc=/300x200/s3.glbimg.com/ddoyYVYUbDf6Po/ddoyYVYUbDf6Po_dzOrBhCDrXLc%3D/ddoyYVYUbDf6Po/v1/...\n```\n\n## Impact\n\nThis behavior may allow attackers to:\n\n- Bypass HMAC URL validation\n- Load images from arbitrary domains\n- Abuse Thumbor deployments as an open proxy / image fetcher\n- Circumvent domain restrictions intended by the signed URL mechanism\n\n## Root Cause\n\nUse of `.replace()` without limiting the number of replacements when removing the signature from the URL.\n\nSince the signature is part of the request path, using a global replacement allows attackers to place additional occurrences of the same substring to influence the validated string.\n\n## Suggested Fix\n\nRemove only the first occurrence of the signature or explicitly parse the URL components instead of performing global string replacement.\n\nExample:\n\n```python\nurl.replace(f\"/{self.context.request.hash}/\", \"\", 1)\n```\n\nAlternatively, reconstruct the unsigned URL deterministically from the parsed request components.",
  "id": "GHSA-mw3h-qjxj-6xg9",
  "modified": "2026-07-31T18:51:54Z",
  "published": "2026-07-31T18:51:54Z",
  "references": [
    {
      "type": "WEB",
      "url": "https://github.com/thumbor/thumbor/security/advisories/GHSA-mw3h-qjxj-6xg9"
    },
    {
      "type": "WEB",
      "url": "https://github.com/thumbor/thumbor/commit/e3ae3e2500537b4d735df4144129a649374bb70b"
    },
    {
      "type": "PACKAGE",
      "url": "https://github.com/thumbor/thumbor"
    },
    {
      "type": "WEB",
      "url": "https://github.com/thumbor/thumbor/releases/tag/7.8.0"
    }
  ],
  "schema_version": "1.4.0",
  "severity": [
    {
      "score": "CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:L",
      "type": "CVSS_V3"
    }
  ],
  "summary": "Thumbor has HMAC validation bypass via multiple .replace() calls when removing URL signature"
}

Sightings

Author	Source	Type	Date	Other

Nomenclature

Seen: The vulnerability was mentioned, discussed, or observed by the user.
Confirmed: The vulnerability has been validated from an analyst's perspective.
Published Proof of Concept: A public proof of concept is available for this vulnerability.
Exploited: The vulnerability was observed as exploited by the user who reported the sighting.
Patched: The vulnerability was observed as successfully patched by the user who reported the sighting.
Not exploited: The vulnerability was not observed as exploited by the user who reported the sighting.
Not confirmed: The user expressed doubt about the validity of the vulnerability.
Not patched: The vulnerability was not observed as successfully patched by the user who reported the sighting.

Detection rules are retrieved from Rulezet.

The MITRE ATT&CK techniques below are AI-generated suggestions, inferred from the description of the vulnerability by the CIRCL/vulnerability-attack-technique-classification-roberta-base model, served locally by ML-Gateway. They have not been verified by an analyst and are provided for guidance only.

Action not permitted

CVE-2026-53501 (GCVE-0-2026-53501)

GHSA-MW3H-QJXJ-6XG9

HMAC validation bypass via multiple .replace() calls when removing URL signature

Summary

Details

Example

Impact

Root Cause

Suggested Fix

Tags

Sightings

Nomenclature

HMAC validation bypass via multiple `.replace()` calls when removing URL signature