Description

An attacker that has valid credentials can use a Sieve script with the editheader extension to trigger a use-after-free in the mail editing code, and to write memory contents beyond the intended buffer into the delivered mail. This causes memory leak and opportunity to do memory corruption during mail delivery, which can crash the delivery process and may allow execution of arbitrary code in the context of that process. Disable the Sieve editheader extension. Update to non-vulnerable version. No publicly available exploits are known.

Executive Summary

Technical Details

Mitigations

References

Additional Metadata

{
  "score": "9.1",
  "pubDate": "2026-08-28T12:16:29.260Z",
  "pubdate": "2026-08-28T12:16:29.260Z",
  "executiveSummary": "This vulnerability involves a critical use-after-free defect within the Sieve mail filtering system, specifically impacting the 'editheader' extension.\nThe flaw allows an authenticated attacker to trigger memory corruption during the mail delivery process.\nBy manipulating Sieve scripts, an attacker can cause the application to access previously deallocated memory, resulting in out-of-bounds memory writes into the processed mail content.\nThe potential impact includes information disclosure via memory leakage and the possibility of arbitrary code execution within the security context of the delivery process.\nSuccessful exploitation requires valid user credentials, which are necessary to upload or modify Sieve scripts.\nThe vulnerability poses a severe risk to mail server integrity, as arbitrary code execution could allow for full system compromise or unauthorized access to sensitive mail data.\nDefensive measures currently rely on administrative intervention, as the primary recommendation is the immediate disabling of the vulnerable extension until a patched version is deployed.",
  "technicalDetails": "The root cause of this vulnerability lies in an improper memory management implementation within the mail editing logic associated with the Sieve 'editheader' extension.\nThe vulnerability manifests as a use-after-free (UAF) condition. During the parsing or execution of a crafted Sieve script, the mail delivery component fails to maintain correct reference counting or object lifecycle tracking for internal buffers used to store header information.\nWhen the 'editheader' extension is invoked, the system allocates memory to perform modifications. Due to the flaw, an object pointer is prematurely freed or invalidated, while subsequent operations continue to reference this memory address.\nThe attack flow requires an attacker with valid credentials to upload a malicious Sieve script designed to trigger the specific sequence of header modifications that leads to the UAF state.\nUpon triggering the use-after-free, the attacker influences the allocator to reallocate the freed memory block with controlled data. This effectively results in a 'write-what-where' condition or an out-of-bounds write.\nThis manipulation allows the attacker to inject arbitrary data into the outgoing mail buffer. Beyond simply corrupting the mail header metadata, this memory corruption can lead to the overwrite of sensitive function pointers or internal state variables if the allocator reuse is predictable.\nIf control flow hijacking is achieved through these overwritten pointers, the attacker may divert execution to malicious payloads, leading to arbitrary code execution in the context of the mail delivery process.\nFurthermore, the uncontrolled access to memory contents beyond the designated buffers leads to the leakage of internal memory segments into the resulting email headers, potentially exposing sensitive information from the application heap, such as cryptographic keys, session tokens, or other users' mail data.\nThe vulnerability is limited to environments where the 'editheader' extension is enabled and reachable via user-defined Sieve scripts. As this occurs during the mail delivery phase, the exploitation process is inherently tied to the processing of incoming or filtered messages, which may be automated if the attacker can force delivery of an email to trigger the script execution."
}